Sync to upstream/release/532 (#545)

Analysis/include/Luau/Constraint.h

@@ -0,0 +1,82 @@
+// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+#pragma once
+
+#include "Luau/Location.h"
+#include "Luau/NotNull.h"
+#include "Luau/Variant.h"
+
+#include <memory>
+#include <vector>
+
+namespace Luau
+{
+
+struct Scope2;
+struct TypeVar;
+using TypeId = const TypeVar*;
+
+struct TypePackVar;
+using TypePackId = const TypePackVar*;
+
+// subType <: superType
+struct SubtypeConstraint
+{
+    TypeId subType;
+    TypeId superType;
+};
+
+// subPack <: superPack
+struct PackSubtypeConstraint
+{
+    TypePackId subPack;
+    TypePackId superPack;
+};
+
+// subType ~ gen superType
+struct GeneralizationConstraint
+{
+    TypeId generalizedType;
+    TypeId sourceType;
+    Scope2* scope;
+};
+
+// subType ~ inst superType
+struct InstantiationConstraint
+{
+    TypeId subType;
+    TypeId superType;
+};
+
+using ConstraintV = Variant<SubtypeConstraint, PackSubtypeConstraint, GeneralizationConstraint, InstantiationConstraint>;
+using ConstraintPtr = std::unique_ptr<struct Constraint>;
+
+struct Constraint
+{
+    Constraint(ConstraintV&& c, Location location);
+
+    Constraint(const Constraint&) = delete;
+    Constraint& operator=(const Constraint&) = delete;
+
+    ConstraintV c;
+    Location location;
+    std::vector<NotNull<Constraint>> dependencies;
+};
+
+inline Constraint& asMutable(const Constraint& c)
+{
+    return const_cast<Constraint&>(c);
+}
+
+template<typename T>
+T* getMutable(Constraint& c)
+{
+    return ::Luau::get_if<T>(&c.c);
+}
+
+template<typename T>
+const T* get(const Constraint& c)
+{
+    return getMutable<T>(asMutable(c));
+}
+
+} // namespace Luau

Analysis/include/Luau/ConstraintGraphBuilder.h

@@ -4,9 +4,12 @@
 
 #include <memory>
 #include <vector>
+#include <unordered_map>
 
 #include "Luau/Ast.h"
+#include "Luau/Constraint.h"
 #include "Luau/Module.h"
+#include "Luau/NotNull.h"
 #include "Luau/Symbol.h"
 #include "Luau/TypeVar.h"
 #include "Luau/Variant.h"
@@ -14,69 +17,6 @@
 namespace Luau
 {
 
-struct Scope2;
-
-// subType <: superType
-struct SubtypeConstraint
-{
-    TypeId subType;
-    TypeId superType;
-};
-
-// subPack <: superPack
-struct PackSubtypeConstraint
-{
-    TypePackId subPack;
-    TypePackId superPack;
-};
-
-// subType ~ gen superType
-struct GeneralizationConstraint
-{
-    TypeId subType;
-    TypeId superType;
-    Scope2* scope;
-};
-
-// subType ~ inst superType
-struct InstantiationConstraint
-{
-    TypeId subType;
-    TypeId superType;
-};
-
-using ConstraintV = Variant<SubtypeConstraint, PackSubtypeConstraint, GeneralizationConstraint, InstantiationConstraint>;
-using ConstraintPtr = std::unique_ptr<struct Constraint>;
-
-struct Constraint
-{
-    Constraint(ConstraintV&& c);
-    Constraint(ConstraintV&& c, std::vector<Constraint*> dependencies);
-
-    Constraint(const Constraint&) = delete;
-    Constraint& operator=(const Constraint&) = delete;
-
-    ConstraintV c;
-    std::vector<Constraint*> dependencies;
-};
-
-inline Constraint& asMutable(const Constraint& c)
-{
-    return const_cast<Constraint&>(c);
-}
-
-template<typename T>
-T* getMutable(Constraint& c)
-{
-    return ::Luau::get_if<T>(&c.c);
-}
-
-template<typename T>
-const T* get(const Constraint& c)
-{
-    return getMutable<T>(asMutable(c));
-}
-
 struct Scope2
 {
     // The parent scope of this scope. Null if there is no parent (i.e. this
@@ -102,6 +42,11 @@ struct ConstraintGraphBuilder
     TypeArena* const arena;
     // The root scope of the module we're generating constraints for.
     Scope2* rootScope;
+    // A mapping of AST node to TypeId.
+    DenseHashMap<const AstExpr*, TypeId> astTypes{nullptr};
+    // A mapping of AST node to TypePackId.
+    DenseHashMap<const AstExpr*, TypePackId> astTypePacks{nullptr};
+    DenseHashMap<const AstExpr*, TypeId> astOriginalCallTypes{nullptr};
 
     explicit ConstraintGraphBuilder(TypeArena* arena);
 
@@ -128,8 +73,9 @@ struct ConstraintGraphBuilder
      * Adds a new constraint with no dependencies to a given scope.
      * @param scope the scope to add the constraint to. Must not be null.
      * @param cv the constraint variant to add.
+     * @param location the location to attribute to the constraint.
      */
-    void addConstraint(Scope2* scope, ConstraintV cv);
+    void addConstraint(Scope2* scope, ConstraintV cv, Location location);
 
     /**
      * Adds a constraint to a given scope.
@@ -148,15 +94,48 @@ struct ConstraintGraphBuilder
     void visit(Scope2* scope, AstStat* stat);
     void visit(Scope2* scope, AstStatBlock* block);
     void visit(Scope2* scope, AstStatLocal* local);
-    void visit(Scope2* scope, AstStatLocalFunction* local);
+    void visit(Scope2* scope, AstStatLocalFunction* function);
-    void visit(Scope2* scope, AstStatReturn* local);
+    void visit(Scope2* scope, AstStatFunction* function);
+    void visit(Scope2* scope, AstStatReturn* ret);
+    void visit(Scope2* scope, AstStatAssign* assign);
+    void visit(Scope2* scope, AstStatIf* ifStatement);
+
+    TypePackId checkExprList(Scope2* scope, const AstArray<AstExpr*>& exprs);
 
     TypePackId checkPack(Scope2* scope, AstArray<AstExpr*> exprs);
     TypePackId checkPack(Scope2* scope, AstExpr* expr);
 
+    /**
+     * Checks an expression that is expected to evaluate to one type.
+     * @param scope the scope the expression is contained within.
+     * @param expr the expression to check.
+     * @return the type of the expression.
+     */
     TypeId check(Scope2* scope, AstExpr* expr);
+
+    TypeId checkExprTable(Scope2* scope, AstExprTable* expr);
+    TypeId check(Scope2* scope, AstExprIndexName* indexName);
+
+    std::pair<TypeId, Scope2*> checkFunctionSignature(Scope2* parent, AstExprFunction* fn);
+
+    /**
+     * Checks the body of a function expression.
+     * @param scope the interior scope of the body of the function.
+     * @param fn the function expression to check.
+     */
+    void checkFunctionBody(Scope2* scope, AstExprFunction* fn);
 };
 
-std::vector<const Constraint*> collectConstraints(Scope2* rootScope);
+/**
+ * Collects a vector of borrowed constraints from the scope and all its child
+ * scopes. It is important to only call this function when you're done adding
+ * constraints to the scope or its descendants, lest the borrowed pointers
+ * become invalid due to a container reallocation.
+ * @param rootScope the root scope of the scope graph to collect constraints
+ * from.
+ * @return a list of pointers to constraints contained within the scope graph.
+ * None of these pointers should be null.
+ */
+std::vector<NotNull<Constraint>> collectConstraints(Scope2* rootScope);
 
 } // namespace Luau

Analysis/include/Luau/ConstraintSolver.h

@@ -4,7 +4,8 @@
 
 #include "Luau/Error.h"
 #include "Luau/Variant.h"
-#include "Luau/ConstraintGraphBuilder.h"
+#include "Luau/Constraint.h"
+#include "Luau/ConstraintSolverLogger.h"
 #include "Luau/TypeVar.h"
 
 #include <vector>
@@ -20,39 +21,81 @@ struct ConstraintSolver
 {
     TypeArena* arena;
     InternalErrorReporter iceReporter;
-    // The entire set of constraints that the solver is trying to resolve.
+    // The entire set of constraints that the solver is trying to resolve. It
-    std::vector<const Constraint*> constraints;
+    // is important to not add elements to this vector, lest the underlying
+    // storage that we retain pointers to be mutated underneath us.
+    const std::vector<NotNull<Constraint>> constraints;
     Scope2* rootScope;
-    std::vector<TypeError> errors;
 
     // This includes every constraint that has not been fully solved.
     // A constraint can be both blocked and unsolved, for instance.
-    std::unordered_set<const Constraint*> unsolvedConstraints;
+    std::vector<NotNull<const Constraint>> unsolvedConstraints;
 
     // A mapping of constraint pointer to how many things the constraint is
     // blocked on. Can be empty or 0 for constraints that are not blocked on
     // anything.
-    std::unordered_map<const Constraint*, size_t> blockedConstraints;
+    std::unordered_map<NotNull<const Constraint>, size_t> blockedConstraints;
     // A mapping of type/pack pointers to the constraints they block.
-    std::unordered_map<BlockedConstraintId, std::vector<const Constraint*>> blocked;
+    std::unordered_map<BlockedConstraintId, std::vector<NotNull<const Constraint>>> blocked;
+
+    ConstraintSolverLogger logger;
 
     explicit ConstraintSolver(TypeArena* arena, Scope2* rootScope);
 
     /**
      * Attempts to dispatch all pending constraints and reach a type solution
-     * that satisfies all of the constraints, recording any errors that are
+     * that satisfies all of the constraints.
-     * encountered.
      **/
     void run();
 
     bool done();
 
-    bool tryDispatch(const Constraint* c);
+    bool tryDispatch(NotNull<const Constraint> c, bool force);
-    bool tryDispatch(const SubtypeConstraint& c);
+    bool tryDispatch(const SubtypeConstraint& c, NotNull<const Constraint> constraint, bool force);
-    bool tryDispatch(const PackSubtypeConstraint& c);
+    bool tryDispatch(const PackSubtypeConstraint& c, NotNull<const Constraint> constraint, bool force);
-    bool tryDispatch(const GeneralizationConstraint& c);
+    bool tryDispatch(const GeneralizationConstraint& c, NotNull<const Constraint> constraint, bool force);
-    bool tryDispatch(const InstantiationConstraint& c, const Constraint* constraint);
+    bool tryDispatch(const InstantiationConstraint& c, NotNull<const Constraint> constraint, bool force);
 
+    void block(NotNull<const Constraint> target, NotNull<const Constraint> constraint);
+    /**
+     * Block a constraint on the resolution of a TypeVar.
+     * @returns false always.  This is just to allow tryDispatch to return the result of block()
+     */
+    bool block(TypeId target, NotNull<const Constraint> constraint);
+    bool block(TypePackId target, NotNull<const Constraint> constraint);
+
+    void unblock(NotNull<const Constraint> progressed);
+    void unblock(TypeId progressed);
+    void unblock(TypePackId progressed);
+
+    /**
+     * @returns true if the TypeId is in a blocked state.
+     */
+    bool isBlocked(TypeId ty);
+
+    /**
+     * Returns whether the constraint is blocked on anything.
+     * @param constraint the constraint to check.
+     */
+    bool isBlocked(NotNull<const Constraint> constraint);
+
+    /**
+     * Creates a new Unifier and performs a single unification operation. Commits
+     * the result.
+     * @param subType the sub-type to unify.
+     * @param superType the super-type to unify.
+     */
+    void unify(TypeId subType, TypeId superType, Location location);
+
+    /**
+     * Creates a new Unifier and performs a single unification operation. Commits
+     * the result.
+     * @param subPack the sub-type pack to unify.
+     * @param superPack the super-type pack to unify.
+     */
+    void unify(TypePackId subPack, TypePackId superPack, Location location);
+
+private:
     /**
      * Marks a constraint as being blocked on a type or type pack. The constraint
      * solver will not attempt to dispatch blocked constraints until their
@@ -60,10 +103,7 @@ struct ConstraintSolver
      * @param target the type or type pack pointer that the constraint is blocked on.
      * @param constraint the constraint to block.
      **/
-    void block_(BlockedConstraintId target, const Constraint* constraint);
+    void block_(BlockedConstraintId target, NotNull<const Constraint> constraint);
-    void block(const Constraint* target, const Constraint* constraint);
-    void block(TypeId target, const Constraint* constraint);
-    void block(TypePackId target, const Constraint* constraint);
 
     /**
      * Informs the solver that progress has been made on a type or type pack. The
@@ -72,33 +112,6 @@ struct ConstraintSolver
      * @param progressed the type or type pack pointer that has progressed.
      **/
     void unblock_(BlockedConstraintId progressed);
-    void unblock(const Constraint* progressed);
-    void unblock(TypeId progressed);
-    void unblock(TypePackId progressed);
-
-    /**
-     * Returns whether the constraint is blocked on anything.
-     * @param constraint the constraint to check.
-     */
-    bool isBlocked(const Constraint* constraint);
-
-    void reportErrors(const std::vector<TypeError>& errors);
-
-    /**
-     * Creates a new Unifier and performs a single unification operation. Commits
-     * the result and reports errors if necessary.
-     * @param subType the sub-type to unify.
-     * @param superType the super-type to unify.
-     */
-    void unify(TypeId subType, TypeId superType);
-
-    /**
-     * Creates a new Unifier and performs a single unification operation. Commits
-     * the result and reports errors if necessary.
-     * @param subPack the sub-type pack to unify.
-     * @param superPack the super-type pack to unify.
-     */
-    void unify(TypePackId subPack, TypePackId superPack);
 };
 
 void dump(Scope2* rootScope, struct ToStringOptions& opts);

Analysis/include/Luau/ConstraintSolverLogger.h

@@ -0,0 +1,26 @@
+// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+
+#include "Luau/ConstraintGraphBuilder.h"
+#include "Luau/ToString.h"
+
+#include <optional>
+#include <string>
+#include <vector>
+
+namespace Luau
+{
+
+struct ConstraintSolverLogger
+{
+    std::string compileOutput();
+    void captureBoundarySnapshot(const Scope2* rootScope, std::vector<NotNull<const Constraint>>& unsolvedConstraints);
+    void prepareStepSnapshot(const Scope2* rootScope, NotNull<const Constraint> current, std::vector<NotNull<const Constraint>>& unsolvedConstraints);
+    void commitPreparedStepSnapshot();
+
+private:
+    std::vector<std::string> snapshots;
+    std::optional<std::string> preparedSnapshot;
+    ToStringOptions opts;
+};
+
+} // namespace Luau

Analysis/include/Luau/Frontend.h

@@ -66,7 +66,7 @@ struct SourceNode
     }
 
     ModuleName name;
-    std::unordered_set<ModuleName> requires;
+    std::unordered_set<ModuleName> requireSet;
     std::vector<std::pair<ModuleName, Location>> requireLocations;
     bool dirtySourceModule = true;
     bool dirtyModule = true;
@@ -186,7 +186,7 @@ public:
 
     std::unordered_map<ModuleName, SourceNode> sourceNodes;
     std::unordered_map<ModuleName, SourceModule> sourceModules;
-    std::unordered_map<ModuleName, RequireTraceResult> requires;
+    std::unordered_map<ModuleName, RequireTraceResult> requireTrace;
 
     Stats stats = {};
 };

Analysis/include/Luau/Module.h

@@ -69,6 +69,7 @@ struct Module
     std::vector<std::pair<Location, std::unique_ptr<Scope2>>> scope2s; // never empty
 
     DenseHashMap<const AstExpr*, TypeId> astTypes{nullptr};
+    DenseHashMap<const AstExpr*, TypePackId> astTypePacks{nullptr};
     DenseHashMap<const AstExpr*, TypeId> astExpectedTypes{nullptr};
     DenseHashMap<const AstExpr*, TypeId> astOriginalCallTypes{nullptr};
     DenseHashMap<const AstExpr*, TypeId> astOverloadResolvedTypes{nullptr};

Analysis/include/Luau/Normalize.h

@@ -10,6 +10,7 @@ namespace Luau
 struct InternalErrorReporter;
 
 bool isSubtype(TypeId superTy, TypeId subTy, InternalErrorReporter& ice);
+bool isSubtype(TypePackId superTy, TypePackId subTy, InternalErrorReporter& ice);
 
 std::pair<TypeId, bool> normalize(TypeId ty, TypeArena& arena, InternalErrorReporter& ice);
 std::pair<TypeId, bool> normalize(TypeId ty, const ModulePtr& module, InternalErrorReporter& ice);

Analysis/include/Luau/NotNull.h

@@ -10,19 +10,21 @@ namespace Luau
 
 /** A non-owning, non-null pointer to a T.
  *
- * A NotNull<T> is notionally identical to a T* with the added restriction that it
+ * A NotNull<T> is notionally identical to a T* with the added restriction that
- * can never store nullptr.
+ * it can never store nullptr.
  *
- * The sole conversion rule from T* to NotNull<T> is the single-argument constructor, which
+ * The sole conversion rule from T* to NotNull<T> is the single-argument
- * is intentionally marked explicit. This constructor performs a runtime test to verify
+ * constructor, which is intentionally marked explicit. This constructor
- * that the passed pointer is never nullptr.
+ * performs a runtime test to verify that the passed pointer is never nullptr.
  *
- * Pointer arithmetic, increment, decrement, and array indexing are all forbidden.
+ * Pointer arithmetic, increment, decrement, and array indexing are all
+ * forbidden.
  *
- * An implicit coersion from NotNull<T> to T* is afforded, as are the pointer indirection and member
+ * An implicit coersion from NotNull<T> to T* is afforded, as are the pointer
- * access operators. (*p and p->prop)
+ * indirection and member access operators. (*p and p->prop)
  *
- * The explicit delete statement is permitted on a NotNull<T> through this implicit conversion.
+ * The explicit delete statement is permitted (but not recommended) on a
+ * NotNull<T> through this implicit conversion.
  */
 template <typename T>
 struct NotNull
@@ -36,6 +38,11 @@ struct NotNull
     explicit NotNull(std::nullptr_t) = delete;
     void operator=(std::nullptr_t) = delete;
 
+    template <typename U>
+    NotNull(NotNull<U> other)
+        : ptr(other.get())
+    {}
+
     operator T*() const noexcept
     {
         return ptr;
@@ -56,6 +63,12 @@ struct NotNull
     T& operator+(int) = delete;
     T& operator-(int) = delete;
 
+    T* get() const noexcept
+    {
+        return ptr;
+    }
+
+private:
     T* ptr;
 };
 
@@ -68,7 +81,7 @@ template <typename T> struct hash<Luau::NotNull<T>>
 {
     size_t operator()(const Luau::NotNull<T>& p) const
     {
-        return std::hash<T*>()(p.ptr);
+        return std::hash<T*>()(p.get());
     }
 };
 

Analysis/include/Luau/Quantify.h

@@ -6,9 +6,10 @@
 namespace Luau
 {
 
+struct TypeArena;
 struct Scope2;
 
 void quantify(TypeId ty, TypeLevel level);
-void quantify(TypeId ty, Scope2* scope);
+TypeId quantify(TypeArena* arena, TypeId ty, Scope2* scope);
 
 } // namespace Luau

Analysis/include/Luau/RequireTracer.h

@@ -19,7 +19,7 @@ struct RequireTraceResult
 {
     DenseHashMap<const AstExpr*, ModuleInfo> exprs{nullptr};
 
-    std::vector<std::pair<ModuleName, Location>> requires;
+    std::vector<std::pair<ModuleName, Location>> requireList;
 };
 
 RequireTraceResult traceRequires(FileResolver* fileResolver, AstStatBlock* root, const ModuleName& currentModuleName);

Analysis/include/Luau/TypeChecker2.h

@@ -0,0 +1,13 @@
+// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+
+#pragma once
+
+#include "Luau/Ast.h"
+#include "Luau/Module.h"
+
+namespace Luau
+{
+
+void check(const SourceModule& sourceModule, Module* module);
+
+} // namespace Luau

Analysis/include/Luau/TypeInfer.h

@@ -138,25 +138,25 @@ struct TypeChecker
     void checkBlockWithoutRecursionCheck(const ScopePtr& scope, const AstStatBlock& statement);
     void checkBlockTypeAliases(const ScopePtr& scope, std::vector<AstStat*>& sorted);
 
-    ExprResult<TypeId> checkExpr(
+    WithPredicate<TypeId> checkExpr(
         const ScopePtr& scope, const AstExpr& expr, std::optional<TypeId> expectedType = std::nullopt, bool forceSingleton = false);
-    ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprLocal& expr);
+    WithPredicate<TypeId> checkExpr(const ScopePtr& scope, const AstExprLocal& expr);
-    ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprGlobal& expr);
+    WithPredicate<TypeId> checkExpr(const ScopePtr& scope, const AstExprGlobal& expr);
-    ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprVarargs& expr);
+    WithPredicate<TypeId> checkExpr(const ScopePtr& scope, const AstExprVarargs& expr);
-    ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprCall& expr);
+    WithPredicate<TypeId> checkExpr(const ScopePtr& scope, const AstExprCall& expr);
-    ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprIndexName& expr);
+    WithPredicate<TypeId> checkExpr(const ScopePtr& scope, const AstExprIndexName& expr);
-    ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprIndexExpr& expr);
+    WithPredicate<TypeId> checkExpr(const ScopePtr& scope, const AstExprIndexExpr& expr);
-    ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprFunction& expr, std::optional<TypeId> expectedType = std::nullopt);
+    WithPredicate<TypeId> checkExpr(const ScopePtr& scope, const AstExprFunction& expr, std::optional<TypeId> expectedType = std::nullopt);
-    ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprTable& expr, std::optional<TypeId> expectedType = std::nullopt);
+    WithPredicate<TypeId> checkExpr(const ScopePtr& scope, const AstExprTable& expr, std::optional<TypeId> expectedType = std::nullopt);
-    ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprUnary& expr);
+    WithPredicate<TypeId> checkExpr(const ScopePtr& scope, const AstExprUnary& expr);
     TypeId checkRelationalOperation(
         const ScopePtr& scope, const AstExprBinary& expr, TypeId lhsType, TypeId rhsType, const PredicateVec& predicates = {});
     TypeId checkBinaryOperation(
         const ScopePtr& scope, const AstExprBinary& expr, TypeId lhsType, TypeId rhsType, const PredicateVec& predicates = {});
-    ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprBinary& expr);
+    WithPredicate<TypeId> checkExpr(const ScopePtr& scope, const AstExprBinary& expr);
-    ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprTypeAssertion& expr);
+    WithPredicate<TypeId> checkExpr(const ScopePtr& scope, const AstExprTypeAssertion& expr);
-    ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprError& expr);
+    WithPredicate<TypeId> checkExpr(const ScopePtr& scope, const AstExprError& expr);
-    ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprIfElse& expr, std::optional<TypeId> expectedType = std::nullopt);
+    WithPredicate<TypeId> checkExpr(const ScopePtr& scope, const AstExprIfElse& expr, std::optional<TypeId> expectedType = std::nullopt);
 
     TypeId checkExprTable(const ScopePtr& scope, const AstExprTable& expr, const std::vector<std::pair<TypeId, TypeId>>& fieldTypes,
         std::optional<TypeId> expectedType);
@@ -179,11 +179,11 @@ struct TypeChecker
     void checkArgumentList(
         const ScopePtr& scope, Unifier& state, TypePackId paramPack, TypePackId argPack, const std::vector<Location>& argLocations);
 
-    ExprResult<TypePackId> checkExprPack(const ScopePtr& scope, const AstExpr& expr);
+    WithPredicate<TypePackId> checkExprPack(const ScopePtr& scope, const AstExpr& expr);
-    ExprResult<TypePackId> checkExprPack(const ScopePtr& scope, const AstExprCall& expr);
+    WithPredicate<TypePackId> checkExprPack(const ScopePtr& scope, const AstExprCall& expr);
     std::vector<std::optional<TypeId>> getExpectedTypesForCall(const std::vector<TypeId>& overloads, size_t argumentCount, bool selfCall);
-    std::optional<ExprResult<TypePackId>> checkCallOverload(const ScopePtr& scope, const AstExprCall& expr, TypeId fn, TypePackId retPack,
+    std::optional<WithPredicate<TypePackId>> checkCallOverload(const ScopePtr& scope, const AstExprCall& expr, TypeId fn, TypePackId retPack,
-        TypePackId argPack, TypePack* args, const std::vector<Location>* argLocations, const ExprResult<TypePackId>& argListResult,
+        TypePackId argPack, TypePack* args, const std::vector<Location>* argLocations, const WithPredicate<TypePackId>& argListResult,
         std::vector<TypeId>& overloadsThatMatchArgCount, std::vector<TypeId>& overloadsThatDont, std::vector<OverloadErrorEntry>& errors);
     bool handleSelfCallMismatch(const ScopePtr& scope, const AstExprCall& expr, TypePack* args, const std::vector<Location>& argLocations,
         const std::vector<OverloadErrorEntry>& errors);
@@ -191,7 +191,7 @@ struct TypeChecker
         const std::vector<Location>& argLocations, const std::vector<TypeId>& overloads, const std::vector<TypeId>& overloadsThatMatchArgCount,
         const std::vector<OverloadErrorEntry>& errors);
 
-    ExprResult<TypePackId> checkExprList(const ScopePtr& scope, const Location& location, const AstArray<AstExpr*>& exprs,
+    WithPredicate<TypePackId> checkExprList(const ScopePtr& scope, const Location& location, const AstArray<AstExpr*>& exprs,
         bool substituteFreeForNil = false, const std::vector<bool>& lhsAnnotations = {},
         const std::vector<std::optional<TypeId>>& expectedTypes = {});
 
@@ -234,7 +234,7 @@ struct TypeChecker
     ErrorVec canUnify(TypeId subTy, TypeId superTy, const Location& location);
     ErrorVec canUnify(TypePackId subTy, TypePackId superTy, const Location& location);
 
-    void unifyLowerBound(TypePackId subTy, TypePackId superTy, const Location& location);
+    void unifyLowerBound(TypePackId subTy, TypePackId superTy, TypeLevel demotedLevel, const Location& location);
 
     std::optional<TypeId> findMetatableEntry(TypeId type, std::string entry, const Location& location);
     std::optional<TypeId> findTablePropertyRespectingMeta(TypeId lhsType, Name name, const Location& location);
@@ -412,7 +412,6 @@ public:
     const TypeId booleanType;
     const TypeId threadType;
     const TypeId anyType;
-    const TypeId optionalNumberType;
 
     const TypePackId anyTypePack;
 

Analysis/include/Luau/TypeVar.h

@@ -84,6 +84,24 @@ using Tags = std::vector<std::string>;
 
 using ModuleName = std::string;
 
+/** A TypeVar that cannot be computed.
+ *
+ * BlockedTypeVars essentially serve as a way to encode partial ordering on the
+ * constraint graph. Until a BlockedTypeVar is unblocked by its owning
+ * constraint, nothing at all can be said about it. Constraints that need to
+ * process a BlockedTypeVar cannot be dispatched.
+ *
+ * Whenever a BlockedTypeVar is added to the graph, we also record a constraint
+ * that will eventually unblock it.
+ */
+struct BlockedTypeVar
+{
+    BlockedTypeVar();
+    int index;
+
+    static int nextIndex;
+};
+
 struct PrimitiveTypeVar
 {
     enum Type
@@ -231,29 +249,29 @@ struct FunctionDefinition
 // TODO: Do we actually need this? We'll find out later if we can delete this.
 // Does not exactly belong in TypeVar.h, but this is the only way to appease the compiler.
 template<typename T>
-struct ExprResult
+struct WithPredicate
 {
     T type;
     PredicateVec predicates;
 };
 
-using MagicFunction = std::function<std::optional<ExprResult<TypePackId>>(
+using MagicFunction = std::function<std::optional<WithPredicate<TypePackId>>(
-    struct TypeChecker&, const std::shared_ptr<struct Scope>&, const class AstExprCall&, ExprResult<TypePackId>)>;
+    struct TypeChecker&, const std::shared_ptr<struct Scope>&, const class AstExprCall&, WithPredicate<TypePackId>)>;
 
 struct FunctionTypeVar
 {
     // Global monomorphic function
-    FunctionTypeVar(TypePackId argTypes, TypePackId retType, std::optional<FunctionDefinition> defn = {}, bool hasSelf = false);
+    FunctionTypeVar(TypePackId argTypes, TypePackId retTypes, std::optional<FunctionDefinition> defn = {}, bool hasSelf = false);
 
     // Global polymorphic function
-    FunctionTypeVar(std::vector<TypeId> generics, std::vector<TypePackId> genericPacks, TypePackId argTypes, TypePackId retType,
+    FunctionTypeVar(std::vector<TypeId> generics, std::vector<TypePackId> genericPacks, TypePackId argTypes, TypePackId retTypes,
         std::optional<FunctionDefinition> defn = {}, bool hasSelf = false);
 
     // Local monomorphic function
-    FunctionTypeVar(TypeLevel level, TypePackId argTypes, TypePackId retType, std::optional<FunctionDefinition> defn = {}, bool hasSelf = false);
+    FunctionTypeVar(TypeLevel level, TypePackId argTypes, TypePackId retTypes, std::optional<FunctionDefinition> defn = {}, bool hasSelf = false);
 
     // Local polymorphic function
-    FunctionTypeVar(TypeLevel level, std::vector<TypeId> generics, std::vector<TypePackId> genericPacks, TypePackId argTypes, TypePackId retType,
+    FunctionTypeVar(TypeLevel level, std::vector<TypeId> generics, std::vector<TypePackId> genericPacks, TypePackId argTypes, TypePackId retTypes,
         std::optional<FunctionDefinition> defn = {}, bool hasSelf = false);
 
     TypeLevel level;
@@ -263,7 +281,7 @@ struct FunctionTypeVar
     std::vector<TypePackId> genericPacks;
     TypePackId argTypes;
     std::vector<std::optional<FunctionArgument>> argNames;
-    TypePackId retType;
+    TypePackId retTypes;
     std::optional<FunctionDefinition> definition;
     MagicFunction magicFunction = nullptr; // Function pointer, can be nullptr.
     bool hasSelf;
@@ -442,7 +460,7 @@ struct LazyTypeVar
 
 using ErrorTypeVar = Unifiable::Error;
 
-using TypeVariant = Unifiable::Variant<TypeId, PrimitiveTypeVar, ConstrainedTypeVar, SingletonTypeVar, FunctionTypeVar, TableTypeVar,
+using TypeVariant = Unifiable::Variant<TypeId, PrimitiveTypeVar, ConstrainedTypeVar, BlockedTypeVar, SingletonTypeVar, FunctionTypeVar, TableTypeVar,
     MetatableTypeVar, ClassTypeVar, AnyTypeVar, UnionTypeVar, IntersectionTypeVar, LazyTypeVar>;
 
 struct TypeVar final
@@ -555,7 +573,6 @@ struct SingletonTypes
     const TypeId trueType;
     const TypeId falseType;
     const TypeId anyType;
-    const TypeId optionalNumberType;
 
     const TypePackId anyTypePack;
 

Analysis/include/Luau/Unifier.h

@@ -110,7 +110,7 @@ private:
     void tryUnifyWithConstrainedSuperTypeVar(TypeId subTy, TypeId superTy);
 
 public:
-    void unifyLowerBound(TypePackId subTy, TypePackId superTy);
+    void unifyLowerBound(TypePackId subTy, TypePackId superTy, TypeLevel demotedLevel);
 
     // Report an "infinite type error" if the type "needle" already occurs within "haystack"
     void occursCheck(TypeId needle, TypeId haystack);

Analysis/include/Luau/VisitTypeVar.h

@@ -209,7 +209,7 @@ struct GenericTypeVarVisitor
             if (visit(ty, *ftv))
             {
                 traverse(ftv->argTypes);
-                traverse(ftv->retType);
+                traverse(ftv->retTypes);
             }
         }
 

Analysis/src/Autocomplete.cpp

@@ -13,7 +13,6 @@
 #include <unordered_set>
 #include <utility>
 
-LUAU_FASTFLAGVARIABLE(LuauIfElseExprFixCompletionIssue, false);
 LUAU_FASTFLAG(LuauSelfCallAutocompleteFix2)
 
 static const std::unordered_set<std::string> kStatementStartingKeywords = {
@@ -268,14 +267,14 @@ static TypeCorrectKind checkTypeCorrectKind(const Module& module, TypeArena* typ
     auto checkFunctionType = [typeArena, &canUnify, &expectedType](const FunctionTypeVar* ftv) {
         if (FFlag::LuauSelfCallAutocompleteFix2)
         {
-            if (std::optional<TypeId> firstRetTy = first(ftv->retType))
+            if (std::optional<TypeId> firstRetTy = first(ftv->retTypes))
                 return checkTypeMatch(typeArena, *firstRetTy, expectedType);
 
             return false;
         }
         else
         {
-            auto [retHead, retTail] = flatten(ftv->retType);
+            auto [retHead, retTail] = flatten(ftv->retTypes);
 
             if (!retHead.empty() && canUnify(retHead.front(), expectedType))
                 return true;
@@ -454,7 +453,7 @@ static void autocompleteProps(const Module& module, TypeArena* typeArena, TypeId
             }
             else if (auto indexFunction = get<FunctionTypeVar>(followed))
             {
-                std::optional<TypeId> indexFunctionResult = first(indexFunction->retType);
+                std::optional<TypeId> indexFunctionResult = first(indexFunction->retTypes);
                 if (indexFunctionResult)
                     autocompleteProps(module, typeArena, rootTy, *indexFunctionResult, indexType, nodes, result, seen);
             }
@@ -493,7 +492,7 @@ static void autocompleteProps(const Module& module, TypeArena* typeArena, TypeId
                     autocompleteProps(module, typeArena, rootTy, followed, indexType, nodes, result, seen);
                 else if (auto indexFunction = get<FunctionTypeVar>(followed))
                 {
-                    std::optional<TypeId> indexFunctionResult = first(indexFunction->retType);
+                    std::optional<TypeId> indexFunctionResult = first(indexFunction->retTypes);
                     if (indexFunctionResult)
                         autocompleteProps(module, typeArena, rootTy, *indexFunctionResult, indexType, nodes, result, seen);
                 }
@@ -742,7 +741,7 @@ static std::optional<TypeId> findTypeElementAt(AstType* astType, TypeId ty, Posi
         if (auto element = findTypeElementAt(type->argTypes, ftv->argTypes, position))
             return element;
 
-        if (auto element = findTypeElementAt(type->returnTypes, ftv->retType, position))
+        if (auto element = findTypeElementAt(type->returnTypes, ftv->retTypes, position))
             return element;
     }
 
@@ -958,7 +957,7 @@ AutocompleteEntryMap autocompleteTypeNames(const Module& module, Position positi
                     {
                         if (const FunctionTypeVar* ftv = get<FunctionTypeVar>(follow(*it)))
                         {
-                            if (auto ty = tryGetTypePackTypeAt(ftv->retType, tailPos))
+                            if (auto ty = tryGetTypePackTypeAt(ftv->retTypes, tailPos))
                                 inferredType = *ty;
                         }
                     }
@@ -1050,7 +1049,7 @@ AutocompleteEntryMap autocompleteTypeNames(const Module& module, Position positi
             {
                 if (const FunctionTypeVar* ftv = tryGetExpectedFunctionType(module, node))
                 {
-                    if (auto ty = tryGetTypePackTypeAt(ftv->retType, i))
+                    if (auto ty = tryGetTypePackTypeAt(ftv->retTypes, i))
                         tryAddTypeCorrectSuggestion(result, startScope, topType, *ty, position);
                 }
 
@@ -1067,7 +1066,7 @@ AutocompleteEntryMap autocompleteTypeNames(const Module& module, Position positi
                 {
                     if (const FunctionTypeVar* ftv = tryGetExpectedFunctionType(module, node))
                     {
-                        if (auto ty = tryGetTypePackTypeAt(ftv->retType, ~0u))
+                        if (auto ty = tryGetTypePackTypeAt(ftv->retTypes, ~0u))
                             tryAddTypeCorrectSuggestion(result, startScope, topType, *ty, position);
                     }
                 }
@@ -1266,7 +1265,7 @@ static bool autocompleteIfElseExpression(
     if (!parent)
         return false;
 
-    if (FFlag::LuauIfElseExprFixCompletionIssue && node->is<AstExprIfElse>())
+    if (node->is<AstExprIfElse>())
     {
         // Don't try to complete when the current node is an if-else expression (i.e. only try to complete when the node is a child of an if-else
         // expression.

Analysis/src/BuiltinDefinitions.cpp

@@ -19,16 +19,16 @@ LUAU_FASTFLAGVARIABLE(LuauSetMetaTableArgsCheck, false)
 namespace Luau
 {
 
-static std::optional<ExprResult<TypePackId>> magicFunctionSelect(
+static std::optional<WithPredicate<TypePackId>> magicFunctionSelect(
-    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult);
+    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, WithPredicate<TypePackId> withPredicate);
-static std::optional<ExprResult<TypePackId>> magicFunctionSetMetaTable(
+static std::optional<WithPredicate<TypePackId>> magicFunctionSetMetaTable(
-    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult);
+    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, WithPredicate<TypePackId> withPredicate);
-static std::optional<ExprResult<TypePackId>> magicFunctionAssert(
+static std::optional<WithPredicate<TypePackId>> magicFunctionAssert(
-    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult);
+    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, WithPredicate<TypePackId> withPredicate);
-static std::optional<ExprResult<TypePackId>> magicFunctionPack(
+static std::optional<WithPredicate<TypePackId>> magicFunctionPack(
-    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult);
+    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, WithPredicate<TypePackId> withPredicate);
-static std::optional<ExprResult<TypePackId>> magicFunctionRequire(
+static std::optional<WithPredicate<TypePackId>> magicFunctionRequire(
-    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult);
+    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, WithPredicate<TypePackId> withPredicate);
 
 TypeId makeUnion(TypeArena& arena, std::vector<TypeId>&& types)
 {
@@ -263,10 +263,10 @@ void registerBuiltinTypes(TypeChecker& typeChecker)
     attachMagicFunction(getGlobalBinding(typeChecker, "require"), magicFunctionRequire);
 }
 
-static std::optional<ExprResult<TypePackId>> magicFunctionSelect(
+static std::optional<WithPredicate<TypePackId>> magicFunctionSelect(
-    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult)
+    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, WithPredicate<TypePackId> withPredicate)
 {
-    auto [paramPack, _predicates] = exprResult;
+    auto [paramPack, _predicates] = withPredicate;
 
     (void)scope;
 
@@ -287,10 +287,10 @@ static std::optional<ExprResult<TypePackId>> magicFunctionSelect(
             if (size_t(offset) < v.size())
             {
                 std::vector<TypeId> result(v.begin() + offset, v.end());
-                return ExprResult<TypePackId>{typechecker.currentModule->internalTypes.addTypePack(TypePack{std::move(result), tail})};
+                return WithPredicate<TypePackId>{typechecker.currentModule->internalTypes.addTypePack(TypePack{std::move(result), tail})};
             }
             else if (tail)
-                return ExprResult<TypePackId>{*tail};
+                return WithPredicate<TypePackId>{*tail};
         }
 
         typechecker.reportError(TypeError{arg1->location, GenericError{"bad argument #1 to select (index out of range)"}});
@@ -298,16 +298,16 @@ static std::optional<ExprResult<TypePackId>> magicFunctionSelect(
     else if (AstExprConstantString* str = arg1->as<AstExprConstantString>())
     {
         if (str->value.size == 1 && str->value.data[0] == '#')
-            return ExprResult<TypePackId>{typechecker.currentModule->internalTypes.addTypePack({typechecker.numberType})};
+            return WithPredicate<TypePackId>{typechecker.currentModule->internalTypes.addTypePack({typechecker.numberType})};
     }
 
     return std::nullopt;
 }
 
-static std::optional<ExprResult<TypePackId>> magicFunctionSetMetaTable(
+static std::optional<WithPredicate<TypePackId>> magicFunctionSetMetaTable(
-    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult)
+    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, WithPredicate<TypePackId> withPredicate)
 {
-    auto [paramPack, _predicates] = exprResult;
+    auto [paramPack, _predicates] = withPredicate;
 
     TypeArena& arena = typechecker.currentModule->internalTypes;
 
@@ -343,7 +343,7 @@ static std::optional<ExprResult<TypePackId>> magicFunctionSetMetaTable(
 
             if (FFlag::LuauSetMetaTableArgsCheck && expr.args.size < 1)
             {
-                return ExprResult<TypePackId>{};
+                return WithPredicate<TypePackId>{};
             }
 
             if (!FFlag::LuauSetMetaTableArgsCheck || !expr.self)
@@ -356,7 +356,7 @@ static std::optional<ExprResult<TypePackId>> magicFunctionSetMetaTable(
                 }
             }
 
-            return ExprResult<TypePackId>{arena.addTypePack({mtTy})};
+            return WithPredicate<TypePackId>{arena.addTypePack({mtTy})};
         }
     }
     else if (get<AnyTypeVar>(target) || get<ErrorTypeVar>(target) || isTableIntersection(target))
@@ -367,13 +367,13 @@ static std::optional<ExprResult<TypePackId>> magicFunctionSetMetaTable(
         typechecker.reportError(TypeError{expr.location, GenericError{"setmetatable should take a table"}});
     }
 
-    return ExprResult<TypePackId>{arena.addTypePack({target})};
+    return WithPredicate<TypePackId>{arena.addTypePack({target})};
 }
 
-static std::optional<ExprResult<TypePackId>> magicFunctionAssert(
+static std::optional<WithPredicate<TypePackId>> magicFunctionAssert(
-    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult)
+    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, WithPredicate<TypePackId> withPredicate)
 {
-    auto [paramPack, predicates] = exprResult;
+    auto [paramPack, predicates] = withPredicate;
 
     TypeArena& arena = typechecker.currentModule->internalTypes;
 
@@ -382,7 +382,7 @@ static std::optional<ExprResult<TypePackId>> magicFunctionAssert(
     {
         std::optional<TypeId> fst = first(*tail);
         if (!fst)
-            return ExprResult<TypePackId>{paramPack};
+            return WithPredicate<TypePackId>{paramPack};
         head.push_back(*fst);
     }
 
@@ -397,13 +397,13 @@ static std::optional<ExprResult<TypePackId>> magicFunctionAssert(
             head[0] = *newhead;
     }
 
-    return ExprResult<TypePackId>{arena.addTypePack(TypePack{std::move(head), tail})};
+    return WithPredicate<TypePackId>{arena.addTypePack(TypePack{std::move(head), tail})};
 }
 
-static std::optional<ExprResult<TypePackId>> magicFunctionPack(
+static std::optional<WithPredicate<TypePackId>> magicFunctionPack(
-    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult)
+    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, WithPredicate<TypePackId> withPredicate)
 {
-    auto [paramPack, _predicates] = exprResult;
+    auto [paramPack, _predicates] = withPredicate;
 
     TypeArena& arena = typechecker.currentModule->internalTypes;
 
@@ -436,7 +436,7 @@ static std::optional<ExprResult<TypePackId>> magicFunctionPack(
     TypeId packedTable = arena.addType(
         TableTypeVar{{{"n", {typechecker.numberType}}}, TableIndexer(typechecker.numberType, result), scope->level, TableState::Sealed});
 
-    return ExprResult<TypePackId>{arena.addTypePack({packedTable})};
+    return WithPredicate<TypePackId>{arena.addTypePack({packedTable})};
 }
 
 static bool checkRequirePath(TypeChecker& typechecker, AstExpr* expr)
@@ -461,8 +461,8 @@ static bool checkRequirePath(TypeChecker& typechecker, AstExpr* expr)
     return good;
 }
 
-static std::optional<ExprResult<TypePackId>> magicFunctionRequire(
+static std::optional<WithPredicate<TypePackId>> magicFunctionRequire(
-    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult)
+    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, WithPredicate<TypePackId> withPredicate)
 {
     TypeArena& arena = typechecker.currentModule->internalTypes;
 
@@ -476,7 +476,7 @@ static std::optional<ExprResult<TypePackId>> magicFunctionRequire(
         return std::nullopt;
 
     if (auto moduleInfo = typechecker.resolver->resolveModuleInfo(typechecker.currentModuleName, expr))
-        return ExprResult<TypePackId>{arena.addTypePack({typechecker.checkRequire(scope, *moduleInfo, expr.location)})};
+        return WithPredicate<TypePackId>{arena.addTypePack({typechecker.checkRequire(scope, *moduleInfo, expr.location)})};
 
     return std::nullopt;
 }

Analysis/src/Clone.cpp

@@ -47,6 +47,7 @@ struct TypeCloner
     void operator()(const Unifiable::Generic& t);
     void operator()(const Unifiable::Bound<TypeId>& t);
     void operator()(const Unifiable::Error& t);
+    void operator()(const BlockedTypeVar& t);
     void operator()(const PrimitiveTypeVar& t);
     void operator()(const ConstrainedTypeVar& t);
     void operator()(const SingletonTypeVar& t);
@@ -158,6 +159,11 @@ void TypeCloner::operator()(const Unifiable::Error& t)
     defaultClone(t);
 }
 
+void TypeCloner::operator()(const BlockedTypeVar& t)
+{
+    defaultClone(t);
+}
+
 void TypeCloner::operator()(const PrimitiveTypeVar& t)
 {
     defaultClone(t);
@@ -200,7 +206,7 @@ void TypeCloner::operator()(const FunctionTypeVar& t)
     ftv->tags = t.tags;
     ftv->argTypes = clone(t.argTypes, dest, cloneState);
     ftv->argNames = t.argNames;
-    ftv->retType = clone(t.retType, dest, cloneState);
+    ftv->retTypes = clone(t.retTypes, dest, cloneState);
     ftv->hasNoGenerics = t.hasNoGenerics;
 }
 
@@ -391,7 +397,7 @@ TypeId shallowClone(TypeId ty, TypeArena& dest, const TxnLog* log)
 
     if (const FunctionTypeVar* ftv = get<FunctionTypeVar>(ty))
     {
-        FunctionTypeVar clone = FunctionTypeVar{ftv->level, ftv->argTypes, ftv->retType, ftv->definition, ftv->hasSelf};
+        FunctionTypeVar clone = FunctionTypeVar{ftv->level, ftv->argTypes, ftv->retTypes, ftv->definition, ftv->hasSelf};
         clone.generics = ftv->generics;
         clone.genericPacks = ftv->genericPacks;
         clone.magicFunction = ftv->magicFunction;

Analysis/src/Constraint.cpp

@@ -0,0 +1,14 @@
+// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+
+#include "Luau/Constraint.h"
+
+namespace Luau
+{
+
+Constraint::Constraint(ConstraintV&& c, Location location)
+    : c(std::move(c))
+    , location(location)
+{
+}
+
+} // namespace Luau

Analysis/src/ConstraintGraphBuilder.cpp

@@ -5,16 +5,7 @@
 namespace Luau
 {
 
-Constraint::Constraint(ConstraintV&& c)
+const AstStat* getFallthrough(const AstStat* node); // TypeInfer.cpp
-    : c(std::move(c))
-{
-}
-
-Constraint::Constraint(ConstraintV&& c, std::vector<Constraint*> dependencies)
-    : c(std::move(c))
-    , dependencies(dependencies)
-{
-}
 
 std::optional<TypeId> Scope2::lookup(Symbol sym)
 {
@@ -68,10 +59,10 @@ Scope2* ConstraintGraphBuilder::childScope(Location location, Scope2* parent)
     return borrow;
 }
 
-void ConstraintGraphBuilder::addConstraint(Scope2* scope, ConstraintV cv)
+void ConstraintGraphBuilder::addConstraint(Scope2* scope, ConstraintV cv, Location location)
 {
     LUAU_ASSERT(scope);
-    scope->constraints.emplace_back(new Constraint{std::move(cv)});
+    scope->constraints.emplace_back(new Constraint{std::move(cv), location});
 }
 
 void ConstraintGraphBuilder::addConstraint(Scope2* scope, std::unique_ptr<Constraint> c)
@@ -99,10 +90,18 @@ void ConstraintGraphBuilder::visit(Scope2* scope, AstStat* stat)
         visit(scope, s);
     else if (auto s = stat->as<AstStatLocal>())
         visit(scope, s);
+    else if (auto f = stat->as<AstStatFunction>())
+        visit(scope, f);
     else if (auto f = stat->as<AstStatLocalFunction>())
         visit(scope, f);
     else if (auto r = stat->as<AstStatReturn>())
         visit(scope, r);
+    else if (auto a = stat->as<AstStatAssign>())
+        visit(scope, a);
+    else if (auto e = stat->as<AstStatExpr>())
+        checkPack(scope, e->expr);
+    else if (auto i = stat->as<AstStatIf>())
+        visit(scope, i);
     else
         LUAU_ASSERT(0);
 }
@@ -121,12 +120,30 @@ void ConstraintGraphBuilder::visit(Scope2* scope, AstStatLocal* local)
         scope->bindings[local] = ty;
     }
 
-    for (size_t i = 0; i < local->vars.size; ++i)
+    for (size_t i = 0; i < local->values.size; ++i)
     {
-        if (i < local->values.size)
+        if (local->values.data[i]->is<AstExprConstantNil>())
+        {
+            // HACK: we leave nil-initialized things floating under the assumption that they will later be populated.
+            // See the test TypeInfer/infer_locals_with_nil_value.
+            // Better flow awareness should make this obsolete.
+        }
+        else if (i == local->values.size - 1)
+        {
+            TypePackId exprPack = checkPack(scope, local->values.data[i]);
+
+            if (i < local->vars.size)
+            {
+                std::vector<TypeId> tailValues{varTypes.begin() + i, varTypes.end()};
+                TypePackId tailPack = arena->addTypePack(std::move(tailValues));
+                addConstraint(scope, PackSubtypeConstraint{exprPack, tailPack}, local->location);
+            }
+        }
+        else
         {
             TypeId exprType = check(scope, local->values.data[i]);
-            addConstraint(scope, SubtypeConstraint{varTypes[i], exprType});
+            if (i < varTypes.size())
+                addConstraint(scope, SubtypeConstraint{varTypes[i], exprType}, local->vars.data[i]->location);
         }
     }
 }
@@ -138,7 +155,7 @@ void addConstraints(Constraint* constraint, Scope2* scope)
     scope->constraints.reserve(scope->constraints.size() + scope->constraints.size());
 
     for (const auto& c : scope->constraints)
-        constraint->dependencies.push_back(c.get());
+        constraint->dependencies.push_back(NotNull{c.get()});
 
     for (Scope2* childScope : scope->children)
         addConstraints(constraint, childScope);
@@ -155,31 +172,75 @@ void ConstraintGraphBuilder::visit(Scope2* scope, AstStatLocalFunction* function
 
     TypeId functionType = nullptr;
     auto ty = scope->lookup(function->name);
-    LUAU_ASSERT(!ty.has_value()); // The parser ensures that every local function has a distinct Symbol for its name.
+    if (ty.has_value())
-
-    functionType = freshType(scope);
-    scope->bindings[function->name] = functionType;
-
-    Scope2* innerScope = childScope(function->func->body->location, scope);
-    TypePackId returnType = freshTypePack(scope);
-    innerScope->returnType = returnType;
-
-    std::vector<TypeId> argTypes;
-
-    for (AstLocal* local : function->func->args)
     {
-        TypeId t = freshType(innerScope);
+        // TODO: This is duplicate definition of a local function.  Is this allowed?
-        argTypes.push_back(t);
+        functionType = *ty;
-        innerScope->bindings[local] = t; // TODO annotations
+    }
+    else
+    {
+        functionType = arena->addType(BlockedTypeVar{});
+        scope->bindings[function->name] = functionType;
     }
 
-    for (AstStat* stat : function->func->body->body)
+    auto [actualFunctionType, innerScope] = checkFunctionSignature(scope, function->func);
-        visit(innerScope, stat);
+    innerScope->bindings[function->name] = actualFunctionType;
 
-    FunctionTypeVar actualFunction{arena->addTypePack(argTypes), returnType};
+    checkFunctionBody(innerScope, function->func);
-    TypeId actualFunctionType = arena->addType(std::move(actualFunction));
 
-    std::unique_ptr<Constraint> c{new Constraint{GeneralizationConstraint{functionType, actualFunctionType, innerScope}}};
+    std::unique_ptr<Constraint> c{new Constraint{GeneralizationConstraint{functionType, actualFunctionType, innerScope}, function->location}};
+    addConstraints(c.get(), innerScope);
+
+    addConstraint(scope, std::move(c));
+}
+
+void ConstraintGraphBuilder::visit(Scope2* scope, AstStatFunction* function)
+{
+    // Name could be AstStatLocal, AstStatGlobal, AstStatIndexName.
+    // With or without self
+
+    TypeId functionType = nullptr;
+
+    auto [actualFunctionType, innerScope] = checkFunctionSignature(scope, function->func);
+
+    if (AstExprLocal* localName = function->name->as<AstExprLocal>())
+    {
+        std::optional<TypeId> existingFunctionTy = scope->lookup(localName->local);
+        if (existingFunctionTy)
+        {
+            // Duplicate definition
+            functionType = *existingFunctionTy;
+        }
+        else
+        {
+            functionType = arena->addType(BlockedTypeVar{});
+            scope->bindings[localName->local] = functionType;
+        }
+        innerScope->bindings[localName->local] = actualFunctionType;
+    }
+    else if (AstExprGlobal* globalName = function->name->as<AstExprGlobal>())
+    {
+        std::optional<TypeId> existingFunctionTy = scope->lookup(globalName->name);
+        if (existingFunctionTy)
+        {
+            // Duplicate definition
+            functionType = *existingFunctionTy;
+        }
+        else
+        {
+            functionType = arena->addType(BlockedTypeVar{});
+            rootScope->bindings[globalName->name] = functionType;
+        }
+        innerScope->bindings[globalName->name] = actualFunctionType;
+    }
+    else if (AstExprIndexName* indexName = function->name->as<AstExprIndexName>())
+    {
+        LUAU_ASSERT(0); // not yet implemented
+    }
+
+    checkFunctionBody(innerScope, function->func);
+
+    std::unique_ptr<Constraint> c{new Constraint{GeneralizationConstraint{functionType, actualFunctionType, innerScope}, function->location}};
     addConstraints(c.get(), innerScope);
 
     addConstraint(scope, std::move(c));
@@ -190,7 +251,7 @@ void ConstraintGraphBuilder::visit(Scope2* scope, AstStatReturn* ret)
     LUAU_ASSERT(scope);
 
     TypePackId exprTypes = checkPack(scope, ret->list);
-    addConstraint(scope, PackSubtypeConstraint{exprTypes, scope->returnType});
+    addConstraint(scope, PackSubtypeConstraint{exprTypes, scope->returnType}, ret->location);
 }
 
 void ConstraintGraphBuilder::visit(Scope2* scope, AstStatBlock* block)
@@ -201,6 +262,28 @@ void ConstraintGraphBuilder::visit(Scope2* scope, AstStatBlock* block)
         visit(scope, stat);
 }
 
+void ConstraintGraphBuilder::visit(Scope2* scope, AstStatAssign* assign)
+{
+    TypePackId varPackId = checkExprList(scope, assign->vars);
+    TypePackId valuePack = checkPack(scope, assign->values);
+
+    addConstraint(scope, PackSubtypeConstraint{valuePack, varPackId}, assign->location);
+}
+
+void ConstraintGraphBuilder::visit(Scope2* scope, AstStatIf* ifStatement)
+{
+    check(scope, ifStatement->condition);
+
+    Scope2* thenScope = childScope(ifStatement->thenbody->location, scope);
+    visit(thenScope, ifStatement->thenbody);
+
+    if (ifStatement->elsebody)
+    {
+        Scope2* elseScope = childScope(ifStatement->elsebody->location, scope);
+        visit(elseScope, ifStatement->elsebody);
+    }
+}
+
 TypePackId ConstraintGraphBuilder::checkPack(Scope2* scope, AstArray<AstExpr*> exprs)
 {
     LUAU_ASSERT(scope);
@@ -224,75 +307,256 @@ TypePackId ConstraintGraphBuilder::checkPack(Scope2* scope, AstArray<AstExpr*> e
     return arena->addTypePack(TypePack{std::move(types), last});
 }
 
+TypePackId ConstraintGraphBuilder::checkExprList(Scope2* scope, const AstArray<AstExpr*>& exprs)
+{
+    TypePackId result = arena->addTypePack({});
+    TypePack* resultPack = getMutable<TypePack>(result);
+    LUAU_ASSERT(resultPack);
+
+    for (size_t i = 0; i < exprs.size; ++i)
+    {
+        AstExpr* expr = exprs.data[i];
+        if (i < exprs.size - 1)
+            resultPack->head.push_back(check(scope, expr));
+        else
+            resultPack->tail = checkPack(scope, expr);
+    }
+
+    if (resultPack->head.empty() && resultPack->tail)
+        return *resultPack->tail;
+    else
+        return result;
+}
+
 TypePackId ConstraintGraphBuilder::checkPack(Scope2* scope, AstExpr* expr)
 {
     LUAU_ASSERT(scope);
 
-    // TEMP TEMP TEMP HACK HACK HACK FIXME FIXME
+    TypePackId result = nullptr;
+
+    if (AstExprCall* call = expr->as<AstExprCall>())
+    {
+        std::vector<TypeId> args;
+
+        for (AstExpr* arg : call->args)
+        {
+            args.push_back(check(scope, arg));
+        }
+
+        // TODO self
+
+        TypeId fnType = check(scope, call->func);
+
+        astOriginalCallTypes[call->func] = fnType;
+
+        TypeId instantiatedType = freshType(scope);
+        addConstraint(scope, InstantiationConstraint{instantiatedType, fnType}, expr->location);
+
+        TypePackId rets = freshTypePack(scope);
+        FunctionTypeVar ftv(arena->addTypePack(TypePack{args, {}}), rets);
+        TypeId inferredFnType = arena->addType(ftv);
+
+        addConstraint(scope, SubtypeConstraint{inferredFnType, instantiatedType}, expr->location);
+        result = rets;
+    }
+    else
+    {
         TypeId t = check(scope, expr);
-    return arena->addTypePack({t});
+        result = arena->addTypePack({t});
+    }
+
+    LUAU_ASSERT(result);
+    astTypePacks[expr] = result;
+    return result;
 }
 
 TypeId ConstraintGraphBuilder::check(Scope2* scope, AstExpr* expr)
 {
     LUAU_ASSERT(scope);
 
-    if (auto a = expr->as<AstExprConstantString>())
+    TypeId result = nullptr;
-        return singletonTypes.stringType;
+
-    else if (auto a = expr->as<AstExprConstantNumber>())
+    if (auto group = expr->as<AstExprGroup>())
-        return singletonTypes.numberType;
+        result = check(scope, group->expr);
-    else if (auto a = expr->as<AstExprConstantBool>())
+    else if (expr->is<AstExprConstantString>())
-        return singletonTypes.booleanType;
+        result = singletonTypes.stringType;
-    else if (auto a = expr->as<AstExprConstantNil>())
+    else if (expr->is<AstExprConstantNumber>())
-        return singletonTypes.nilType;
+        result = singletonTypes.numberType;
+    else if (expr->is<AstExprConstantBool>())
+        result = singletonTypes.booleanType;
+    else if (expr->is<AstExprConstantNil>())
+        result = singletonTypes.nilType;
     else if (auto a = expr->as<AstExprLocal>())
     {
         std::optional<TypeId> ty = scope->lookup(a->local);
         if (ty)
-            return *ty;
+            result = *ty;
         else
-            return singletonTypes.errorRecoveryType(singletonTypes.anyType); // FIXME?  Record an error at this point?
+            result = singletonTypes.errorRecoveryType(); // FIXME?  Record an error at this point?
+    }
+    else if (auto g = expr->as<AstExprGlobal>())
+    {
+        std::optional<TypeId> ty = scope->lookup(g->name);
+        if (ty)
+            result = *ty;
+        else
+            result = singletonTypes.errorRecoveryType(); // FIXME?  Record an error at this point?
     }
     else if (auto a = expr->as<AstExprCall>())
     {
-        std::vector<TypeId> args;
+        TypePackId packResult = checkPack(scope, expr);
-
+        if (auto f = first(packResult))
-        for (AstExpr* arg : a->args)
+            return *f;
+        else if (get<FreeTypePack>(packResult))
         {
-            args.push_back(check(scope, arg));
+            TypeId typeResult = freshType(scope);
+            TypePack onePack{{typeResult}, freshTypePack(scope)};
+            TypePackId oneTypePack = arena->addTypePack(std::move(onePack));
+
+            addConstraint(scope, PackSubtypeConstraint{packResult, oneTypePack}, expr->location);
+
+            return typeResult;
         }
-
+    }
-        TypeId fnType = check(scope, a->func);
+    else if (auto a = expr->as<AstExprFunction>())
-        TypeId instantiatedType = freshType(scope);
+    {
-        addConstraint(scope, InstantiationConstraint{instantiatedType, fnType});
+        auto [fnType, functionScope] = checkFunctionSignature(scope, a);
-
+        checkFunctionBody(functionScope, a);
-        TypeId firstRet = freshType(scope);
+        return fnType;
-        TypePackId rets = arena->addTypePack(TypePack{{firstRet}, arena->addTypePack(TypePackVar{FreeTypePack{TypeLevel{}}})});
+    }
-        FunctionTypeVar ftv(arena->addTypePack(TypePack{args, {}}), rets);
+    else if (auto indexName = expr->as<AstExprIndexName>())
-        TypeId inferredFnType = arena->addType(ftv);
+    {
-
+        result = check(scope, indexName);
-        addConstraint(scope, SubtypeConstraint{inferredFnType, instantiatedType});
+    }
-        return firstRet;
+    else if (auto table = expr->as<AstExprTable>())
+    {
+        result = checkExprTable(scope, table);
     }
     else
     {
         LUAU_ASSERT(0);
-        return freshType(scope);
+        result = freshType(scope);
+    }
+
+    LUAU_ASSERT(result);
+    astTypes[expr] = result;
+    return result;
+}
+
+TypeId ConstraintGraphBuilder::check(Scope2* scope, AstExprIndexName* indexName)
+{
+    TypeId obj = check(scope, indexName->expr);
+    TypeId result = freshType(scope);
+
+    TableTypeVar::Props props{{indexName->index.value, Property{result}}};
+    const std::optional<TableIndexer> indexer;
+    TableTypeVar ttv{std::move(props), indexer, TypeLevel{}, TableState::Free};
+
+    TypeId expectedTableType = arena->addType(std::move(ttv));
+
+    addConstraint(scope, SubtypeConstraint{obj, expectedTableType}, indexName->location);
+
+    return result;
+}
+
+TypeId ConstraintGraphBuilder::checkExprTable(Scope2* scope, AstExprTable* expr)
+{
+    TypeId ty = arena->addType(TableTypeVar{});
+    TableTypeVar* ttv = getMutable<TableTypeVar>(ty);
+    LUAU_ASSERT(ttv);
+
+    auto createIndexer = [this, scope, ttv](
+                             TypeId currentIndexType, TypeId currentResultType, Location itemLocation, std::optional<Location> keyLocation) {
+        if (!ttv->indexer)
+        {
+            TypeId indexType = this->freshType(scope);
+            TypeId resultType = this->freshType(scope);
+            ttv->indexer = TableIndexer{indexType, resultType};
+        }
+
+        addConstraint(scope, SubtypeConstraint{ttv->indexer->indexType, currentIndexType}, keyLocation ? *keyLocation : itemLocation);
+        addConstraint(scope, SubtypeConstraint{ttv->indexer->indexResultType, currentResultType}, itemLocation);
+    };
+
+    for (const AstExprTable::Item& item : expr->items)
+    {
+        TypeId itemTy = check(scope, item.value);
+
+        if (item.key)
+        {
+            // Even though we don't need to use the type of the item's key if
+            // it's a string constant, we still want to check it to populate
+            // astTypes.
+            TypeId keyTy = check(scope, item.key);
+
+            if (AstExprConstantString* key = item.key->as<AstExprConstantString>())
+            {
+                ttv->props[key->value.begin()] = {itemTy};
+            }
+            else
+            {
+                createIndexer(keyTy, itemTy, item.value->location, item.key->location);
+            }
+        }
+        else
+        {
+            TypeId numberType = singletonTypes.numberType;
+            createIndexer(numberType, itemTy, item.value->location, std::nullopt);
+        }
+    }
+
+    return ty;
+}
+
+std::pair<TypeId, Scope2*> ConstraintGraphBuilder::checkFunctionSignature(Scope2* parent, AstExprFunction* fn)
+{
+    Scope2* innerScope = childScope(fn->body->location, parent);
+    TypePackId returnType = freshTypePack(innerScope);
+    innerScope->returnType = returnType;
+
+    std::vector<TypeId> argTypes;
+
+    for (AstLocal* local : fn->args)
+    {
+        TypeId t = freshType(innerScope);
+        argTypes.push_back(t);
+        innerScope->bindings[local] = t; // TODO annotations
+    }
+
+    FunctionTypeVar actualFunction{arena->addTypePack(argTypes), returnType};
+    TypeId actualFunctionType = arena->addType(std::move(actualFunction));
+    LUAU_ASSERT(actualFunctionType);
+    astTypes[fn] = actualFunctionType;
+
+    return {actualFunctionType, innerScope};
+}
+
+void ConstraintGraphBuilder::checkFunctionBody(Scope2* scope, AstExprFunction* fn)
+{
+    for (AstStat* stat : fn->body->body)
+        visit(scope, stat);
+
+    // If it is possible for execution to reach the end of the function, the return type must be compatible with ()
+
+    if (nullptr != getFallthrough(fn->body))
+    {
+        TypePackId empty = arena->addTypePack({}); // TODO we could have CSG retain one of these forever
+        addConstraint(scope, PackSubtypeConstraint{scope->returnType, empty}, fn->body->location);
     }
 }
 
-static void collectConstraints(std::vector<const Constraint*>& result, Scope2* scope)
+void collectConstraints(std::vector<NotNull<Constraint>>& result, Scope2* scope)
 {
     for (const auto& c : scope->constraints)
-        result.push_back(c.get());
+        result.push_back(NotNull{c.get()});
 
     for (Scope2* child : scope->children)
         collectConstraints(result, child);
 }
 
-std::vector<const Constraint*> collectConstraints(Scope2* rootScope)
+std::vector<NotNull<Constraint>> collectConstraints(Scope2* rootScope)
 {
-    std::vector<const Constraint*> result;
+    std::vector<NotNull<Constraint>> result;
     collectConstraints(result, rootScope);
     return result;
 }

Analysis/src/ConstraintSolver.cpp

@@ -7,6 +7,7 @@
 #include "Luau/Unifier.h"
 
 LUAU_FASTFLAGVARIABLE(DebugLuauLogSolver, false);
+LUAU_FASTFLAGVARIABLE(DebugLuauLogSolverToJson, false);
 
 namespace Luau
 {
@@ -58,11 +59,11 @@ ConstraintSolver::ConstraintSolver(TypeArena* arena, Scope2* rootScope)
     , constraints(collectConstraints(rootScope))
     , rootScope(rootScope)
 {
-    for (const Constraint* c : constraints)
+    for (NotNull<Constraint> c : constraints)
     {
-        unsolvedConstraints.insert(c);
+        unsolvedConstraints.push_back(c);
 
-        for (const Constraint* dep : c->dependencies)
+        for (NotNull<const Constraint> dep : c->dependencies)
         {
             block(dep, c);
         }
@@ -74,8 +75,6 @@ void ConstraintSolver::run()
     if (done())
         return;
 
-    bool progress = false;
-
     ToStringOptions opts;
 
     if (FFlag::DebugLuauLogSolver)
@@ -84,44 +83,80 @@ void ConstraintSolver::run()
         dump(this, opts);
     }
 
-    do
+    if (FFlag::DebugLuauLogSolverToJson)
     {
-        progress = false;
+        logger.captureBoundarySnapshot(rootScope, unsolvedConstraints);
+    }
 
-        auto it = begin(unsolvedConstraints);
+    auto runSolverPass = [&](bool force) {
-        auto endIt = end(unsolvedConstraints);
+        bool progress = false;
 
-        while (it != endIt)
+        size_t i = 0;
+        while (i < unsolvedConstraints.size())
         {
-            if (isBlocked(*it))
+            NotNull<const Constraint> c = unsolvedConstraints[i];
+            if (!force && isBlocked(c))
             {
-                ++it;
+                ++i;
                 continue;
             }
 
-            std::string saveMe = FFlag::DebugLuauLogSolver ? toString(**it, opts) : std::string{};
+            std::string saveMe = FFlag::DebugLuauLogSolver ? toString(*c, opts) : std::string{};
 
-            bool success = tryDispatch(*it);
+            if (FFlag::DebugLuauLogSolverToJson)
-            progress = progress || success;
+            {
+                logger.prepareStepSnapshot(rootScope, c, unsolvedConstraints);
+            }
+
+            bool success = tryDispatch(c, force);
+
+            progress |= success;
 
-            auto saveIt = it;
-            ++it;
             if (success)
             {
-                unsolvedConstraints.erase(saveIt);
+                unsolvedConstraints.erase(unsolvedConstraints.begin() + i);
+
+                if (FFlag::DebugLuauLogSolverToJson)
+                {
+                    logger.commitPreparedStepSnapshot();
+                }
+
                 if (FFlag::DebugLuauLogSolver)
                 {
+                    if (force)
+                        printf("Force ");
                     printf("Dispatched\n\t%s\n", saveMe.c_str());
                     dump(this, opts);
                 }
             }
+            else
+                ++i;
+
+            if (force && success)
+                return true;
         }
+
+        return progress;
+    };
+
+    bool progress = false;
+    do
+    {
+        progress = runSolverPass(false);
+        if (!progress)
+            progress |= runSolverPass(true);
     } while (progress);
 
     if (FFlag::DebugLuauLogSolver)
+    {
         dumpBindings(rootScope, opts);
+    }
 
-    LUAU_ASSERT(done());
+    if (FFlag::DebugLuauLogSolverToJson)
+    {
+        logger.captureBoundarySnapshot(rootScope, unsolvedConstraints);
+        printf("Logger output:\n%s\n", logger.compileOutput().c_str());
+    }
 }
 
 bool ConstraintSolver::done()
@@ -129,21 +164,21 @@ bool ConstraintSolver::done()
     return unsolvedConstraints.empty();
 }
 
-bool ConstraintSolver::tryDispatch(const Constraint* constraint)
+bool ConstraintSolver::tryDispatch(NotNull<const Constraint> constraint, bool force)
 {
-    if (isBlocked(constraint))
+    if (!force && isBlocked(constraint))
         return false;
 
     bool success = false;
 
     if (auto sc = get<SubtypeConstraint>(*constraint))
-        success = tryDispatch(*sc);
+        success = tryDispatch(*sc, constraint, force);
     else if (auto psc = get<PackSubtypeConstraint>(*constraint))
-        success = tryDispatch(*psc);
+        success = tryDispatch(*psc, constraint, force);
     else if (auto gc = get<GeneralizationConstraint>(*constraint))
-        success = tryDispatch(*gc);
+        success = tryDispatch(*gc, constraint, force);
     else if (auto ic = get<InstantiationConstraint>(*constraint))
-        success = tryDispatch(*ic, constraint);
+        success = tryDispatch(*ic, constraint, force);
     else
         LUAU_ASSERT(0);
 
@@ -155,65 +190,66 @@ bool ConstraintSolver::tryDispatch(const Constraint* constraint)
     return success;
 }
 
-bool ConstraintSolver::tryDispatch(const SubtypeConstraint& c)
+bool ConstraintSolver::tryDispatch(const SubtypeConstraint& c, NotNull<const Constraint> constraint, bool force)
 {
-    unify(c.subType, c.superType);
+    if (isBlocked(c.subType))
+        return block(c.subType, constraint);
+    else if (isBlocked(c.superType))
+        return block(c.superType, constraint);
+
+    unify(c.subType, c.superType, constraint->location);
+
     unblock(c.subType);
     unblock(c.superType);
 
     return true;
 }
 
-bool ConstraintSolver::tryDispatch(const PackSubtypeConstraint& c)
+bool ConstraintSolver::tryDispatch(const PackSubtypeConstraint& c, NotNull<const Constraint> constraint, bool force)
 {
-    unify(c.subPack, c.superPack);
+    unify(c.subPack, c.superPack, constraint->location);
     unblock(c.subPack);
     unblock(c.superPack);
 
     return true;
 }
 
-bool ConstraintSolver::tryDispatch(const GeneralizationConstraint& constraint)
+bool ConstraintSolver::tryDispatch(const GeneralizationConstraint& c, NotNull<const Constraint> constraint, bool force)
 {
-    unify(constraint.subType, constraint.superType);
+    if (isBlocked(c.sourceType))
+        return block(c.sourceType, constraint);
 
-    quantify(constraint.superType, constraint.scope);
+    if (isBlocked(c.generalizedType))
-    unblock(constraint.subType);
+        asMutable(c.generalizedType)->ty.emplace<BoundTypeVar>(c.sourceType);
-    unblock(constraint.superType);
+    else
+        unify(c.generalizedType, c.sourceType, constraint->location);
+
+    TypeId generalized = quantify(arena, c.sourceType, c.scope);
+    *asMutable(c.sourceType) = *generalized;
+
+    unblock(c.generalizedType);
+    unblock(c.sourceType);
 
     return true;
 }
 
-bool ConstraintSolver::tryDispatch(const InstantiationConstraint& c, const Constraint* constraint)
+bool ConstraintSolver::tryDispatch(const InstantiationConstraint& c, NotNull<const Constraint> constraint, bool force)
 {
-    TypeId superType = follow(c.superType);
+    if (isBlocked(c.superType))
-    if (const FunctionTypeVar* ftv = get<FunctionTypeVar>(superType))
+        return block(c.superType, constraint);
-    {
-        if (!ftv->generalized)
-        {
-            block(superType, constraint);
-            return false;
-        }
-    }
-    else if (get<FreeTypeVar>(superType))
-    {
-        block(superType, constraint);
-        return false;
-    }
-    // TODO: Error if it's a primitive or something
 
     Instantiation inst(TxnLog::empty(), arena, TypeLevel{});
 
     std::optional<TypeId> instantiated = inst.substitute(c.superType);
     LUAU_ASSERT(instantiated); // TODO FIXME HANDLE THIS
 
-    unify(c.subType, *instantiated);
+    unify(c.subType, *instantiated, constraint->location);
     unblock(c.subType);
 
     return true;
 }
 
-void ConstraintSolver::block_(BlockedConstraintId target, const Constraint* constraint)
+void ConstraintSolver::block_(BlockedConstraintId target, NotNull<const Constraint> constraint)
 {
     blocked[target].push_back(constraint);
 
@@ -221,19 +257,21 @@ void ConstraintSolver::block_(BlockedConstraintId target, const Constraint* cons
     count += 1;
 }
 
-void ConstraintSolver::block(const Constraint* target, const Constraint* constraint)
+void ConstraintSolver::block(NotNull<const Constraint> target, NotNull<const Constraint> constraint)
 {
     block_(target, constraint);
 }
 
-void ConstraintSolver::block(TypeId target, const Constraint* constraint)
+bool ConstraintSolver::block(TypeId target, NotNull<const Constraint> constraint)
 {
     block_(target, constraint);
+    return false;
 }
 
-void ConstraintSolver::block(TypePackId target, const Constraint* constraint)
+bool ConstraintSolver::block(TypePackId target, NotNull<const Constraint> constraint)
 {
     block_(target, constraint);
+    return false;
 }
 
 void ConstraintSolver::unblock_(BlockedConstraintId progressed)
@@ -243,7 +281,7 @@ void ConstraintSolver::unblock_(BlockedConstraintId progressed)
         return;
 
     // unblocked should contain a value always, because of the above check
-    for (const Constraint* unblockedConstraint : it->second)
+    for (NotNull<const Constraint> unblockedConstraint : it->second)
     {
         auto& count = blockedConstraints[unblockedConstraint];
         // This assertion being hit indicates that `blocked` and
@@ -257,7 +295,7 @@ void ConstraintSolver::unblock_(BlockedConstraintId progressed)
     blocked.erase(it);
 }
 
-void ConstraintSolver::unblock(const Constraint* progressed)
+void ConstraintSolver::unblock(NotNull<const Constraint> progressed)
 {
     return unblock_(progressed);
 }
@@ -272,35 +310,33 @@ void ConstraintSolver::unblock(TypePackId progressed)
     return unblock_(progressed);
 }
 
-bool ConstraintSolver::isBlocked(const Constraint* constraint)
+bool ConstraintSolver::isBlocked(TypeId ty)
+{
+    return nullptr != get<BlockedTypeVar>(follow(ty));
+}
+
+bool ConstraintSolver::isBlocked(NotNull<const Constraint> constraint)
 {
     auto blockedIt = blockedConstraints.find(constraint);
     return blockedIt != blockedConstraints.end() && blockedIt->second > 0;
 }
 
-void ConstraintSolver::reportErrors(const std::vector<TypeError>& errors)
+void ConstraintSolver::unify(TypeId subType, TypeId superType, Location location)
-{
-    this->errors.insert(end(this->errors), begin(errors), end(errors));
-}
-
-void ConstraintSolver::unify(TypeId subType, TypeId superType)
 {
     UnifierSharedState sharedState{&iceReporter};
-    Unifier u{arena, Mode::Strict, Location{}, Covariant, sharedState};
+    Unifier u{arena, Mode::Strict, location, Covariant, sharedState};
 
     u.tryUnify(subType, superType);
     u.log.commit();
-    reportErrors(u.errors);
 }
 
-void ConstraintSolver::unify(TypePackId subPack, TypePackId superPack)
+void ConstraintSolver::unify(TypePackId subPack, TypePackId superPack, Location location)
 {
     UnifierSharedState sharedState{&iceReporter};
-    Unifier u{arena, Mode::Strict, Location{}, Covariant, sharedState};
+    Unifier u{arena, Mode::Strict, location, Covariant, sharedState};
 
     u.tryUnify(subPack, superPack);
     u.log.commit();
-    reportErrors(u.errors);
 }
 
 } // namespace Luau

Analysis/src/ConstraintSolverLogger.cpp

@@ -0,0 +1,139 @@
+// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+
+#include "Luau/ConstraintSolverLogger.h"
+
+namespace Luau
+{
+
+static std::string dumpScopeAndChildren(const Scope2* scope, ToStringOptions& opts)
+{
+    std::string output = "{\"bindings\":{";
+
+    bool comma = false;
+    for (const auto& [name, type] : scope->bindings)
+    {
+        if (comma)
+            output += ",";
+
+        output += "\"";
+        output += name.c_str();
+        output += "\": \"";
+
+        ToStringResult result = toStringDetailed(type, opts);
+        opts.nameMap = std::move(result.nameMap);
+        output += result.name;
+        output += "\"";
+
+        comma = true;
+    }
+
+    output += "},\"children\":[";
+    comma = false;
+
+    for (const Scope2* child : scope->children)
+    {
+        if (comma)
+            output += ",";
+
+        output += dumpScopeAndChildren(child, opts);
+        comma = true;
+    }
+
+    output += "]}";
+    return output;
+}
+
+static std::string dumpConstraintsToDot(std::vector<NotNull<const Constraint>>& constraints, ToStringOptions& opts)
+{
+    std::string result = "digraph Constraints {\\n";
+
+    std::unordered_set<NotNull<const Constraint>> contained;
+    for (NotNull<const Constraint> c : constraints)
+    {
+        contained.insert(c);
+    }
+
+    for (NotNull<const Constraint> c : constraints)
+    {
+        std::string id = std::to_string(reinterpret_cast<size_t>(c.get()));
+        result += id;
+        result += " [label=\\\"";
+        result += toString(*c, opts).c_str();
+        result += "\\\"];\\n";
+
+        for (NotNull<const Constraint> dep : c->dependencies)
+        {
+            if (contained.count(dep) == 0)
+                continue;
+
+            result += std::to_string(reinterpret_cast<size_t>(dep.get()));
+            result += " -> ";
+            result += id;
+            result += ";\\n";
+        }
+    }
+
+    result += "}";
+
+    return result;
+}
+
+std::string ConstraintSolverLogger::compileOutput()
+{
+    std::string output = "[";
+    bool comma = false;
+
+    for (const std::string& snapshot : snapshots)
+    {
+        if (comma)
+            output += ",";
+        output += snapshot;
+
+        comma = true;
+    }
+
+    output += "]";
+    return output;
+}
+
+void ConstraintSolverLogger::captureBoundarySnapshot(const Scope2* rootScope, std::vector<NotNull<const Constraint>>& unsolvedConstraints)
+{
+    std::string snapshot = "{\"type\":\"boundary\",\"rootScope\":";
+
+    snapshot += dumpScopeAndChildren(rootScope, opts);
+    snapshot += ",\"constraintGraph\":\"";
+    snapshot += dumpConstraintsToDot(unsolvedConstraints, opts);
+    snapshot += "\"}";
+
+    snapshots.push_back(std::move(snapshot));
+}
+
+void ConstraintSolverLogger::prepareStepSnapshot(
+    const Scope2* rootScope, NotNull<const Constraint> current, std::vector<NotNull<const Constraint>>& unsolvedConstraints)
+{
+    // LUAU_ASSERT(!preparedSnapshot);
+
+    std::string snapshot = "{\"type\":\"step\",\"rootScope\":";
+
+    snapshot += dumpScopeAndChildren(rootScope, opts);
+    snapshot += ",\"constraintGraph\":\"";
+    snapshot += dumpConstraintsToDot(unsolvedConstraints, opts);
+    snapshot += "\",\"currentId\":\"";
+    snapshot += std::to_string(reinterpret_cast<size_t>(current.get()));
+    snapshot += "\",\"current\":\"";
+    snapshot += toString(*current, opts);
+    snapshot += "\"}";
+
+    preparedSnapshot = std::move(snapshot);
+}
+
+void ConstraintSolverLogger::commitPreparedStepSnapshot()
+{
+    if (preparedSnapshot)
+    {
+        snapshots.push_back(std::move(*preparedSnapshot));
+        preparedSnapshot = std::nullopt;
+    }
+}
+
+} // namespace Luau

Analysis/src/Frontend.cpp

@@ -1,16 +1,17 @@
 // This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
 #include "Luau/Frontend.h"
 
-#include "Luau/Common.h"
 #include "Luau/Clone.h"
+#include "Luau/Common.h"
 #include "Luau/Config.h"
-#include "Luau/FileResolver.h"
 #include "Luau/ConstraintGraphBuilder.h"
 #include "Luau/ConstraintSolver.h"
+#include "Luau/FileResolver.h"
 #include "Luau/Parser.h"
 #include "Luau/Scope.h"
 #include "Luau/StringUtils.h"
 #include "Luau/TimeTrace.h"
+#include "Luau/TypeChecker2.h"
 #include "Luau/TypeInfer.h"
 #include "Luau/Variant.h"
 
@@ -216,7 +217,7 @@ ErrorVec accumulateErrors(
             continue;
 
         const SourceNode& sourceNode = it->second;
-        queue.insert(queue.end(), sourceNode.requires.begin(), sourceNode.requires.end());
+        queue.insert(queue.end(), sourceNode.requireSet.begin(), sourceNode.requireSet.end());
 
         // FIXME: If a module has a syntax error, we won't be able to re-report it here.
         // The solution is probably to move errors from Module to SourceNode
@@ -586,7 +587,7 @@ bool Frontend::parseGraph(std::vector<ModuleName>& buildQueue, CheckResult& chec
             path.push_back(top);
 
             // push children
-            for (const ModuleName& dep : top->requires)
+            for (const ModuleName& dep : top->requireSet)
             {
                 auto it = sourceNodes.find(dep);
                 if (it != sourceNodes.end())
@@ -738,7 +739,7 @@ void Frontend::markDirty(const ModuleName& name, std::vector<ModuleName>* marked
     std::unordered_map<ModuleName, std::vector<ModuleName>> reverseDeps;
     for (const auto& module : sourceNodes)
     {
-        for (const auto& dep : module.second.requires)
+        for (const auto& dep : module.second.requireSet)
             reverseDeps[dep].push_back(module.first);
     }
 
@@ -797,9 +798,14 @@ ModulePtr Frontend::check(const SourceModule& sourceModule, Mode mode, const Sco
     cs.run();
 
     result->scope2s = std::move(cgb.scopes);
+    result->astTypes = std::move(cgb.astTypes);
+    result->astTypePacks = std::move(cgb.astTypePacks);
+    result->astOriginalCallTypes = std::move(cgb.astOriginalCallTypes);
 
     result->clonePublicInterface(iceHandler);
 
+    Luau::check(sourceModule, result.get());
+
     return result;
 }
 
@@ -841,8 +847,8 @@ std::pair<SourceNode*, SourceModule*> Frontend::getSourceNode(CheckResult& check
     SourceModule result = parse(name, source->source, opts);
     result.type = source->type;
 
-    RequireTraceResult& requireTrace = requires[name];
+    RequireTraceResult& require = requireTrace[name];
-    requireTrace = traceRequires(fileResolver, result.root, name);
+    require = traceRequires(fileResolver, result.root, name);
 
     SourceNode& sourceNode = sourceNodes[name];
     SourceModule& sourceModule = sourceModules[name];
@@ -851,7 +857,7 @@ std::pair<SourceNode*, SourceModule*> Frontend::getSourceNode(CheckResult& check
     sourceModule.environmentName = environmentName;
 
     sourceNode.name = name;
-    sourceNode.requires.clear();
+    sourceNode.requireSet.clear();
     sourceNode.requireLocations.clear();
     sourceNode.dirtySourceModule = false;
 
@@ -861,10 +867,10 @@ std::pair<SourceNode*, SourceModule*> Frontend::getSourceNode(CheckResult& check
         sourceNode.dirtyModuleForAutocomplete = true;
     }
 
-    for (const auto& [moduleName, location] : requireTrace.requires)
+    for (const auto& [moduleName, location] : require.requireList)
-        sourceNode.requires.insert(moduleName);
+        sourceNode.requireSet.insert(moduleName);
 
-    sourceNode.requireLocations = requireTrace.requires;
+    sourceNode.requireLocations = require.requireList;
 
     return {&sourceNode, &sourceModule};
 }
@@ -925,8 +931,8 @@ SourceModule Frontend::parse(const ModuleName& name, std::string_view src, const
 std::optional<ModuleInfo> FrontendModuleResolver::resolveModuleInfo(const ModuleName& currentModuleName, const AstExpr& pathExpr)
 {
     // FIXME I think this can be pushed into the FileResolver.
-    auto it = frontend->requires.find(currentModuleName);
+    auto it = frontend->requireTrace.find(currentModuleName);
-    if (it == frontend->requires.end())
+    if (it == frontend->requireTrace.end())
     {
         // CLI-43699
         // If we can't find the current module name, that's because we bypassed the frontend's initializer
@@ -1025,7 +1031,7 @@ void Frontend::clear()
     sourceModules.clear();
     moduleResolver.modules.clear();
     moduleResolverForAutocomplete.modules.clear();
-    requires.clear();
+    requireTrace.clear();
 }
 
 } // namespace Luau

Analysis/src/Instantiation.cpp

@@ -40,7 +40,7 @@ TypeId Instantiation::clean(TypeId ty)
     const FunctionTypeVar* ftv = log->getMutable<FunctionTypeVar>(ty);
     LUAU_ASSERT(ftv);
 
-    FunctionTypeVar clone = FunctionTypeVar{level, ftv->argTypes, ftv->retType, ftv->definition, ftv->hasSelf};
+    FunctionTypeVar clone = FunctionTypeVar{level, ftv->argTypes, ftv->retTypes, ftv->definition, ftv->hasSelf};
     clone.magicFunction = ftv->magicFunction;
     clone.tags = ftv->tags;
     clone.argNames = ftv->argNames;

Analysis/src/Linter.cpp

@@ -2282,7 +2282,7 @@ private:
     size_t getReturnCount(TypeId ty)
     {
         if (auto ftv = get<FunctionTypeVar>(ty))
-            return size(ftv->retType);
+            return size(ftv->retTypes);
 
         if (auto itv = get<IntersectionTypeVar>(ty))
         {
@@ -2291,7 +2291,7 @@ private:
 
             for (TypeId part : itv->parts)
                 if (auto ftv = get<FunctionTypeVar>(follow(part)))
-                    result = std::max(result, size(ftv->retType));
+                    result = std::max(result, size(ftv->retTypes));
 
             return result;
         }

Analysis/src/Normalize.cpp

@@ -17,6 +17,7 @@ LUAU_FASTFLAGVARIABLE(LuauNormalizeCombineTableFix, false);
 LUAU_FASTFLAGVARIABLE(LuauNormalizeFlagIsConservative, false);
 LUAU_FASTFLAGVARIABLE(LuauNormalizeCombineEqFix, false);
 LUAU_FASTFLAGVARIABLE(LuauReplaceReplacer, false);
+LUAU_FASTFLAG(LuauQuantifyConstrained)
 
 namespace Luau
 {
@@ -273,6 +274,18 @@ bool isSubtype(TypeId subTy, TypeId superTy, InternalErrorReporter& ice)
     return ok;
 }
 
+bool isSubtype(TypePackId subPack, TypePackId superPack, InternalErrorReporter& ice)
+{
+    UnifierSharedState sharedState{&ice};
+    TypeArena arena;
+    Unifier u{&arena, Mode::Strict, Location{}, Covariant, sharedState};
+    u.anyIsTop = true;
+
+    u.tryUnify(subPack, superPack);
+    const bool ok = u.errors.empty() && u.log.empty();
+    return ok;
+}
+
 template<typename T>
 static bool areNormal_(const T& t, const std::unordered_set<void*>& seen, InternalErrorReporter& ice)
 {
@@ -390,6 +403,7 @@ struct Normalize final : TypeVarVisitor
     bool visit(TypeId ty, const ConstrainedTypeVar& ctvRef) override
     {
         CHECK_ITERATION_LIMIT(false);
+        LUAU_ASSERT(!ty->normal);
 
         ConstrainedTypeVar* ctv = const_cast<ConstrainedTypeVar*>(&ctvRef);
 
@@ -401,6 +415,12 @@ struct Normalize final : TypeVarVisitor
 
         std::vector<TypeId> newParts = normalizeUnion(parts);
 
+        if (FFlag::LuauQuantifyConstrained)
+        {
+            ctv->parts = std::move(newParts);
+        }
+        else
+        {
             const bool normal = areNormal(newParts, seen, ice);
 
             if (newParts.size() == 1)
@@ -409,6 +429,7 @@ struct Normalize final : TypeVarVisitor
                 *asMutable(ty) = UnionTypeVar{std::move(newParts)};
 
             asMutable(ty)->normal = normal;
+        }
 
         return false;
     }
@@ -421,9 +442,9 @@ struct Normalize final : TypeVarVisitor
             return false;
 
         traverse(ftv.argTypes);
-        traverse(ftv.retType);
+        traverse(ftv.retTypes);
 
-        asMutable(ty)->normal = areNormal(ftv.argTypes, seen, ice) && areNormal(ftv.retType, seen, ice);
+        asMutable(ty)->normal = areNormal(ftv.argTypes, seen, ice) && areNormal(ftv.retTypes, seen, ice);
 
         return false;
     }
@@ -465,6 +486,13 @@ struct Normalize final : TypeVarVisitor
             checkNormal(ttv.indexer->indexResultType);
         }
 
+        // An unsealed table can never be normal, ditto for free tables iff the type it is bound to is also not normal.
+        if (FFlag::LuauQuantifyConstrained)
+        {
+            if (ttv.state == TableState::Generic || ttv.state == TableState::Sealed || (ttv.state == TableState::Free && follow(ty)->normal))
+                asMutable(ty)->normal = normal;
+        }
+        else
             asMutable(ty)->normal = normal;
 
         return false;

Analysis/src/Quantify.cpp

@@ -2,15 +2,32 @@
 
 #include "Luau/Quantify.h"
 
+#include "Luau/ConstraintGraphBuilder.h" // TODO for Scope2; move to separate header
+#include "Luau/TxnLog.h"
+#include "Luau/Substitution.h"
 #include "Luau/VisitTypeVar.h"
 #include "Luau/ConstraintGraphBuilder.h" // TODO for Scope2; move to separate header
 
 LUAU_FASTFLAG(LuauAlwaysQuantify);
 LUAU_FASTFLAG(DebugLuauDeferredConstraintResolution);
+LUAU_FASTFLAGVARIABLE(LuauQuantifyConstrained, false)
 
 namespace Luau
 {
 
+/// @return true if outer encloses inner
+static bool subsumes(Scope2* outer, Scope2* inner)
+{
+    while (inner)
+    {
+        if (inner == outer)
+            return true;
+        inner = inner->parent;
+    }
+
+    return false;
+}
+
 struct Quantifier final : TypeVarOnceVisitor
 {
     TypeLevel level;
@@ -62,6 +79,34 @@ struct Quantifier final : TypeVarOnceVisitor
         return false;
     }
 
+    bool visit(TypeId ty, const ConstrainedTypeVar&) override
+    {
+        if (FFlag::LuauQuantifyConstrained)
+        {
+            ConstrainedTypeVar* ctv = getMutable<ConstrainedTypeVar>(ty);
+
+            seenMutableType = true;
+
+            if (FFlag::DebugLuauDeferredConstraintResolution ? !subsumes(scope, ctv->scope) : !level.subsumes(ctv->level))
+                return false;
+
+            std::vector<TypeId> opts = std::move(ctv->parts);
+
+            // We might transmute, so it's not safe to rely on the builtin traversal logic
+            for (TypeId opt : opts)
+                traverse(opt);
+
+            if (opts.size() == 1)
+                *asMutable(ty) = BoundTypeVar{opts[0]};
+            else
+                *asMutable(ty) = UnionTypeVar{std::move(opts)};
+
+            return false;
+        }
+        else
+            return true;
+    }
+
     bool visit(TypeId ty, const TableTypeVar&) override
     {
         LUAU_ASSERT(getMutable<TableTypeVar>(ty));
@@ -73,8 +118,12 @@ struct Quantifier final : TypeVarOnceVisitor
         if (ttv.state == TableState::Free)
             seenMutableType = true;
 
+        if (!FFlag::LuauQuantifyConstrained)
+        {
             if (ttv.state == TableState::Sealed || ttv.state == TableState::Generic)
                 return false;
+        }
+
         if (FFlag::DebugLuauDeferredConstraintResolution ? !subsumes(scope, ttv.scope) : !level.subsumes(ttv.level))
         {
             if (ttv.state == TableState::Unsealed)
@@ -156,4 +205,104 @@ void quantify(TypeId ty, Scope2* scope)
     ftv->generalized = true;
 }
 
+struct PureQuantifier : Substitution
+{
+    Scope2* scope;
+    std::vector<TypeId> insertedGenerics;
+    std::vector<TypePackId> insertedGenericPacks;
+
+    PureQuantifier(const TxnLog* log, TypeArena* arena, Scope2* scope)
+        : Substitution(log, arena)
+        , scope(scope)
+    {
+    }
+
+    bool isDirty(TypeId ty) override
+    {
+        LUAU_ASSERT(ty == follow(ty));
+
+        if (auto ftv = get<FreeTypeVar>(ty))
+        {
+            return subsumes(scope, ftv->scope);
+        }
+        else if (auto ttv = get<TableTypeVar>(ty))
+        {
+            return ttv->state == TableState::Free && subsumes(scope, ttv->scope);
+        }
+
+        return false;
+    }
+
+    bool isDirty(TypePackId tp) override
+    {
+        if (auto ftp = get<FreeTypePack>(tp))
+        {
+            return subsumes(scope, ftp->scope);
+        }
+
+        return false;
+    }
+
+    TypeId clean(TypeId ty) override
+    {
+        if (auto ftv = get<FreeTypeVar>(ty))
+        {
+            TypeId result = arena->addType(GenericTypeVar{});
+            insertedGenerics.push_back(result);
+            return result;
+        }
+        else if (auto ttv = get<TableTypeVar>(ty))
+        {
+            TypeId result = arena->addType(TableTypeVar{});
+            TableTypeVar* resultTable = getMutable<TableTypeVar>(result);
+            LUAU_ASSERT(resultTable);
+
+            *resultTable = *ttv;
+            resultTable->scope = nullptr;
+            resultTable->state = TableState::Generic;
+
+            return result;
+        }
+
+        return ty;
+    }
+
+    TypePackId clean(TypePackId tp) override
+    {
+        if (auto ftp = get<FreeTypePack>(tp))
+        {
+            TypePackId result = arena->addTypePack(TypePackVar{GenericTypePack{}});
+            insertedGenericPacks.push_back(result);
+            return result;
+        }
+
+        return tp;
+    }
+
+    bool ignoreChildren(TypeId ty) override
+    {
+        return ty->persistent;
+    }
+    bool ignoreChildren(TypePackId ty) override
+    {
+        return ty->persistent;
+    }
+};
+
+TypeId quantify(TypeArena* arena, TypeId ty, Scope2* scope)
+{
+    PureQuantifier quantifier{TxnLog::empty(), arena, scope};
+    std::optional<TypeId> result = quantifier.substitute(ty);
+    LUAU_ASSERT(result);
+
+    FunctionTypeVar* ftv = getMutable<FunctionTypeVar>(*result);
+    LUAU_ASSERT(ftv);
+    ftv->generics.insert(ftv->generics.end(), quantifier.insertedGenerics.begin(), quantifier.insertedGenerics.end());
+    ftv->genericPacks.insert(ftv->genericPacks.end(), quantifier.insertedGenericPacks.begin(), quantifier.insertedGenericPacks.end());
+
+    // TODO: Set hasNoGenerics.
+
+    return *result;
+}
+
 } // namespace Luau

Analysis/src/RequireTracer.cpp

@@ -28,7 +28,7 @@ struct RequireTracer : AstVisitor
         AstExprGlobal* global = expr->func->as<AstExprGlobal>();
 
         if (global && global->name == "require" && expr->args.size >= 1)
-            requires.push_back(expr);
+            requireCalls.push_back(expr);
 
         return true;
     }
@@ -84,9 +84,9 @@ struct RequireTracer : AstVisitor
         ModuleInfo moduleContext{currentModuleName};
 
         // seed worklist with require arguments
-        work.reserve(requires.size());
+        work.reserve(requireCalls.size());
 
-        for (AstExprCall* require : requires)
+        for (AstExprCall* require : requireCalls)
             work.push_back(require->args.data[0]);
 
         // push all dependent expressions to the work stack; note that the vector is modified during traversal
@@ -125,15 +125,15 @@ struct RequireTracer : AstVisitor
         }
 
         // resolve all requires according to their argument
-        result.requires.reserve(requires.size());
+        result.requireList.reserve(requireCalls.size());
 
-        for (AstExprCall* require : requires)
+        for (AstExprCall* require : requireCalls)
         {
             AstExpr* arg = require->args.data[0];
 
             if (const ModuleInfo* info = result.exprs.find(arg))
             {
-                result.requires.push_back({info->name, require->location});
+                result.requireList.push_back({info->name, require->location});
 
                 ModuleInfo infoCopy = *info; // copy *info out since next line invalidates info!
                 result.exprs[require] = std::move(infoCopy);
@@ -151,7 +151,7 @@ struct RequireTracer : AstVisitor
 
     DenseHashMap<AstLocal*, AstExpr*> locals;
     std::vector<AstExpr*> work;
-    std::vector<AstExprCall*> requires;
+    std::vector<AstExprCall*> requireCalls;
 };
 
 RequireTraceResult traceRequires(FileResolver* fileResolver, AstStatBlock* root, const ModuleName& currentModuleName)

Analysis/src/Substitution.cpp

@@ -27,7 +27,7 @@ void Tarjan::visitChildren(TypeId ty, int index)
     if (const FunctionTypeVar* ftv = get<FunctionTypeVar>(ty))
     {
         visitChild(ftv->argTypes);
-        visitChild(ftv->retType);
+        visitChild(ftv->retTypes);
     }
     else if (const TableTypeVar* ttv = get<TableTypeVar>(ty))
     {
@@ -442,7 +442,7 @@ void Substitution::replaceChildren(TypeId ty)
     if (FunctionTypeVar* ftv = getMutable<FunctionTypeVar>(ty))
     {
         ftv->argTypes = replace(ftv->argTypes);
-        ftv->retType = replace(ftv->retType);
+        ftv->retTypes = replace(ftv->retTypes);
     }
     else if (TableTypeVar* ttv = getMutable<TableTypeVar>(ty))
     {

Analysis/src/ToDot.cpp

@@ -154,7 +154,7 @@ void StateDot::visitChildren(TypeId ty, int index)
         finishNode();
 
         visitChild(ftv->argTypes, index, "arg");
-        visitChild(ftv->retType, index, "ret");
+        visitChild(ftv->retTypes, index, "ret");
     }
     else if (const TableTypeVar* ttv = get<TableTypeVar>(ty))
     {

Analysis/src/ToString.cpp

@@ -226,6 +226,11 @@ struct StringifierState
         result.name += s;
     }
 
+    void emit(int i)
+    {
+        emit(std::to_string(i).c_str());
+    }
+
     void indent()
     {
         indentation += 4;
@@ -394,6 +399,13 @@ struct TypeVarStringifier
         state.emit("]]");
     }
 
+    void operator()(TypeId, const BlockedTypeVar& btv)
+    {
+        state.emit("*blocked-");
+        state.emit(btv.index);
+        state.emit("*");
+    }
+
     void operator()(TypeId, const PrimitiveTypeVar& ptv)
     {
         switch (ptv.type)
@@ -480,8 +492,8 @@ struct TypeVarStringifier
 
         if (FFlag::LuauLowerBoundsCalculation)
         {
-            auto retBegin = begin(ftv.retType);
+            auto retBegin = begin(ftv.retTypes);
-            auto retEnd = end(ftv.retType);
+            auto retEnd = end(ftv.retTypes);
             if (retBegin != retEnd)
             {
                 ++retBegin;
@@ -491,7 +503,7 @@ struct TypeVarStringifier
         }
         else
         {
-            if (auto retPack = get<TypePack>(follow(ftv.retType)))
+            if (auto retPack = get<TypePack>(follow(ftv.retTypes)))
             {
                 if (retPack->head.size() == 1 && !retPack->tail)
                     plural = false;
@@ -501,7 +513,7 @@ struct TypeVarStringifier
         if (plural)
             state.emit("(");
 
-        stringify(ftv.retType);
+        stringify(ftv.retTypes);
 
         if (plural)
             state.emit(")");
@@ -1303,14 +1315,14 @@ std::string toStringNamedFunction(const std::string& funcName, const FunctionTyp
 
     state.emit("): ");
 
-    size_t retSize = size(ftv.retType);
+    size_t retSize = size(ftv.retTypes);
-    bool hasTail = !finite(ftv.retType);
+    bool hasTail = !finite(ftv.retTypes);
-    bool wrap = get<TypePack>(follow(ftv.retType)) && (hasTail ? retSize != 0 : retSize != 1);
+    bool wrap = get<TypePack>(follow(ftv.retTypes)) && (hasTail ? retSize != 0 : retSize != 1);
 
     if (wrap)
         state.emit("(");
 
-    tvs.stringify(ftv.retType);
+    tvs.stringify(ftv.retTypes);
 
     if (wrap)
         state.emit(")");
@@ -1385,9 +1397,9 @@ std::string toString(const Constraint& c, ToStringOptions& opts)
     }
     else if (const GeneralizationConstraint* gc = Luau::get_if<GeneralizationConstraint>(&c.c))
     {
-        ToStringResult subStr = toStringDetailed(gc->subType, opts);
+        ToStringResult subStr = toStringDetailed(gc->generalizedType, opts);
         opts.nameMap = std::move(subStr.nameMap);
-        ToStringResult superStr = toStringDetailed(gc->superType, opts);
+        ToStringResult superStr = toStringDetailed(gc->sourceType, opts);
         opts.nameMap = std::move(superStr.nameMap);
         return subStr.name + " ~ gen " + superStr.name;
     }

Analysis/src/TypeAttach.cpp

@@ -94,6 +94,11 @@ public:
         }
     }
 
+    AstType* operator()(const BlockedTypeVar& btv)
+    {
+        return allocator->alloc<AstTypeReference>(Location(), std::nullopt, AstName("*blocked*"));
+    }
+
     AstType* operator()(const ConstrainedTypeVar& ctv)
     {
         AstArray<AstType*> types;
@@ -271,7 +276,7 @@ public:
         }
 
         AstArray<AstType*> returnTypes;
-        const auto& [retVector, retTail] = flatten(ftv.retType);
+        const auto& [retVector, retTail] = flatten(ftv.retTypes);
         returnTypes.size = retVector.size();
         returnTypes.data = static_cast<AstType**>(allocator->allocate(sizeof(AstType*) * returnTypes.size));
         for (size_t i = 0; i < returnTypes.size; ++i)

Analysis/src/TypeChecker2.cpp

@@ -0,0 +1,160 @@
+
+#include "Luau/TypeChecker2.h"
+
+#include <algorithm>
+
+#include "Luau/Ast.h"
+#include "Luau/AstQuery.h"
+#include "Luau/Clone.h"
+#include "Luau/Normalize.h"
+
+namespace Luau
+{
+
+struct TypeChecker2 : public AstVisitor
+{
+    const SourceModule* sourceModule;
+    Module* module;
+    InternalErrorReporter ice; // FIXME accept a pointer from Frontend
+
+    TypeChecker2(const SourceModule* sourceModule, Module* module)
+        : sourceModule(sourceModule)
+        , module(module)
+    {
+    }
+
+    using AstVisitor::visit;
+
+    TypePackId lookupPack(AstExpr* expr)
+    {
+        TypePackId* tp = module->astTypePacks.find(expr);
+        LUAU_ASSERT(tp);
+        return follow(*tp);
+    }
+
+    TypeId lookupType(AstExpr* expr)
+    {
+        TypeId* ty = module->astTypes.find(expr);
+        LUAU_ASSERT(ty);
+        return follow(*ty);
+    }
+
+    bool visit(AstStatAssign* assign) override
+    {
+        size_t count = std::min(assign->vars.size, assign->values.size);
+
+        for (size_t i = 0; i < count; ++i)
+        {
+            AstExpr* lhs = assign->vars.data[i];
+            TypeId* lhsType = module->astTypes.find(lhs);
+            LUAU_ASSERT(lhsType);
+
+            AstExpr* rhs = assign->values.data[i];
+            TypeId* rhsType = module->astTypes.find(rhs);
+            LUAU_ASSERT(rhsType);
+
+            if (!isSubtype(*rhsType, *lhsType, ice))
+            {
+                reportError(TypeMismatch{*lhsType, *rhsType}, rhs->location);
+            }
+        }
+
+        return true;
+    }
+
+    bool visit(AstExprCall* call) override
+    {
+        TypePackId expectedRetType = lookupPack(call);
+        TypeId functionType = lookupType(call->func);
+
+        TypeArena arena;
+        TypePack args;
+        for (const auto& arg : call->args)
+        {
+            TypeId argTy = module->astTypes[arg];
+            LUAU_ASSERT(argTy);
+            args.head.push_back(argTy);
+        }
+
+        TypePackId argsTp = arena.addTypePack(args);
+        FunctionTypeVar ftv{argsTp, expectedRetType};
+        TypeId expectedType = arena.addType(ftv);
+        if (!isSubtype(expectedType, functionType, ice))
+        {
+            unfreeze(module->interfaceTypes);
+            CloneState cloneState;
+            expectedType = clone(expectedType, module->interfaceTypes, cloneState);
+            freeze(module->interfaceTypes);
+            reportError(TypeMismatch{expectedType, functionType}, call->location);
+        }
+
+        return true;
+    }
+
+    bool visit(AstExprIndexName* indexName) override
+    {
+        TypeId leftType = lookupType(indexName->expr);
+        TypeId resultType = lookupType(indexName);
+
+        // leftType must have a property called indexName->index
+
+        if (auto ttv = get<TableTypeVar>(leftType))
+        {
+            auto it = ttv->props.find(indexName->index.value);
+            if (it == ttv->props.end())
+            {
+                reportError(UnknownProperty{leftType, indexName->index.value}, indexName->location);
+            }
+            else if (!isSubtype(resultType, it->second.type, ice))
+            {
+                reportError(TypeMismatch{resultType, it->second.type}, indexName->location);
+            }
+        }
+        else
+        {
+            reportError(UnknownProperty{leftType, indexName->index.value}, indexName->location);
+        }
+
+        return true;
+    }
+
+    bool visit(AstExprConstantNumber* number) override
+    {
+        TypeId actualType = lookupType(number);
+        TypeId numberType = getSingletonTypes().numberType;
+
+        if (!isSubtype(actualType, numberType, ice))
+        {
+            reportError(TypeMismatch{actualType, numberType}, number->location);
+        }
+
+        return true;
+    }
+
+    bool visit(AstExprConstantString* string) override
+    {
+        TypeId actualType = lookupType(string);
+        TypeId stringType = getSingletonTypes().stringType;
+
+        if (!isSubtype(actualType, stringType, ice))
+        {
+            reportError(TypeMismatch{actualType, stringType}, string->location);
+        }
+
+        return true;
+    }
+
+    void reportError(TypeErrorData&& data, const Location& location)
+    {
+        module->errors.emplace_back(location, sourceModule->name, std::move(data));
+    }
+};
+
+void check(const SourceModule& sourceModule, Module* module)
+{
+    TypeChecker2 typeChecker{&sourceModule, module};
+
+    sourceModule.root->visit(&typeChecker);
+}
+
+} // namespace Luau

Analysis/src/TypeInfer.cpp

@@ -18,6 +18,7 @@
 #include "Luau/TypePack.h"
 #include "Luau/TypeUtils.h"
 #include "Luau/TypeVar.h"
+#include "Luau/VisitTypeVar.h"
 
 #include <algorithm>
 #include <iterator>
@@ -30,7 +31,6 @@ LUAU_FASTINTVARIABLE(LuauCheckRecursionLimit, 300)
 LUAU_FASTINTVARIABLE(LuauVisitRecursionLimit, 500)
 LUAU_FASTFLAG(LuauKnowsTheDataModel3)
 LUAU_FASTFLAG(LuauAutocompleteDynamicLimits)
-LUAU_FASTFLAGVARIABLE(LuauExpectedPropTypeFromIndexer, false)
 LUAU_FASTFLAGVARIABLE(LuauLowerBoundsCalculation, false)
 LUAU_FASTFLAGVARIABLE(DebugLuauFreezeDuringUnification, false)
 LUAU_FASTFLAGVARIABLE(LuauSelfCallAutocompleteFix2, false)
@@ -42,9 +42,9 @@ LUAU_FASTFLAG(LuauNormalizeFlagIsConservative)
 LUAU_FASTFLAGVARIABLE(LuauReturnTypeInferenceInNonstrict, false)
 LUAU_FASTFLAGVARIABLE(LuauRecursionLimitException, false);
 LUAU_FASTFLAGVARIABLE(LuauApplyTypeFunctionFix, false);
-LUAU_FASTFLAGVARIABLE(LuauSuccessTypingForEqualityOperations, false)
 LUAU_FASTFLAGVARIABLE(LuauAlwaysQuantify, false);
 LUAU_FASTFLAGVARIABLE(LuauReportErrorsOnIndexerKeyMismatch, false)
+LUAU_FASTFLAG(LuauQuantifyConstrained)
 LUAU_FASTFLAGVARIABLE(LuauFalsyPredicateReturnsNilInstead, false)
 LUAU_FASTFLAGVARIABLE(LuauNonCopyableTypeVarFields, false)
 
@@ -260,7 +260,6 @@ TypeChecker::TypeChecker(ModuleResolver* resolver, InternalErrorReporter* iceHan
     , booleanType(getSingletonTypes().booleanType)
     , threadType(getSingletonTypes().threadType)
     , anyType(getSingletonTypes().anyType)
-    , optionalNumberType(getSingletonTypes().optionalNumberType)
     , anyTypePack(getSingletonTypes().anyTypePack)
     , duplicateTypeAliases{{false, {}}}
 {
@@ -679,7 +678,7 @@ static std::optional<Predicate> tryGetTypeGuardPredicate(const AstExprBinary& ex
 
 void TypeChecker::check(const ScopePtr& scope, const AstStatIf& statement)
 {
-    ExprResult<TypeId> result = checkExpr(scope, *statement.condition);
+    WithPredicate<TypeId> result = checkExpr(scope, *statement.condition);
 
     ScopePtr ifScope = childScope(scope, statement.thenbody->location);
     resolve(result.predicates, ifScope, true);
@@ -712,7 +711,7 @@ ErrorVec TypeChecker::canUnify(TypePackId subTy, TypePackId superTy, const Locat
 
 void TypeChecker::check(const ScopePtr& scope, const AstStatWhile& statement)
 {
-    ExprResult<TypeId> result = checkExpr(scope, *statement.condition);
+    WithPredicate<TypeId> result = checkExpr(scope, *statement.condition);
 
     ScopePtr whileScope = childScope(scope, statement.body->location);
     resolve(result.predicates, whileScope, true);
@@ -728,16 +727,64 @@ void TypeChecker::check(const ScopePtr& scope, const AstStatRepeat& statement)
     checkExpr(repScope, *statement.condition);
 }
 
-void TypeChecker::unifyLowerBound(TypePackId subTy, TypePackId superTy, const Location& location)
+void TypeChecker::unifyLowerBound(TypePackId subTy, TypePackId superTy, TypeLevel demotedLevel, const Location& location)
 {
     Unifier state = mkUnifier(location);
-    state.unifyLowerBound(subTy, superTy);
+    state.unifyLowerBound(subTy, superTy, demotedLevel);
 
     state.log.commit();
 
     reportErrors(state.errors);
 }
 
+struct Demoter : Substitution
+{
+    Demoter(TypeArena* arena)
+        : Substitution(TxnLog::empty(), arena)
+    {
+    }
+
+    bool isDirty(TypeId ty) override
+    {
+        return get<FreeTypeVar>(ty);
+    }
+
+    bool isDirty(TypePackId tp) override
+    {
+        return get<FreeTypePack>(tp);
+    }
+
+    TypeId clean(TypeId ty) override
+    {
+        auto ftv = get<FreeTypeVar>(ty);
+        LUAU_ASSERT(ftv);
+        return addType(FreeTypeVar{demotedLevel(ftv->level)});
+    }
+
+    TypePackId clean(TypePackId tp) override
+    {
+        auto ftp = get<FreeTypePack>(tp);
+        LUAU_ASSERT(ftp);
+        return addTypePack(TypePackVar{FreeTypePack{demotedLevel(ftp->level)}});
+    }
+
+    TypeLevel demotedLevel(TypeLevel level)
+    {
+        return TypeLevel{level.level + 5000, level.subLevel};
+    }
+
+    void demote(std::vector<std::optional<TypeId>>& expectedTypes)
+    {
+        if (!FFlag::LuauQuantifyConstrained)
+            return;
+        for (std::optional<TypeId>& ty : expectedTypes)
+        {
+            if (ty)
+                ty = substitute(*ty);
+        }
+    }
+};
+
 void TypeChecker::check(const ScopePtr& scope, const AstStatReturn& return_)
 {
     std::vector<std::optional<TypeId>> expectedTypes;
@@ -760,11 +807,14 @@ void TypeChecker::check(const ScopePtr& scope, const AstStatReturn& return_)
         }
     }
 
+    Demoter demoter{&currentModule->internalTypes};
+    demoter.demote(expectedTypes);
+
     TypePackId retPack = checkExprList(scope, return_.location, return_.list, false, {}, expectedTypes).type;
 
     if (FFlag::LuauReturnTypeInferenceInNonstrict ? FFlag::LuauLowerBoundsCalculation : useConstrainedIntersections())
     {
-        unifyLowerBound(retPack, scope->returnType, return_.location);
+        unifyLowerBound(retPack, scope->returnType, demoter.demotedLevel(scope->level), return_.location);
         return;
     }
 
@@ -1230,7 +1280,7 @@ void TypeChecker::check(const ScopePtr& scope, const AstStatForIn& forin)
         unify(retPack, varPack, forin.location);
     }
     else
-        unify(iterFunc->retType, varPack, forin.location);
+        unify(iterFunc->retTypes, varPack, forin.location);
 
     check(loopScope, *forin.body);
 }
@@ -1611,7 +1661,7 @@ void TypeChecker::check(const ScopePtr& scope, const AstStatDeclareFunction& glo
     currentModule->getModuleScope()->bindings[global.name] = Binding{fnType, global.location};
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExpr& expr, std::optional<TypeId> expectedType, bool forceSingleton)
+WithPredicate<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExpr& expr, std::optional<TypeId> expectedType, bool forceSingleton)
 {
     RecursionCounter _rc(&checkRecursionCount);
     if (FInt::LuauCheckRecursionLimit > 0 && checkRecursionCount >= FInt::LuauCheckRecursionLimit)
@@ -1620,7 +1670,7 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExpr&
         return {errorRecoveryType(scope)};
     }
 
-    ExprResult<TypeId> result;
+    WithPredicate<TypeId> result;
 
     if (auto a = expr.as<AstExprGroup>())
         result = checkExpr(scope, *a->expr, expectedType);
@@ -1682,7 +1732,7 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExpr&
     return result;
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprLocal& expr)
+WithPredicate<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprLocal& expr)
 {
     std::optional<LValue> lvalue = tryGetLValue(expr);
     LUAU_ASSERT(lvalue); // Guaranteed to not be nullopt - AstExprLocal is an LValue.
@@ -1696,7 +1746,7 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprLo
     return {errorRecoveryType(scope)};
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprGlobal& expr)
+WithPredicate<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprGlobal& expr)
 {
     std::optional<LValue> lvalue = tryGetLValue(expr);
     LUAU_ASSERT(lvalue); // Guaranteed to not be nullopt - AstExprGlobal is an LValue.
@@ -1708,7 +1758,7 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprGl
     return {errorRecoveryType(scope)};
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprVarargs& expr)
+WithPredicate<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprVarargs& expr)
 {
     TypePackId varargPack = checkExprPack(scope, expr).type;
 
@@ -1738,9 +1788,9 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprVa
         ice("Unknown TypePack type in checkExpr(AstExprVarargs)!");
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprCall& expr)
+WithPredicate<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprCall& expr)
 {
-    ExprResult<TypePackId> result = checkExprPack(scope, expr);
+    WithPredicate<TypePackId> result = checkExprPack(scope, expr);
     TypePackId retPack = follow(result.type);
 
     if (auto pack = get<TypePack>(retPack))
@@ -1770,7 +1820,7 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprCa
         ice("Unknown TypePack type!", expr.location);
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprIndexName& expr)
+WithPredicate<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprIndexName& expr)
 {
     Name name = expr.index.value;
 
@@ -2031,7 +2081,7 @@ TypeId TypeChecker::stripFromNilAndReport(TypeId ty, const Location& location)
     return ty;
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprIndexExpr& expr)
+WithPredicate<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprIndexExpr& expr)
 {
     TypeId ty = checkLValue(scope, expr);
 
@@ -2042,7 +2092,7 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprIn
     return {ty};
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprFunction& expr, std::optional<TypeId> expectedType)
+WithPredicate<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprFunction& expr, std::optional<TypeId> expectedType)
 {
     auto [funTy, funScope] = checkFunctionSignature(scope, 0, expr, std::nullopt, expectedType);
 
@@ -2108,8 +2158,7 @@ TypeId TypeChecker::checkExprTable(
                         if (errors.empty())
                             exprType = expectedProp.type;
                     }
-                    else if (expectedTable->indexer && (FFlag::LuauExpectedPropTypeFromIndexer ? maybeString(expectedTable->indexer->indexType)
+                    else if (expectedTable->indexer && maybeString(expectedTable->indexer->indexType))
-                                                                                               : isString(expectedTable->indexer->indexType)))
                     {
                         ErrorVec errors = tryUnify(exprType, expectedTable->indexer->indexResultType, k->location);
                         if (errors.empty())
@@ -2147,7 +2196,7 @@ TypeId TypeChecker::checkExprTable(
     return addType(table);
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprTable& expr, std::optional<TypeId> expectedType)
+WithPredicate<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprTable& expr, std::optional<TypeId> expectedType)
 {
     RecursionCounter _rc(&checkRecursionCount);
     if (FInt::LuauCheckRecursionLimit > 0 && checkRecursionCount >= FInt::LuauCheckRecursionLimit)
@@ -2201,7 +2250,7 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprTa
                 {
                     if (auto prop = expectedTable->props.find(key->value.data); prop != expectedTable->props.end())
                         expectedResultType = prop->second.type;
-                    else if (FFlag::LuauExpectedPropTypeFromIndexer && expectedIndexType && maybeString(*expectedIndexType))
+                    else if (expectedIndexType && maybeString(*expectedIndexType))
                         expectedResultType = expectedIndexResultType;
                 }
                 else if (expectedUnion)
@@ -2236,9 +2285,9 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprTa
     return {checkExprTable(scope, expr, fieldTypes, expectedType)};
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprUnary& expr)
+WithPredicate<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprUnary& expr)
 {
-    ExprResult<TypeId> result = checkExpr(scope, *expr.expr);
+    WithPredicate<TypeId> result = checkExpr(scope, *expr.expr);
     TypeId operandType = follow(result.type);
 
     switch (expr.op)
@@ -2466,8 +2515,6 @@ TypeId TypeChecker::checkRelationalOperation(
         std::optional<TypeId> leftMetatable = isString(lhsType) ? std::nullopt : getMetatable(follow(lhsType));
         std::optional<TypeId> rightMetatable = isString(rhsType) ? std::nullopt : getMetatable(follow(rhsType));
 
-        if (FFlag::LuauSuccessTypingForEqualityOperations)
-        {
         if (leftMetatable != rightMetatable)
         {
             bool matches = false;
@@ -2510,17 +2557,7 @@ TypeId TypeChecker::checkRelationalOperation(
                 return errorRecoveryType(booleanType);
             }
         }
-        }
+
-        else
-        {
-            if (bool(leftMetatable) != bool(rightMetatable) && leftMetatable != rightMetatable)
-            {
-                reportError(
-                    expr.location, GenericError{format("Types %s and %s cannot be compared with %s because they do not have the same metatable",
-                                       toString(lhsType).c_str(), toString(rhsType).c_str(), toString(expr.op).c_str())});
-                return errorRecoveryType(booleanType);
-            }
-        }
 
         if (leftMetatable)
         {
@@ -2532,7 +2569,7 @@ TypeId TypeChecker::checkRelationalOperation(
                     if (isEquality)
                     {
                         Unifier state = mkUnifier(expr.location);
-                        state.tryUnify(addTypePack({booleanType}), ftv->retType);
+                        state.tryUnify(addTypePack({booleanType}), ftv->retTypes);
 
                         if (!state.errors.empty())
                         {
@@ -2721,7 +2758,7 @@ TypeId TypeChecker::checkBinaryOperation(
     }
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprBinary& expr)
+WithPredicate<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprBinary& expr)
 {
     if (expr.op == AstExprBinary::And)
     {
@@ -2752,8 +2789,8 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprBi
         if (auto predicate = tryGetTypeGuardPredicate(expr))
             return {booleanType, {std::move(*predicate)}};
 
-        ExprResult<TypeId> lhs = checkExpr(scope, *expr.left, std::nullopt, /*forceSingleton=*/true);
+        WithPredicate<TypeId> lhs = checkExpr(scope, *expr.left, std::nullopt, /*forceSingleton=*/true);
-        ExprResult<TypeId> rhs = checkExpr(scope, *expr.right, std::nullopt, /*forceSingleton=*/true);
+        WithPredicate<TypeId> rhs = checkExpr(scope, *expr.right, std::nullopt, /*forceSingleton=*/true);
 
         PredicateVec predicates;
 
@@ -2770,18 +2807,18 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprBi
     }
     else
     {
-        ExprResult<TypeId> lhs = checkExpr(scope, *expr.left);
+        WithPredicate<TypeId> lhs = checkExpr(scope, *expr.left);
-        ExprResult<TypeId> rhs = checkExpr(scope, *expr.right);
+        WithPredicate<TypeId> rhs = checkExpr(scope, *expr.right);
 
         // Intentionally discarding predicates with other operators.
         return {checkBinaryOperation(scope, expr, lhs.type, rhs.type, lhs.predicates)};
     }
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprTypeAssertion& expr)
+WithPredicate<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprTypeAssertion& expr)
 {
     TypeId annotationType = resolveType(scope, *expr.annotation);
-    ExprResult<TypeId> result = checkExpr(scope, *expr.expr, annotationType);
+    WithPredicate<TypeId> result = checkExpr(scope, *expr.expr, annotationType);
 
     // Note: As an optimization, we try 'number <: number | string' first, as that is the more likely case.
     if (canUnify(annotationType, result.type, expr.location).empty())
@@ -2794,7 +2831,7 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprTy
     return {errorRecoveryType(annotationType), std::move(result.predicates)};
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprError& expr)
+WithPredicate<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprError& expr)
 {
     const size_t oldSize = currentModule->errors.size();
 
@@ -2808,17 +2845,17 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprEr
     return {errorRecoveryType(scope)};
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprIfElse& expr, std::optional<TypeId> expectedType)
+WithPredicate<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprIfElse& expr, std::optional<TypeId> expectedType)
 {
-    ExprResult<TypeId> result = checkExpr(scope, *expr.condition);
+    WithPredicate<TypeId> result = checkExpr(scope, *expr.condition);
 
     ScopePtr trueScope = childScope(scope, expr.trueExpr->location);
     resolve(result.predicates, trueScope, true);
-    ExprResult<TypeId> trueType = checkExpr(trueScope, *expr.trueExpr, expectedType);
+    WithPredicate<TypeId> trueType = checkExpr(trueScope, *expr.trueExpr, expectedType);
 
     ScopePtr falseScope = childScope(scope, expr.falseExpr->location);
     resolve(result.predicates, falseScope, false);
-    ExprResult<TypeId> falseType = checkExpr(falseScope, *expr.falseExpr, expectedType);
+    WithPredicate<TypeId> falseType = checkExpr(falseScope, *expr.falseExpr, expectedType);
 
     if (falseType.type == trueType.type)
         return {trueType.type};
@@ -3170,7 +3207,7 @@ std::pair<TypeId, ScopePtr> TypeChecker::checkFunctionSignature(
         retPack = anyTypePack;
     else if (expectedFunctionType)
     {
-        auto [head, tail] = flatten(expectedFunctionType->retType);
+        auto [head, tail] = flatten(expectedFunctionType->retTypes);
 
         // Do not infer 'nil' as function return type
         if (!tail && head.size() == 1 && isNil(head[0]))
@@ -3354,7 +3391,7 @@ void TypeChecker::checkFunctionBody(const ScopePtr& scope, TypeId ty, const AstE
 
         if (useConstrainedIntersections())
         {
-            TypePackId retPack = follow(funTy->retType);
+            TypePackId retPack = follow(funTy->retTypes);
             // It is possible for a function to have no annotation and no return statement, and yet still have an ascribed return type
             // if it is expected to conform to some other interface. (eg the function may be a lambda passed as a callback)
             if (!hasReturn(function.body) && !function.returnAnnotation.has_value() && get<FreeTypePack>(retPack))
@@ -3367,20 +3404,20 @@ void TypeChecker::checkFunctionBody(const ScopePtr& scope, TypeId ty, const AstE
         else
         {
             // We explicitly don't follow here to check if we have a 'true' free type instead of bound one
-            if (get_if<FreeTypePack>(&funTy->retType->ty))
+            if (get_if<FreeTypePack>(&funTy->retTypes->ty))
-                *asMutable(funTy->retType) = TypePack{{}, std::nullopt};
+                *asMutable(funTy->retTypes) = TypePack{{}, std::nullopt};
         }
 
         bool reachesImplicitReturn = getFallthrough(function.body) != nullptr;
 
-        if (reachesImplicitReturn && !allowsNoReturnValues(follow(funTy->retType)))
+        if (reachesImplicitReturn && !allowsNoReturnValues(follow(funTy->retTypes)))
         {
             // If we're in nonstrict mode we want to only report this missing return
             // statement if there are type annotations on the function. In strict mode
             // we report it regardless.
             if (!isNonstrictMode() || function.returnAnnotation)
             {
-                reportError(getEndLocation(function), FunctionExitsWithoutReturning{funTy->retType});
+                reportError(getEndLocation(function), FunctionExitsWithoutReturning{funTy->retTypes});
             }
         }
     }
@@ -3388,7 +3425,7 @@ void TypeChecker::checkFunctionBody(const ScopePtr& scope, TypeId ty, const AstE
         ice("Checking non functional type");
 }
 
-ExprResult<TypePackId> TypeChecker::checkExprPack(const ScopePtr& scope, const AstExpr& expr)
+WithPredicate<TypePackId> TypeChecker::checkExprPack(const ScopePtr& scope, const AstExpr& expr)
 {
     if (auto a = expr.as<AstExprCall>())
         return checkExprPack(scope, *a);
@@ -3654,7 +3691,7 @@ void TypeChecker::checkArgumentList(
     }
 }
 
-ExprResult<TypePackId> TypeChecker::checkExprPack(const ScopePtr& scope, const AstExprCall& expr)
+WithPredicate<TypePackId> TypeChecker::checkExprPack(const ScopePtr& scope, const AstExprCall& expr)
 {
     // evaluate type of function
     // decompose an intersection into its component overloads
@@ -3722,7 +3759,7 @@ ExprResult<TypePackId> TypeChecker::checkExprPack(const ScopePtr& scope, const A
 
     std::vector<std::optional<TypeId>> expectedTypes = getExpectedTypesForCall(overloads, expr.args.size, expr.self);
 
-    ExprResult<TypePackId> argListResult = checkExprList(scope, expr.location, expr.args, false, {}, expectedTypes);
+    WithPredicate<TypePackId> argListResult = checkExprList(scope, expr.location, expr.args, false, {}, expectedTypes);
     TypePackId argPack = argListResult.type;
 
     if (get<Unifiable::Error>(argPack))
@@ -3766,7 +3803,7 @@ ExprResult<TypePackId> TypeChecker::checkExprPack(const ScopePtr& scope, const A
     if (!overload && !overloadsThatDont.empty())
         overload = get<FunctionTypeVar>(overloadsThatDont[0]);
     if (overload)
-        return {errorRecoveryTypePack(overload->retType)};
+        return {errorRecoveryTypePack(overload->retTypes)};
 
     return {errorRecoveryTypePack(retPack)};
 }
@@ -3775,7 +3812,7 @@ std::vector<std::optional<TypeId>> TypeChecker::getExpectedTypesForCall(const st
 {
     std::vector<std::optional<TypeId>> expectedTypes;
 
-    auto assignOption = [this, &expectedTypes](size_t index, std::optional<TypeId> ty) {
+    auto assignOption = [this, &expectedTypes](size_t index, TypeId ty) {
         if (index == expectedTypes.size())
         {
             expectedTypes.push_back(ty);
@@ -3790,7 +3827,7 @@ std::vector<std::optional<TypeId>> TypeChecker::getExpectedTypesForCall(const st
             }
             else
             {
-                std::vector<TypeId> result = reduceUnion({*el, *ty});
+                std::vector<TypeId> result = reduceUnion({*el, ty});
                 el = result.size() == 1 ? result[0] : addType(UnionTypeVar{std::move(result)});
             }
         }
@@ -3810,7 +3847,8 @@ std::vector<std::optional<TypeId>> TypeChecker::getExpectedTypesForCall(const st
 
             if (argsTail)
             {
-                if (const VariadicTypePack* vtp = get<VariadicTypePack>(follow(*argsTail)))
+                argsTail = follow(*argsTail);
+                if (const VariadicTypePack* vtp = get<VariadicTypePack>(*argsTail))
                 {
                     while (index < argumentCount)
                         assignOption(index++, vtp->ty);
@@ -3819,11 +3857,14 @@ std::vector<std::optional<TypeId>> TypeChecker::getExpectedTypesForCall(const st
         }
     }
 
+    Demoter demoter{&currentModule->internalTypes};
+    demoter.demote(expectedTypes);
+
     return expectedTypes;
 }
 
-std::optional<ExprResult<TypePackId>> TypeChecker::checkCallOverload(const ScopePtr& scope, const AstExprCall& expr, TypeId fn, TypePackId retPack,
+std::optional<WithPredicate<TypePackId>> TypeChecker::checkCallOverload(const ScopePtr& scope, const AstExprCall& expr, TypeId fn, TypePackId retPack,
-    TypePackId argPack, TypePack* args, const std::vector<Location>* argLocations, const ExprResult<TypePackId>& argListResult,
+    TypePackId argPack, TypePack* args, const std::vector<Location>* argLocations, const WithPredicate<TypePackId>& argListResult,
     std::vector<TypeId>& overloadsThatMatchArgCount, std::vector<TypeId>& overloadsThatDont, std::vector<OverloadErrorEntry>& errors)
 {
     LUAU_ASSERT(argLocations);
@@ -3918,14 +3959,14 @@ std::optional<ExprResult<TypePackId>> TypeChecker::checkCallOverload(const Scope
     if (ftv->magicFunction)
     {
         // TODO: We're passing in the wrong TypePackId. Should be argPack, but a unit test fails otherwise. CLI-40458
-        if (std::optional<ExprResult<TypePackId>> ret = ftv->magicFunction(*this, scope, expr, argListResult))
+        if (std::optional<WithPredicate<TypePackId>> ret = ftv->magicFunction(*this, scope, expr, argListResult))
             return *ret;
     }
 
     Unifier state = mkUnifier(expr.location);
 
     // Unify return types
-    checkArgumentList(scope, state, retPack, ftv->retType, /*argLocations*/ {});
+    checkArgumentList(scope, state, retPack, ftv->retTypes, /*argLocations*/ {});
     if (!state.errors.empty())
     {
         return {};
@@ -3996,7 +4037,7 @@ bool TypeChecker::handleSelfCallMismatch(const ScopePtr& scope, const AstExprCal
                 // we eagerly assume that that's what you actually meant and we commit to it.
                 // This could be incorrect if the function has an additional overload that
                 // actually works.
-                // checkArgumentList(scope, editedState, retPack, ftv->retType, retLocations, CountMismatch::Return);
+                // checkArgumentList(scope, editedState, retPack, ftv->retTypes, retLocations, CountMismatch::Return);
                 return true;
             }
         }
@@ -4027,7 +4068,7 @@ bool TypeChecker::handleSelfCallMismatch(const ScopePtr& scope, const AstExprCal
                     // we eagerly assume that that's what you actually meant and we commit to it.
                     // This could be incorrect if the function has an additional overload that
                     // actually works.
-                    // checkArgumentList(scope, editedState, retPack, ftv->retType, retLocations, CountMismatch::Return);
+                    // checkArgumentList(scope, editedState, retPack, ftv->retTypes, retLocations, CountMismatch::Return);
                     return true;
                 }
             }
@@ -4085,7 +4126,7 @@ void TypeChecker::reportOverloadResolutionError(const ScopePtr& scope, const Ast
         // Unify return types
         if (const FunctionTypeVar* ftv = get<FunctionTypeVar>(overload))
         {
-            checkArgumentList(scope, state, retPack, ftv->retType, {});
+            checkArgumentList(scope, state, retPack, ftv->retTypes, {});
             checkArgumentList(scope, state, argPack, ftv->argTypes, argLocations);
         }
 
@@ -4110,7 +4151,7 @@ void TypeChecker::reportOverloadResolutionError(const ScopePtr& scope, const Ast
     return;
 }
 
-ExprResult<TypePackId> TypeChecker::checkExprList(const ScopePtr& scope, const Location& location, const AstArray<AstExpr*>& exprs,
+WithPredicate<TypePackId> TypeChecker::checkExprList(const ScopePtr& scope, const Location& location, const AstArray<AstExpr*>& exprs,
     bool substituteFreeForNil, const std::vector<bool>& instantiateGenerics, const std::vector<std::optional<TypeId>>& expectedTypes)
 {
     TypePackId pack = addTypePack(TypePack{});
@@ -4400,12 +4441,26 @@ TypeId Anyification::clean(TypeId ty)
         return res;
     }
     else if (auto ctv = get<ConstrainedTypeVar>(ty))
+    {
+        if (FFlag::LuauQuantifyConstrained)
+        {
+            std::vector<TypeId> copy = ctv->parts;
+            for (TypeId& ty : copy)
+                ty = replace(ty);
+            TypeId res = copy.size() == 1 ? copy[0] : addType(UnionTypeVar{std::move(copy)});
+            auto [t, ok] = normalize(res, *arena, *iceHandler);
+            if (!ok)
+                normalizationTooComplex = true;
+            return t;
+        }
+        else
         {
             auto [t, ok] = normalize(ty, *arena, *iceHandler);
             if (!ok)
                 normalizationTooComplex = true;
             return t;
         }
+    }
     else
         return anyType;
 }

Analysis/src/TypeUtils.cpp

@@ -66,7 +66,7 @@ std::optional<TypeId> findTablePropertyRespectingMeta(ErrorVec& errors, TypeId t
         }
         else if (const auto& itf = get<FunctionTypeVar>(index))
         {
-            std::optional<TypeId> r = first(follow(itf->retType));
+            std::optional<TypeId> r = first(follow(itf->retTypes));
             if (!r)
                 return getSingletonTypes().nilType;
             else

Analysis/src/TypeVar.cpp

@@ -29,8 +29,8 @@ LUAU_FASTFLAG(LuauNonCopyableTypeVarFields)
 namespace Luau
 {
 
-std::optional<ExprResult<TypePackId>> magicFunctionFormat(
+std::optional<WithPredicate<TypePackId>> magicFunctionFormat(
-    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult);
+    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, WithPredicate<TypePackId> withPredicate);
 
 TypeId follow(TypeId t)
 {
@@ -408,41 +408,48 @@ bool hasLength(TypeId ty, DenseHashSet<TypeId>& seen, int* recursionCount)
     return false;
 }
 
-FunctionTypeVar::FunctionTypeVar(TypePackId argTypes, TypePackId retType, std::optional<FunctionDefinition> defn, bool hasSelf)
+BlockedTypeVar::BlockedTypeVar()
+    : index(++nextIndex)
+{
+}
+
+int BlockedTypeVar::nextIndex = 0;
+
+FunctionTypeVar::FunctionTypeVar(TypePackId argTypes, TypePackId retTypes, std::optional<FunctionDefinition> defn, bool hasSelf)
     : argTypes(argTypes)
-    , retType(retType)
+    , retTypes(retTypes)
     , definition(std::move(defn))
     , hasSelf(hasSelf)
 {
 }
 
-FunctionTypeVar::FunctionTypeVar(TypeLevel level, TypePackId argTypes, TypePackId retType, std::optional<FunctionDefinition> defn, bool hasSelf)
+FunctionTypeVar::FunctionTypeVar(TypeLevel level, TypePackId argTypes, TypePackId retTypes, std::optional<FunctionDefinition> defn, bool hasSelf)
     : level(level)
     , argTypes(argTypes)
-    , retType(retType)
+    , retTypes(retTypes)
     , definition(std::move(defn))
     , hasSelf(hasSelf)
 {
 }
 
-FunctionTypeVar::FunctionTypeVar(std::vector<TypeId> generics, std::vector<TypePackId> genericPacks, TypePackId argTypes, TypePackId retType,
+FunctionTypeVar::FunctionTypeVar(std::vector<TypeId> generics, std::vector<TypePackId> genericPacks, TypePackId argTypes, TypePackId retTypes,
     std::optional<FunctionDefinition> defn, bool hasSelf)
     : generics(generics)
     , genericPacks(genericPacks)
     , argTypes(argTypes)
-    , retType(retType)
+    , retTypes(retTypes)
     , definition(std::move(defn))
     , hasSelf(hasSelf)
 {
 }
 
 FunctionTypeVar::FunctionTypeVar(TypeLevel level, std::vector<TypeId> generics, std::vector<TypePackId> genericPacks, TypePackId argTypes,
-    TypePackId retType, std::optional<FunctionDefinition> defn, bool hasSelf)
+    TypePackId retTypes, std::optional<FunctionDefinition> defn, bool hasSelf)
     : level(level)
     , generics(generics)
     , genericPacks(genericPacks)
     , argTypes(argTypes)
-    , retType(retType)
+    , retTypes(retTypes)
     , definition(std::move(defn))
     , hasSelf(hasSelf)
 {
@@ -488,7 +495,7 @@ bool areEqual(SeenSet& seen, const FunctionTypeVar& lhs, const FunctionTypeVar&
     if (!areEqual(seen, *lhs.argTypes, *rhs.argTypes))
         return false;
 
-    if (!areEqual(seen, *lhs.retType, *rhs.retType))
+    if (!areEqual(seen, *lhs.retTypes, *rhs.retTypes))
         return false;
 
     return true;
@@ -678,7 +685,6 @@ static TypeVar trueType_{SingletonTypeVar{BooleanSingleton{true}}, /*persistent*
 static TypeVar falseType_{SingletonTypeVar{BooleanSingleton{false}}, /*persistent*/ true};
 static TypeVar anyType_{AnyTypeVar{}, /*persistent*/ true};
 static TypeVar errorType_{ErrorTypeVar{}, /*persistent*/ true};
-static TypeVar optionalNumberType_{UnionTypeVar{{&numberType_, &nilType_}}, /*persistent*/ true};
 
 static TypePackVar anyTypePack_{VariadicTypePack{&anyType_}, true};
 static TypePackVar errorTypePack_{Unifiable::Error{}};
@@ -692,7 +698,6 @@ SingletonTypes::SingletonTypes()
     , trueType(&trueType_)
     , falseType(&falseType_)
     , anyType(&anyType_)
-    , optionalNumberType(&optionalNumberType_)
     , anyTypePack(&anyTypePack_)
     , arena(new TypeArena)
 {
@@ -825,7 +830,7 @@ void persist(TypeId ty)
         else if (auto ftv = get<FunctionTypeVar>(t))
         {
             persist(ftv->argTypes);
-            persist(ftv->retType);
+            persist(ftv->retTypes);
         }
         else if (auto ttv = get<TableTypeVar>(t))
         {
@@ -1100,10 +1105,10 @@ static std::vector<TypeId> parseFormatString(TypeChecker& typechecker, const cha
     return result;
 }
 
-std::optional<ExprResult<TypePackId>> magicFunctionFormat(
+std::optional<WithPredicate<TypePackId>> magicFunctionFormat(
-    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult)
+    TypeChecker& typechecker, const ScopePtr& scope, const AstExprCall& expr, WithPredicate<TypePackId> withPredicate)
 {
-    auto [paramPack, _predicates] = exprResult;
+    auto [paramPack, _predicates] = withPredicate;
 
     TypeArena& arena = typechecker.currentModule->internalTypes;
 
@@ -1142,7 +1147,7 @@ std::optional<ExprResult<TypePackId>> magicFunctionFormat(
     if (expected.size() != actualParamSize && (!tail || expected.size() < actualParamSize))
         typechecker.reportError(TypeError{expr.location, CountMismatch{expected.size(), actualParamSize}});
 
-    return ExprResult<TypePackId>{arena.addTypePack({typechecker.stringType})};
+    return WithPredicate<TypePackId>{arena.addTypePack({typechecker.stringType})};
 }
 
 std::vector<TypeId> filterMap(TypeId type, TypeIdPredicate predicate)

Analysis/src/Unifier.cpp

@@ -22,6 +22,7 @@ LUAU_FASTFLAG(LuauLowerBoundsCalculation);
 LUAU_FASTFLAG(LuauErrorRecoveryType);
 LUAU_FASTFLAGVARIABLE(LuauSubtypingAddOptPropsToUnsealedTables, false)
 LUAU_FASTFLAGVARIABLE(LuauTxnLogRefreshFunctionPointers, false)
+LUAU_FASTFLAG(LuauQuantifyConstrained)
 
 namespace Luau
 {
@@ -1288,13 +1289,13 @@ void Unifier::tryUnifyFunctions(TypeId subTy, TypeId superTy, bool isFunctionCal
             reportError(TypeError{location, TypeMismatch{superTy, subTy, "", innerState.errors.front()}});
 
         innerState.ctx = CountMismatch::Result;
-        innerState.tryUnify_(subFunction->retType, superFunction->retType);
+        innerState.tryUnify_(subFunction->retTypes, superFunction->retTypes);
 
         if (!reported)
         {
             if (auto e = hasUnificationTooComplex(innerState.errors))
                 reportError(*e);
-            else if (!innerState.errors.empty() && size(superFunction->retType) == 1 && finite(superFunction->retType))
+            else if (!innerState.errors.empty() && size(superFunction->retTypes) == 1 && finite(superFunction->retTypes))
                 reportError(TypeError{location, TypeMismatch{superTy, subTy, "Return type is not compatible.", innerState.errors.front()}});
             else if (!innerState.errors.empty() && innerState.firstPackErrorPos)
                 reportError(
@@ -1312,7 +1313,7 @@ void Unifier::tryUnifyFunctions(TypeId subTy, TypeId superTy, bool isFunctionCal
         tryUnify_(superFunction->argTypes, subFunction->argTypes, isFunctionCall);
 
         ctx = CountMismatch::Result;
-        tryUnify_(subFunction->retType, superFunction->retType);
+        tryUnify_(subFunction->retTypes, superFunction->retTypes);
     }
 
     if (FFlag::LuauTxnLogRefreshFunctionPointers)
@@ -2177,7 +2178,7 @@ static void tryUnifyWithAny(std::vector<TypeId>& queue, Unifier& state, DenseHas
         else if (auto fun = state.log.getMutable<FunctionTypeVar>(ty))
         {
             queueTypePack(queue, seenTypePacks, state, fun->argTypes, anyTypePack);
-            queueTypePack(queue, seenTypePacks, state, fun->retType, anyTypePack);
+            queueTypePack(queue, seenTypePacks, state, fun->retTypes, anyTypePack);
         }
         else if (auto table = state.log.getMutable<TableTypeVar>(ty))
         {
@@ -2322,7 +2323,7 @@ void Unifier::tryUnifyWithConstrainedSuperTypeVar(TypeId subTy, TypeId superTy)
     superC->parts.push_back(subTy);
 }
 
-void Unifier::unifyLowerBound(TypePackId subTy, TypePackId superTy)
+void Unifier::unifyLowerBound(TypePackId subTy, TypePackId superTy, TypeLevel demotedLevel)
 {
     // The duplication between this and regular typepack unification is tragic.
 
@@ -2357,7 +2358,7 @@ void Unifier::unifyLowerBound(TypePackId subTy, TypePackId superTy)
             if (!freeTailPack)
                 return;
 
-            TypeLevel level = freeTailPack->level;
+            TypeLevel level = FFlag::LuauQuantifyConstrained ? demotedLevel : freeTailPack->level;
 
             TypePack* tp = getMutable<TypePack>(log.replace(tailPack, TypePack{}));
 

Compiler/src/BytecodeBuilder.cpp

@@ -1075,6 +1075,8 @@ void BytecodeBuilder::validate() const
         LUAU_ASSERT(i <= insns.size());
     }
 
+    std::vector<uint8_t> openCaptures;
+
     // second pass: validate the rest of the bytecode
     for (size_t i = 0; i < insns.size();)
     {
@@ -1121,6 +1123,8 @@ void BytecodeBuilder::validate() const
 
         case LOP_CLOSEUPVALS:
             VREG(LUAU_INSN_A(insn));
+            while (openCaptures.size() && openCaptures.back() >= LUAU_INSN_A(insn))
+                openCaptures.pop_back();
             break;
 
         case LOP_GETIMPORT:
@@ -1388,8 +1392,12 @@ void BytecodeBuilder::validate() const
             switch (LUAU_INSN_A(insn))
             {
             case LCT_VAL:
+                VREG(LUAU_INSN_B(insn));
+                break;
+
             case LCT_REF:
                 VREG(LUAU_INSN_B(insn));
+                openCaptures.push_back(LUAU_INSN_B(insn));
                 break;
 
             case LCT_UPVAL:
@@ -1409,6 +1417,12 @@ void BytecodeBuilder::validate() const
         LUAU_ASSERT(i <= insns.size());
     }
 
+    // all CAPTURE REF instructions must have a CLOSEUPVALS instruction after them in the bytecode stream
+    // this doesn't guarantee safety as it doesn't perform basic block based analysis, but if this fails
+    // then the bytecode is definitely unsafe to run since the compiler won't generate backwards branches
+    // except for loop edges
+    LUAU_ASSERT(openCaptures.empty());
+
 #undef VREG
 #undef VREGEND
 #undef VUPVAL

Compiler/src/Compiler.cpp

@@ -246,6 +246,14 @@ struct Compiler
             f.canInline = true;
             f.stackSize = stackSize;
             f.costModel = modelCost(func->body, func->args.data, func->args.size);
+
+            // track functions that only ever return a single value so that we can convert multret calls to fixedret calls
+            if (allPathsEndWithReturn(func->body))
+            {
+                ReturnVisitor returnVisitor(this);
+                stat->visit(&returnVisitor);
+                f.returnsOne = returnVisitor.returnsOne;
+            }
         }
 
         upvals.clear(); // note: instead of std::move above, we copy & clear to preserve capacity for future pushes
@@ -260,6 +268,19 @@ struct Compiler
     {
         if (AstExprCall* expr = node->as<AstExprCall>())
         {
+            // Optimization: convert multret calls to functions that always return one value to fixedret calls; this facilitates inlining
+            if (options.optimizationLevel >= 2)
+            {
+                AstExprFunction* func = getFunctionExpr(expr->func);
+                Function* fi = func ? functions.find(func) : nullptr;
+
+                if (fi && fi->returnsOne)
+                {
+                    compileExprTemp(node, target);
+                    return false;
+                }
+            }
+
             // We temporarily swap out regTop to have targetTop work correctly...
             // This is a crude hack but it's necessary for correctness :(
             RegScope rs(this, target);
@@ -447,7 +468,9 @@ struct Compiler
                 return false;
             }
 
-        // TODO: we can compile multret functions if all returns of the function are multret as well
+        // we can't inline multret functions because the caller expects L->top to be adjusted:
+        // - inlined return compiles to a JUMP, and we don't have an instruction that adjusts L->top arbitrarily
+        // - even if we did, right now all L->top adjustments are immediately consumed by the next instruction, and for now we want to preserve that
         if (multRet)
         {
             bytecode.addDebugRemark("inlining failed: can't convert fixed returns to multret");
@@ -492,7 +515,7 @@ struct Compiler
         size_t oldLocals = localStack.size();
 
         // note that we push the frame early; this is needed to block recursive inline attempts
-        inlineFrames.push_back({func, target, targetCount});
+        inlineFrames.push_back({func, oldLocals, target, targetCount});
 
         // evaluate all arguments; note that we don't emit code for constant arguments (relying on constant folding)
         for (size_t i = 0; i < func->args.size; ++i)
@@ -593,6 +616,8 @@ struct Compiler
         {
             for (size_t i = 0; i < targetCount; ++i)
                 bytecode.emitABC(LOP_LOADNIL, uint8_t(target + i), 0, 0);
+
+            closeLocals(oldLocals);
         }
 
         popLocals(oldLocals);
@@ -2355,6 +2380,8 @@ struct Compiler
 
         compileExprListTemp(stat->list, frame.target, frame.targetCount, /* targetTop= */ false);
 
+        closeLocals(frame.localOffset);
+
         if (!fallthrough)
         {
             size_t jumpLabel = bytecode.emitLabel();
@@ -3316,6 +3343,48 @@ struct Compiler
         std::vector<AstLocal*> upvals;
     };
 
+    struct ReturnVisitor: AstVisitor
+    {
+        Compiler* self;
+        bool returnsOne = true;
+
+        ReturnVisitor(Compiler* self)
+            : self(self)
+        {
+        }
+
+        bool visit(AstExpr* expr) override
+        {
+            return false;
+        }
+
+        bool visit(AstStatReturn* stat) override
+        {
+            if (stat->list.size == 1)
+            {
+                AstExpr* value = stat->list.data[0];
+
+                if (AstExprCall* expr = value->as<AstExprCall>())
+                {
+                    AstExprFunction* func = self->getFunctionExpr(expr->func);
+                    Function* fi = func ? self->functions.find(func) : nullptr;
+
+                    returnsOne &= fi && fi->returnsOne;
+                }
+                else if (value->is<AstExprVarargs>())
+                {
+                    returnsOne = false;
+                }
+            }
+            else
+            {
+                returnsOne = false;
+            }
+
+            return false;
+        }
+    };
+
     struct RegScope
     {
         RegScope(Compiler* self)
@@ -3351,6 +3420,7 @@ struct Compiler
         uint64_t costModel = 0;
         unsigned int stackSize = 0;
         bool canInline = false;
+        bool returnsOne = false;
     };
 
     struct Local
@@ -3384,6 +3454,8 @@ struct Compiler
     {
         AstExprFunction* func;
 
+        size_t localOffset;
+
         uint8_t target;
         uint8_t targetCount;
 

Sources.cmake

@@ -65,12 +65,13 @@ target_sources(Luau.CodeGen PRIVATE
 target_sources(Luau.Analysis PRIVATE
     Analysis/include/Luau/AstQuery.h
     Analysis/include/Luau/Autocomplete.h
-    Analysis/include/Luau/NotNull.h
     Analysis/include/Luau/BuiltinDefinitions.h
     Analysis/include/Luau/Clone.h
     Analysis/include/Luau/Config.h
+    Analysis/include/Luau/Constraint.h
     Analysis/include/Luau/ConstraintGraphBuilder.h
     Analysis/include/Luau/ConstraintSolver.h
+    Analysis/include/Luau/ConstraintSolverLogger.h
     Analysis/include/Luau/Documentation.h
     Analysis/include/Luau/Error.h
     Analysis/include/Luau/FileResolver.h
@@ -97,6 +98,7 @@ target_sources(Luau.Analysis PRIVATE
     Analysis/include/Luau/TxnLog.h
     Analysis/include/Luau/TypeArena.h
     Analysis/include/Luau/TypeAttach.h
+    Analysis/include/Luau/TypeChecker2.h
     Analysis/include/Luau/TypedAllocator.h
     Analysis/include/Luau/TypeInfer.h
     Analysis/include/Luau/TypePack.h
@@ -113,8 +115,10 @@ target_sources(Luau.Analysis PRIVATE
     Analysis/src/BuiltinDefinitions.cpp
     Analysis/src/Clone.cpp
     Analysis/src/Config.cpp
+    Analysis/src/Constraint.cpp
     Analysis/src/ConstraintGraphBuilder.cpp
     Analysis/src/ConstraintSolver.cpp
+    Analysis/src/ConstraintSolverLogger.cpp
     Analysis/src/Error.cpp
     Analysis/src/Frontend.cpp
     Analysis/src/Instantiation.cpp
@@ -136,6 +140,7 @@ target_sources(Luau.Analysis PRIVATE
     Analysis/src/TxnLog.cpp
     Analysis/src/TypeArena.cpp
     Analysis/src/TypeAttach.cpp
+    Analysis/src/TypeChecker2.cpp
     Analysis/src/TypedAllocator.cpp
     Analysis/src/TypeInfer.cpp
     Analysis/src/TypePack.cpp
@@ -245,7 +250,6 @@ if(TARGET Luau.UnitTest)
         tests/AstQuery.test.cpp
         tests/AstVisitor.test.cpp
         tests/Autocomplete.test.cpp
-        tests/NotNull.test.cpp
         tests/BuiltinDefinitions.test.cpp
         tests/Compiler.test.cpp
         tests/Config.test.cpp

VM/src/lobject.h

@@ -418,7 +418,7 @@ typedef struct Table
     CommonHeader;
 
 
-    uint8_t flags;      /* 1<<p means tagmethod(p) is not present */
+    uint8_t tmcache;    /* 1<<p means tagmethod(p) is not present */
     uint8_t readonly;   /* sandboxing feature to prohibit writes to table */
     uint8_t safeenv;    /* environment doesn't share globals with other scripts */
     uint8_t lsizenode;  /* log2 of size of `node' array */

VM/src/ltable.cpp

@@ -45,7 +45,7 @@ static_assert(TKey{{NULL}, {0}, LUA_TNIL, MAXSIZE - 1}.next == MAXSIZE - 1, "not
 static_assert(TKey{{NULL}, {0}, LUA_TNIL, -(MAXSIZE - 1)}.next == -(MAXSIZE - 1), "not enough bits for next");
 
 // reset cache of absent metamethods, cache is updated in luaT_gettm
-#define invalidateTMcache(t) t->flags = 0
+#define invalidateTMcache(t) t->tmcache = 0
 
 // empty hash data points to dummynode so that we can always dereference it
 const LuaNode luaH_dummynode = {
@@ -479,7 +479,7 @@ Table* luaH_new(lua_State* L, int narray, int nhash)
     Table* t = luaM_newgco(L, Table, sizeof(Table), L->activememcat);
     luaC_init(L, t, LUA_TTABLE);
     t->metatable = NULL;
-    t->flags = cast_byte(~0);
+    t->tmcache = cast_byte(~0);
     t->array = NULL;
     t->sizearray = 0;
     t->lastfree = 0;
@@ -778,7 +778,7 @@ Table* luaH_clone(lua_State* L, Table* tt)
     Table* t = luaM_newgco(L, Table, sizeof(Table), L->activememcat);
     luaC_init(L, t, LUA_TTABLE);
     t->metatable = tt->metatable;
-    t->flags = tt->flags;
+    t->tmcache = tt->tmcache;
     t->array = NULL;
     t->sizearray = 0;
     t->lsizenode = 0;
@@ -835,5 +835,5 @@ void luaH_clear(Table* tt)
     }
 
     /* back to empty -> no tag methods present */
-    tt->flags = cast_byte(~0);
+    tt->tmcache = cast_byte(~0);
 }

VM/src/ltm.cpp

@@ -89,7 +89,7 @@ const TValue* luaT_gettm(Table* events, TMS event, TString* ename)
     LUAU_ASSERT(event <= TM_EQ);
     if (ttisnil(tm))
     {                                              /* no tag method? */
-        events->flags |= cast_byte(1u << event); /* cache this fact */
+        events->tmcache |= cast_byte(1u << event); /* cache this fact */
         return NULL;
     }
     else

VM/src/ltm.h

@@ -41,10 +41,10 @@ typedef enum
 } TMS;
 // clang-format on
 
-#define gfasttm(g, et, e) ((et) == NULL ? NULL : ((et)->flags & (1u << (e))) ? NULL : luaT_gettm(et, e, (g)->tmname[e]))
+#define gfasttm(g, et, e) ((et) == NULL ? NULL : ((et)->tmcache & (1u << (e))) ? NULL : luaT_gettm(et, e, (g)->tmname[e]))
 
 #define fasttm(l, et, e) gfasttm(l->global, et, e)
-#define fastnotm(et, e) ((et) == NULL || ((et)->flags & (1u << (e))))
+#define fastnotm(et, e) ((et) == NULL || ((et)->tmcache & (1u << (e))))
 
 LUAI_DATA const char* const luaT_typenames[];
 LUAI_DATA const char* const luaT_eventname[];

tests/Autocomplete.test.cpp

@@ -1992,6 +1992,7 @@ local fp: @1= f
 
     auto ac = autocomplete('1');
 
+    REQUIRE_EQ("({| x: number, y: number |}) -> number", toString(requireType("f")));
     CHECK(ac.entryMap.count("({ x: number, y: number }) -> number"));
 }
 
@@ -2620,7 +2621,6 @@ a = if temp then even elseif true then temp else e@9
 
 TEST_CASE_FIXTURE(ACFixture, "autocomplete_if_else_regression")
 {
-    ScopedFastFlag FFlagLuauIfElseExprFixCompletionIssue("LuauIfElseExprFixCompletionIssue", true);
     check(R"(
 local abcdef = 0;
 local temp = false

tests/Compiler.test.cpp

@@ -4992,6 +4992,147 @@ RETURN R1 1
 )");
 }
 
+TEST_CASE("InlineCapture")
+{
+    // if the argument is captured by a nested closure, normally we can rely on capture by value
+    CHECK_EQ("\n" + compileFunction(R"(
+local function foo(a)
+    return function() return a end
+end
+
+local x = ...
+local y = foo(x)
+return y
+)",
+                        2, 2),
+        R"(
+DUPCLOSURE R0 K0
+GETVARARGS R1 1
+NEWCLOSURE R2 P1
+CAPTURE VAL R1
+RETURN R2 1
+)");
+
+    // if the argument is a constant, we move it to a register so that capture by value can happen
+    CHECK_EQ("\n" + compileFunction(R"(
+local function foo(a)
+    return function() return a end
+end
+
+local y = foo(42)
+return y
+)",
+                        2, 2),
+        R"(
+DUPCLOSURE R0 K0
+LOADN R2 42
+NEWCLOSURE R1 P1
+CAPTURE VAL R2
+RETURN R1 1
+)");
+
+    // if the argument is an externally mutated variable, we copy it to an argument and capture it by value
+    CHECK_EQ("\n" + compileFunction(R"(
+local function foo(a)
+    return function() return a end
+end
+
+local x x = 42
+local y = foo(x)
+return y
+)",
+                        2, 2),
+        R"(
+DUPCLOSURE R0 K0
+LOADNIL R1
+LOADN R1 42
+MOVE R3 R1
+NEWCLOSURE R2 P1
+CAPTURE VAL R3
+RETURN R2 1
+)");
+
+    // finally, if the argument is mutated internally, we must capture it by reference and close the upvalue
+    CHECK_EQ("\n" + compileFunction(R"(
+local function foo(a)
+    a = a or 42
+    return function() return a end
+end
+
+local y = foo()
+return y
+)",
+                        2, 2),
+        R"(
+DUPCLOSURE R0 K0
+LOADNIL R2
+ORK R2 R2 K1
+NEWCLOSURE R1 P1
+CAPTURE REF R2
+CLOSEUPVALS R2
+RETURN R1 1
+)");
+
+    // note that capture might need to be performed during the fallthrough block
+    CHECK_EQ("\n" + compileFunction(R"(
+local function foo(a)
+    a = a or 42
+    print(function() return a end)
+end
+
+local x = ...
+local y = foo(x)
+return y
+)",
+                        2, 2),
+        R"(
+DUPCLOSURE R0 K0
+GETVARARGS R1 1
+MOVE R3 R1
+ORK R3 R3 K1
+GETIMPORT R4 3
+NEWCLOSURE R5 P1
+CAPTURE REF R3
+CALL R4 1 0
+LOADNIL R2
+CLOSEUPVALS R3
+RETURN R2 1
+)");
+
+    // note that mutation and capture might be inside internal control flow
+    // TODO: this has an oddly redundant CLOSEUPVALS after JUMP; it's not due to inlining, and is an artifact of how StatBlock/StatReturn interact
+    // fixing this would reduce the number of redundant CLOSEUPVALS a bit but it only affects bytecode size as these instructions aren't executed
+    CHECK_EQ("\n" + compileFunction(R"(
+local function foo(a)
+    if not a then
+        local b b = 42
+        return function() return b end
+    end
+end
+
+local x = ...
+local y = foo(x)
+return y, x
+)",
+                        2, 2),
+        R"(
+DUPCLOSURE R0 K0
+GETVARARGS R1 1
+JUMPIF R1 L0
+LOADNIL R3
+LOADN R3 42
+NEWCLOSURE R2 P1
+CAPTURE REF R3
+CLOSEUPVALS R3
+JUMP L1
+CLOSEUPVALS R3
+L0: LOADNIL R2
+L1: MOVE R3 R2
+MOVE R4 R1
+RETURN R3 2
+)");
+}
+
 TEST_CASE("InlineFallthrough")
 {
     // if the function doesn't return, we still fill the results with nil
@@ -5044,27 +5185,6 @@ RETURN R1 -1
 )");
 }
 
-TEST_CASE("InlineCapture")
-{
-    // can't inline function with nested functions that capture locals because they might be constants
-    CHECK_EQ("\n" + compileFunction(R"(
-local function foo(a)
-    local function bar()
-        return a
-    end
-    return bar()
-end
-)",
-                        1, 2),
-        R"(
-NEWCLOSURE R1 P0
-CAPTURE VAL R0
-MOVE R2 R1
-CALL R2 0 -1
-RETURN R2 -1
-)");
-}
-
 TEST_CASE("InlineArgMismatch")
 {
     // when inlining a function, we must respect all the usual rules
@@ -5491,6 +5611,96 @@ RETURN R2 1
 )");
 }
 
+TEST_CASE("InlineMultret")
+{
+    // inlining a function in multret context is prohibited since we can't adjust L->top outside of CALL/GETVARARGS
+    CHECK_EQ("\n" + compileFunction(R"(
+local function foo(a)
+    return a()
+end
+
+return foo(42)
+)",
+                        1, 2),
+        R"(
+DUPCLOSURE R0 K0
+MOVE R1 R0
+LOADN R2 42
+CALL R1 1 -1
+RETURN R1 -1
+)");
+
+    // however, if we can deduce statically that a function always returns a single value, the inlining will work
+    CHECK_EQ("\n" + compileFunction(R"(
+local function foo(a)
+    return a
+end
+
+return foo(42)
+)",
+                        1, 2),
+        R"(
+DUPCLOSURE R0 K0
+LOADN R1 42
+RETURN R1 1
+)");
+
+    // this analysis will also propagate through other functions
+    CHECK_EQ("\n" + compileFunction(R"(
+local function foo(a)
+    return a
+end
+
+local function bar(a)
+    return foo(a)
+end
+
+return bar(42)
+)",
+                        2, 2),
+        R"(
+DUPCLOSURE R0 K0
+DUPCLOSURE R1 K1
+LOADN R2 42
+RETURN R2 1
+)");
+
+    // we currently don't do this analysis fully for recursive functions since they can't be inlined anyway
+    CHECK_EQ("\n" + compileFunction(R"(
+local function foo(a)
+    return foo(a)
+end
+
+return foo(42)
+)",
+                        1, 2),
+        R"(
+DUPCLOSURE R0 K0
+CAPTURE VAL R0
+MOVE R1 R0
+LOADN R2 42
+CALL R1 1 -1
+RETURN R1 -1
+)");
+
+    // and unfortunately we can't do this analysis for builtins or method calls due to getfenv
+    CHECK_EQ("\n" + compileFunction(R"(
+local function foo(a)
+    return math.abs(a)
+end
+
+return foo(42)
+)",
+                        1, 2),
+        R"(
+DUPCLOSURE R0 K0
+MOVE R1 R0
+LOADN R2 42
+CALL R1 1 -1
+RETURN R1 -1
+)");
+}
+
 TEST_CASE("ReturnConsecutive")
 {
     // we can return a single local directly

tests/ConstraintGraphBuilder.test.cpp

@@ -17,13 +17,13 @@ TEST_CASE_FIXTURE(ConstraintGraphBuilderFixture, "hello_world")
     )");
 
     cgb.visit(block);
-    std::vector<const Constraint*> constraints = collectConstraints(cgb.rootScope);
+    auto constraints = collectConstraints(cgb.rootScope);
 
     REQUIRE(2 == constraints.size());
 
     ToStringOptions opts;
-    CHECK("a <: string" == toString(*constraints[0], opts));
+    CHECK("string <: a" == toString(*constraints[0], opts));
-    CHECK("b <: a" == toString(*constraints[1], opts));
+    CHECK("a <: b" == toString(*constraints[1], opts));
 }
 
 TEST_CASE_FIXTURE(ConstraintGraphBuilderFixture, "primitives")
@@ -36,15 +36,34 @@ TEST_CASE_FIXTURE(ConstraintGraphBuilderFixture, "primitives")
     )");
 
     cgb.visit(block);
-    std::vector<const Constraint*> constraints = collectConstraints(cgb.rootScope);
+    auto constraints = collectConstraints(cgb.rootScope);
 
-    REQUIRE(4 == constraints.size());
+    REQUIRE(3 == constraints.size());
 
     ToStringOptions opts;
-    CHECK("a <: string" == toString(*constraints[0], opts));
+    CHECK("string <: a" == toString(*constraints[0], opts));
-    CHECK("b <: number" == toString(*constraints[1], opts));
+    CHECK("number <: b" == toString(*constraints[1], opts));
-    CHECK("c <: boolean" == toString(*constraints[2], opts));
+    CHECK("boolean <: c" == toString(*constraints[2], opts));
-    CHECK("d <: nil" == toString(*constraints[3], opts));
+}
+
+TEST_CASE_FIXTURE(ConstraintGraphBuilderFixture, "nil_primitive")
+{
+    AstStatBlock* block = parse(R"(
+        local function a() return nil end
+        local b = a()
+    )");
+
+    cgb.visit(block);
+    auto constraints = collectConstraints(cgb.rootScope);
+
+    ToStringOptions opts;
+    REQUIRE(5 <= constraints.size());
+
+    CHECK("*blocked-1* ~ gen () -> (a...)" == toString(*constraints[0], opts));
+    CHECK("b ~ inst *blocked-1*" == toString(*constraints[1], opts));
+    CHECK("() -> (c...) <: b" == toString(*constraints[2], opts));
+    CHECK("c... <: d" == toString(*constraints[3], opts));
+    CHECK("nil <: a..." == toString(*constraints[4], opts));
 }
 
 TEST_CASE_FIXTURE(ConstraintGraphBuilderFixture, "function_application")
@@ -55,15 +74,15 @@ TEST_CASE_FIXTURE(ConstraintGraphBuilderFixture, "function_application")
     )");
 
     cgb.visit(block);
-    std::vector<const Constraint*> constraints = collectConstraints(cgb.rootScope);
+    auto constraints = collectConstraints(cgb.rootScope);
 
     REQUIRE(4 == constraints.size());
 
     ToStringOptions opts;
-    CHECK("a <: string" == toString(*constraints[0], opts));
+    CHECK("string <: a" == toString(*constraints[0], opts));
     CHECK("b ~ inst a" == toString(*constraints[1], opts));
-    CHECK("(string) -> (c, d...) <: b" == toString(*constraints[2], opts));
+    CHECK("(string) -> (c...) <: b" == toString(*constraints[2], opts));
-    CHECK("e <: c" == toString(*constraints[3], opts));
+    CHECK("c... <: d" == toString(*constraints[3], opts));
 }
 
 TEST_CASE_FIXTURE(ConstraintGraphBuilderFixture, "local_function_definition")
@@ -75,13 +94,13 @@ TEST_CASE_FIXTURE(ConstraintGraphBuilderFixture, "local_function_definition")
     )");
 
     cgb.visit(block);
-    std::vector<const Constraint*> constraints = collectConstraints(cgb.rootScope);
+    auto constraints = collectConstraints(cgb.rootScope);
 
     REQUIRE(2 == constraints.size());
 
     ToStringOptions opts;
-    CHECK("a ~ gen (b) -> (c...)" == toString(*constraints[0], opts));
+    CHECK("*blocked-1* ~ gen (a) -> (b...)" == toString(*constraints[0], opts));
-    CHECK("b <: c..." == toString(*constraints[1], opts));
+    CHECK("a <: b..." == toString(*constraints[1], opts));
 }
 
 TEST_CASE_FIXTURE(ConstraintGraphBuilderFixture, "recursive_function")
@@ -93,15 +112,15 @@ TEST_CASE_FIXTURE(ConstraintGraphBuilderFixture, "recursive_function")
     )");
 
     cgb.visit(block);
-    std::vector<const Constraint*> constraints = collectConstraints(cgb.rootScope);
+    auto constraints = collectConstraints(cgb.rootScope);
 
     REQUIRE(4 == constraints.size());
 
     ToStringOptions opts;
-    CHECK("a ~ gen (b) -> (c...)" == toString(*constraints[0], opts));
+    CHECK("*blocked-1* ~ gen (a) -> (b...)" == toString(*constraints[0], opts));
-    CHECK("d ~ inst a" == toString(*constraints[1], opts));
+    CHECK("c ~ inst (a) -> (b...)" == toString(*constraints[1], opts));
-    CHECK("(b) -> (e, f...) <: d" == toString(*constraints[2], opts));
+    CHECK("(a) -> (d...) <: c" == toString(*constraints[2], opts));
-    CHECK("e <: c..." == toString(*constraints[3], opts));
+    CHECK("d... <: b..." == toString(*constraints[3], opts));
 }
 
 TEST_SUITE_END();

tests/Fixture.cpp

@@ -345,7 +345,7 @@ void Fixture::dumpErrors(std::ostream& os, const std::vector<TypeError>& errors)
         if (error.location.begin.line >= lines.size())
         {
             os << "\tSource not available?" << std::endl;
-            return;
+            continue;
         }
 
         std::string_view theLine = lines[error.location.begin.line];
@@ -430,6 +430,7 @@ ConstraintGraphBuilderFixture::ConstraintGraphBuilderFixture()
     : Fixture()
     , forceTheFlag{"DebugLuauDeferredConstraintResolution", true}
 {
+    BlockedTypeVar::nextIndex = 0;
 }
 
 ModuleName fromString(std::string_view name)

tests/Frontend.test.cpp

@@ -97,8 +97,8 @@ TEST_CASE_FIXTURE(FrontendFixture, "find_a_require")
     NaiveFileResolver naiveFileResolver;
 
     auto res = traceRequires(&naiveFileResolver, program, "");
-    CHECK_EQ(1, res.requires.size());
+    CHECK_EQ(1, res.requireList.size());
-    CHECK_EQ(res.requires[0].first, "Modules/Foo/Bar");
+    CHECK_EQ(res.requireList[0].first, "Modules/Foo/Bar");
 }
 
 // It could be argued that this should not work.
@@ -113,7 +113,7 @@ TEST_CASE_FIXTURE(FrontendFixture, "find_a_require_inside_a_function")
     NaiveFileResolver naiveFileResolver;
 
     auto res = traceRequires(&naiveFileResolver, program, "");
-    CHECK_EQ(1, res.requires.size());
+    CHECK_EQ(1, res.requireList.size());
 }
 
 TEST_CASE_FIXTURE(FrontendFixture, "real_source")
@@ -138,7 +138,7 @@ TEST_CASE_FIXTURE(FrontendFixture, "real_source")
     NaiveFileResolver naiveFileResolver;
 
     auto res = traceRequires(&naiveFileResolver, program, "");
-    CHECK_EQ(8, res.requires.size());
+    CHECK_EQ(8, res.requireList.size());
 }
 
 TEST_CASE_FIXTURE(FrontendFixture, "automatically_check_dependent_scripts")

tests/Module.test.cpp

@@ -102,7 +102,7 @@ TEST_CASE_FIXTURE(Fixture, "deepClone_cyclic_table")
     const FunctionTypeVar* ftv = get<FunctionTypeVar>(methodType);
     REQUIRE(ftv != nullptr);
 
-    std::optional<TypeId> methodReturnType = first(ftv->retType);
+    std::optional<TypeId> methodReturnType = first(ftv->retTypes);
     REQUIRE(methodReturnType);
 
     CHECK_EQ(methodReturnType, counterCopy);

tests/NonstrictMode.test.cpp

@@ -13,6 +13,57 @@ using namespace Luau;
 
 TEST_SUITE_BEGIN("NonstrictModeTests");
 
+TEST_CASE_FIXTURE(Fixture, "globals")
+{
+    CheckResult result = check(R"(
+        --!nonstrict
+        foo = true
+        foo = "now i'm a string!"
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+    CHECK_EQ("any", toString(requireType("foo")));
+}
+
+TEST_CASE_FIXTURE(Fixture, "globals2")
+{
+    ScopedFastFlag sff[]{
+        {"LuauReturnTypeInferenceInNonstrict", true},
+        {"LuauLowerBoundsCalculation", true},
+    };
+
+    CheckResult result = check(R"(
+        --!nonstrict
+        foo = function() return 1 end
+        foo = "now i'm a string!"
+    )");
+
+    LUAU_REQUIRE_ERROR_COUNT(1, result);
+
+    TypeMismatch* tm = get<TypeMismatch>(result.errors[0]);
+    REQUIRE(tm);
+    CHECK_EQ("() -> number", toString(tm->wantedType));
+    CHECK_EQ("string", toString(tm->givenType));
+    CHECK_EQ("() -> number", toString(requireType("foo")));
+}
+
+TEST_CASE_FIXTURE(Fixture, "globals_everywhere")
+{
+    CheckResult result = check(R"(
+        --!nonstrict
+        foo = 1
+
+        if true then
+            bar = 2
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ("any", toString(requireType("foo")));
+    CHECK_EQ("any", toString(requireType("bar")));
+}
+
 TEST_CASE_FIXTURE(BuiltinsFixture, "function_returns_number_or_string")
 {
     ScopedFastFlag sff[]{{"LuauReturnTypeInferenceInNonstrict", true}, {"LuauLowerBoundsCalculation", true}};
@@ -51,7 +102,7 @@ TEST_CASE_FIXTURE(Fixture, "infer_nullary_function")
     REQUIRE_EQ("any", toString(args[0]));
     REQUIRE_EQ("any", toString(args[1]));
 
-    auto rets = flatten(ftv->retType).first;
+    auto rets = flatten(ftv->retTypes).first;
     REQUIRE_EQ(0, rets.size());
 }
 

tests/Normalize.test.cpp

@@ -837,6 +837,7 @@ TEST_CASE_FIXTURE(Fixture, "intersection_inside_a_table_inside_another_intersect
 {
     ScopedFastFlag flags[] = {
         {"LuauLowerBoundsCalculation", true},
+        {"LuauQuantifyConstrained", true},
     };
 
     // We use a function and inferred parameter types to prevent intermediate normalizations from being performed.
@@ -867,16 +868,17 @@ TEST_CASE_FIXTURE(Fixture, "intersection_inside_a_table_inside_another_intersect
     CHECK("{+ y: number +}" == toString(args[2]));
     CHECK("{+ z: string +}" == toString(args[3]));
 
-    std::vector<TypeId> ret = flatten(ftv->retType).first;
+    std::vector<TypeId> ret = flatten(ftv->retTypes).first;
 
     REQUIRE(1 == ret.size());
-    CHECK("{| x: a & {- w: boolean, y: number, z: string -} |}" == toString(ret[0]));
+    CHECK("{| x: a & {+ w: boolean, y: number, z: string +} |}" == toString(ret[0]));
 }
 
 TEST_CASE_FIXTURE(Fixture, "intersection_inside_a_table_inside_another_intersection_3")
 {
     ScopedFastFlag flags[] = {
         {"LuauLowerBoundsCalculation", true},
+        {"LuauQuantifyConstrained", true},
     };
 
     // We use a function and inferred parameter types to prevent intermediate normalizations from being performed.
@@ -906,16 +908,17 @@ TEST_CASE_FIXTURE(Fixture, "intersection_inside_a_table_inside_another_intersect
     CHECK("t1 where t1 = {+ y: t1 +}" == toString(args[1]));
     CHECK("{+ z: string +}" == toString(args[2]));
 
-    std::vector<TypeId> ret = flatten(ftv->retType).first;
+    std::vector<TypeId> ret = flatten(ftv->retTypes).first;
 
     REQUIRE(1 == ret.size());
-    CHECK("{| x: {- x: boolean, y: t1, z: string -} |} where t1 = {+ y: t1 +}" == toString(ret[0]));
+    CHECK("{| x: {+ x: boolean, y: t1, z: string +} |} where t1 = {+ y: t1 +}" == toString(ret[0]));
 }
 
 TEST_CASE_FIXTURE(Fixture, "intersection_inside_a_table_inside_another_intersection_4")
 {
     ScopedFastFlag flags[] = {
         {"LuauLowerBoundsCalculation", true},
+        {"LuauQuantifyConstrained", true},
     };
 
     // We use a function and inferred parameter types to prevent intermediate normalizations from being performed.
@@ -944,13 +947,13 @@ TEST_CASE_FIXTURE(Fixture, "intersection_inside_a_table_inside_another_intersect
 
     REQUIRE(3 == args.size());
     CHECK("{+ x: boolean +}" == toString(args[0]));
-    CHECK("{+ y: t1 +} where t1 = {| x: {- x: boolean, y: t1, z: string -} |}" == toString(args[1]));
+    CHECK("{+ y: t1 +} where t1 = {| x: {+ x: boolean, y: t1, z: string +} |}" == toString(args[1]));
     CHECK("{+ z: string +}" == toString(args[2]));
 
-    std::vector<TypeId> ret = flatten(ftv->retType).first;
+    std::vector<TypeId> ret = flatten(ftv->retTypes).first;
 
     REQUIRE(1 == ret.size());
-    CHECK("t1 where t1 = {| x: {- x: boolean, y: t1, z: string -} |}" == toString(ret[0]));
+    CHECK("t1 where t1 = {| x: {+ x: boolean, y: t1, z: string +} |}" == toString(ret[0]));
 }
 
 TEST_CASE_FIXTURE(Fixture, "nested_table_normalization_with_non_table__no_ice")
@@ -1062,4 +1065,29 @@ export type t0 = (((any)&({_:l0.t0,n0:t0,_G:any,}))&({_:any,}))&(((any)&({_:l0.t
     LUAU_REQUIRE_ERRORS(result);
 }
 
+TEST_CASE_FIXTURE(BuiltinsFixture, "normalization_does_not_convert_ever")
+{
+    ScopedFastFlag sff[]{
+        {"LuauLowerBoundsCalculation", true},
+        {"LuauQuantifyConstrained", true},
+    };
+
+    CheckResult result = check(R"(
+        --!strict
+        local function f()
+            if math.random() > 0.5 then
+                return true
+            end
+            type Ret = typeof(f())
+            if math.random() > 0.5 then
+                return "something"
+            end
+            return "something" :: Ret
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+    CHECK_EQ("() -> boolean | string", toString(requireType("f")));
+}
+
 TEST_SUITE_END();

tests/NotNull.test.cpp

@@ -75,9 +75,9 @@ TEST_CASE("basic_stuff")
     t->y = 3.14f;
 
     const NotNull<Test> u = t;
-    // u->x = 44; // nope
+    u->x = 44;
     int v = u->x;
-    CHECK(v == 5);
+    CHECK(v == 44);
 
     bar(a);
 
@@ -96,8 +96,11 @@ TEST_CASE("basic_stuff")
 TEST_CASE("hashable")
 {
     std::unordered_map<NotNull<int>, const char*> map;
-    NotNull<int> a{new int(8)};
+    int a_ = 8;
-    NotNull<int> b{new int(10)};
+    int b_ = 10;
+
+    NotNull<int> a{&a_};
+    NotNull<int> b{&b_};
 
     std::string hello = "hello";
     std::string world = "world";
@@ -108,9 +111,47 @@ TEST_CASE("hashable")
     CHECK_EQ(2, map.size());
     CHECK_EQ(hello.c_str(), map[a]);
     CHECK_EQ(world.c_str(), map[b]);
+}
 
-    delete a;
+TEST_CASE("const")
-    delete b;
+{
+    int p = 0;
+    int q = 0;
+
+    NotNull<int> n{&p};
+
+    *n = 123;
+
+    NotNull<const int> m = n; // Conversion from NotNull<T> to NotNull<const T> is allowed
+
+    CHECK(123 == *m); // readonly access of m is ok
+
+    // *m = 321; // nope.  m points at const data.
+
+    // NotNull<int> o = m; // nope.  Conversion from NotNull<const T> to NotNull<T> is forbidden
+
+    NotNull<int> n2{&q};
+    m = n2; // ok.  m points to const data, but is not itself const
+
+    const NotNull<int> m2 = n;
+    // m2 = n2; // nope.  m2 is const.
+    *m2 = 321; // ok.  m2 is const, but points to mutable data
+
+    CHECK(321 == *n);
+}
+
+TEST_CASE("const_compatibility")
+{
+    int* raw = new int(8);
+
+    NotNull<int> a(raw);
+    NotNull<const int> b(raw);
+    NotNull<const int> c = a;
+    // NotNull<int> d = c; // nope - no conversion from const to non-const
+
+    CHECK_EQ(*c, 8);
+
+    delete raw;
 }
 
 TEST_SUITE_END();

tests/TypeInfer.annotations.test.cpp

@@ -70,7 +70,7 @@ TEST_CASE_FIXTURE(Fixture, "function_return_annotations_are_checked")
     const FunctionTypeVar* ftv = get<FunctionTypeVar>(fiftyType);
     REQUIRE(ftv != nullptr);
 
-    TypePackId retPack = ftv->retType;
+    TypePackId retPack = ftv->retTypes;
     const TypePack* tp = get<TypePack>(retPack);
     REQUIRE(tp != nullptr);
 

tests/TypeInfer.functions.test.cpp

@@ -45,7 +45,7 @@ TEST_CASE_FIXTURE(Fixture, "infer_return_type")
     const FunctionTypeVar* takeFiveType = get<FunctionTypeVar>(requireType("take_five"));
     REQUIRE(takeFiveType != nullptr);
 
-    std::vector<TypeId> retVec = flatten(takeFiveType->retType).first;
+    std::vector<TypeId> retVec = flatten(takeFiveType->retTypes).first;
     REQUIRE(!retVec.empty());
 
     REQUIRE_EQ(*follow(retVec[0]), *typeChecker.numberType);
@@ -345,7 +345,7 @@ TEST_CASE_FIXTURE(Fixture, "local_function")
     const FunctionTypeVar* ftv = get<FunctionTypeVar>(h);
     REQUIRE(ftv != nullptr);
 
-    std::optional<TypeId> rt = first(ftv->retType);
+    std::optional<TypeId> rt = first(ftv->retTypes);
     REQUIRE(bool(rt));
 
     TypeId retType = follow(*rt);
@@ -361,7 +361,7 @@ TEST_CASE_FIXTURE(Fixture, "func_expr_doesnt_leak_free")
     LUAU_REQUIRE_NO_ERRORS(result);
     const Luau::FunctionTypeVar* fn = get<FunctionTypeVar>(requireType("p"));
     REQUIRE(fn);
-    auto ret = first(fn->retType);
+    auto ret = first(fn->retTypes);
     REQUIRE(ret);
     REQUIRE(get<GenericTypeVar>(follow(*ret)));
 }
@@ -460,7 +460,7 @@ TEST_CASE_FIXTURE(Fixture, "complicated_return_types_require_an_explicit_annotat
 
     const FunctionTypeVar* functionType = get<FunctionTypeVar>(requireType("most_of_the_natural_numbers"));
 
-    std::optional<TypeId> retType = first(functionType->retType);
+    std::optional<TypeId> retType = first(functionType->retTypes);
     REQUIRE(retType);
     CHECK(get<UnionTypeVar>(*retType));
 }
@@ -1619,4 +1619,56 @@ TEST_CASE_FIXTURE(Fixture, "weird_fail_to_unify_type_pack")
     LUAU_REQUIRE_NO_ERRORS(result);
 }
 
+TEST_CASE_FIXTURE(Fixture, "quantify_constrained_types")
+{
+    ScopedFastFlag sff[]{
+        {"LuauLowerBoundsCalculation", true},
+        {"LuauQuantifyConstrained", true},
+    };
+
+    CheckResult result = check(R"(
+        --!strict
+        local function foo(f)
+            f(5)
+            f("hi")
+            local function g()
+                return f
+            end
+            local h = g()
+            h(true)
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ("<a...>((boolean | number | string) -> (a...)) -> ()", toString(requireType("foo")));
+}
+
+TEST_CASE_FIXTURE(Fixture, "call_o_with_another_argument_after_foo_was_quantified")
+{
+    ScopedFastFlag sff[]{
+        {"LuauLowerBoundsCalculation", true},
+        {"LuauQuantifyConstrained", true},
+    };
+
+    CheckResult result = check(R"(
+        local function f(o)
+            local t = {}
+            t[o] = true
+
+            local function foo(o)
+                o.m1(5)
+                t[o] = nil
+            end
+
+            o.m1("hi")
+
+            return t
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+    // TODO: check the normalized type of f
+}
+
 TEST_SUITE_END();

tests/TypeInfer.generics.test.cpp

@@ -224,7 +224,7 @@ TEST_CASE_FIXTURE(Fixture, "infer_generic_function")
     const FunctionTypeVar* idFun = get<FunctionTypeVar>(idType);
     REQUIRE(idFun);
     auto [args, varargs] = flatten(idFun->argTypes);
-    auto [rets, varrets] = flatten(idFun->retType);
+    auto [rets, varrets] = flatten(idFun->retTypes);
 
     CHECK_EQ(idFun->generics.size(), 1);
     CHECK_EQ(idFun->genericPacks.size(), 0);
@@ -247,7 +247,7 @@ TEST_CASE_FIXTURE(Fixture, "infer_generic_local_function")
     const FunctionTypeVar* idFun = get<FunctionTypeVar>(idType);
     REQUIRE(idFun);
     auto [args, varargs] = flatten(idFun->argTypes);
-    auto [rets, varrets] = flatten(idFun->retType);
+    auto [rets, varrets] = flatten(idFun->retTypes);
 
     CHECK_EQ(idFun->generics.size(), 1);
     CHECK_EQ(idFun->genericPacks.size(), 0);
@@ -882,7 +882,7 @@ TEST_CASE_FIXTURE(Fixture, "correctly_instantiate_polymorphic_member_functions")
     const FunctionTypeVar* foo = get<FunctionTypeVar>(follow(fooProp->type));
     REQUIRE(bool(foo));
 
-    std::optional<TypeId> ret_ = first(foo->retType);
+    std::optional<TypeId> ret_ = first(foo->retTypes);
     REQUIRE(bool(ret_));
     TypeId ret = follow(*ret_);
 

tests/TypeInfer.operators.test.cpp

@@ -90,7 +90,7 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "primitive_arith_no_metatable")
 
     const FunctionTypeVar* functionType = get<FunctionTypeVar>(requireType("add"));
 
-    std::optional<TypeId> retType = first(functionType->retType);
+    std::optional<TypeId> retType = first(functionType->retTypes);
     REQUIRE(retType.has_value());
     CHECK_EQ(typeChecker.numberType, follow(*retType));
     CHECK_EQ(requireType("n"), typeChecker.numberType);
@@ -777,8 +777,6 @@ TEST_CASE_FIXTURE(Fixture, "infer_any_in_all_modes_when_lhs_is_unknown")
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "equality_operations_succeed_if_any_union_branch_succeeds")
 {
-    ScopedFastFlag sff("LuauSuccessTypingForEqualityOperations", true);
-
     CheckResult result = check(R"(
         local mm = {}
         type Foo = typeof(setmetatable({}, mm))

tests/TypeInfer.provisional.test.cpp

@@ -472,6 +472,7 @@ TEST_CASE_FIXTURE(Fixture, "constrained_is_level_dependent")
     ScopedFastFlag sff[]{
         {"LuauLowerBoundsCalculation", true},
         {"LuauNormalizeFlagIsConservative", true},
+        {"LuauQuantifyConstrained", true},
     };
 
     CheckResult result = check(R"(
@@ -494,8 +495,8 @@ TEST_CASE_FIXTURE(Fixture, "constrained_is_level_dependent")
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
-    // TODO: We're missing generics a... and b...
+    // TODO: We're missing generics b...
-    CHECK_EQ("(t1) -> {| [t1]: boolean |} where t1 = t2 ; t2 = {+ m1: (t1) -> (a...), m2: (t2) -> (b...) +}", toString(requireType("f")));
+    CHECK_EQ("<a...>(t1) -> {| [t1]: boolean |} where t1 = t2 ; t2 = {+ m1: (t1) -> (a...), m2: (t2) -> (b...) +}", toString(requireType("f")));
 }
 
 TEST_SUITE_END();

tests/TypeInfer.refinements.test.cpp

@@ -13,8 +13,8 @@ using namespace Luau;
 
 namespace
 {
-std::optional<ExprResult<TypePackId>> magicFunctionInstanceIsA(
+std::optional<WithPredicate<TypePackId>> magicFunctionInstanceIsA(
-    TypeChecker& typeChecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult)
+    TypeChecker& typeChecker, const ScopePtr& scope, const AstExprCall& expr, WithPredicate<TypePackId> withPredicate)
 {
     if (expr.args.size != 1)
         return std::nullopt;
@@ -32,7 +32,7 @@ std::optional<ExprResult<TypePackId>> magicFunctionInstanceIsA(
     unfreeze(typeChecker.globalTypes);
     TypePackId booleanPack = typeChecker.globalTypes.addTypePack({typeChecker.booleanType});
     freeze(typeChecker.globalTypes);
-    return ExprResult<TypePackId>{booleanPack, {IsAPredicate{std::move(*lvalue), expr.location, tfun->type}}};
+    return WithPredicate<TypePackId>{booleanPack, {IsAPredicate{std::move(*lvalue), expr.location, tfun->type}}};
 }
 
 struct RefinementClassFixture : Fixture

tests/TypeInfer.tables.test.cpp

@@ -642,7 +642,7 @@ TEST_CASE_FIXTURE(Fixture, "indexers_quantification_2")
     const TableTypeVar* argType = get<TableTypeVar>(follow(argVec[0]));
     REQUIRE(argType != nullptr);
 
-    std::vector<TypeId> retVec = flatten(ftv->retType).first;
+    std::vector<TypeId> retVec = flatten(ftv->retTypes).first;
 
     const TableTypeVar* retType = get<TableTypeVar>(follow(retVec[0]));
     REQUIRE(retType != nullptr);
@@ -691,7 +691,7 @@ TEST_CASE_FIXTURE(Fixture, "infer_indexer_from_value_property_in_literal")
     const FunctionTypeVar* fType = get<FunctionTypeVar>(requireType("f"));
     REQUIRE(fType != nullptr);
 
-    auto retType_ = first(fType->retType);
+    auto retType_ = first(fType->retTypes);
     REQUIRE(bool(retType_));
 
     auto retType = get<TableTypeVar>(follow(*retType_));
@@ -1881,7 +1881,7 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "quantifying_a_bound_var_works")
     REQUIRE(prop.type);
     const FunctionTypeVar* ftv = get<FunctionTypeVar>(follow(prop.type));
     REQUIRE(ftv);
-    const TypePack* res = get<TypePack>(follow(ftv->retType));
+    const TypePack* res = get<TypePack>(follow(ftv->retTypes));
     REQUIRE(res);
     REQUIRE(res->head.size() == 1);
     const MetatableTypeVar* mtv = get<MetatableTypeVar>(follow(res->head[0]));
@@ -2584,7 +2584,7 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "dont_quantify_table_that_belongs_to_outer_sc
     const FunctionTypeVar* newType = get<FunctionTypeVar>(follow(counterType->props["new"].type));
     REQUIRE(newType);
 
-    std::optional<TypeId> newRetType = *first(newType->retType);
+    std::optional<TypeId> newRetType = *first(newType->retTypes);
     REQUIRE(newRetType);
 
     const MetatableTypeVar* newRet = get<MetatableTypeVar>(follow(*newRetType));
@@ -2977,7 +2977,6 @@ TEST_CASE_FIXTURE(Fixture, "mixed_tables_with_implicit_numbered_keys")
 
 TEST_CASE_FIXTURE(Fixture, "expected_indexer_value_type_extra")
 {
-    ScopedFastFlag luauExpectedPropTypeFromIndexer{"LuauExpectedPropTypeFromIndexer", true};
     ScopedFastFlag luauSubtypingAddOptPropsToUnsealedTables{"LuauSubtypingAddOptPropsToUnsealedTables", true};
 
     CheckResult result = check(R"(
@@ -2992,8 +2991,6 @@ TEST_CASE_FIXTURE(Fixture, "expected_indexer_value_type_extra")
 
 TEST_CASE_FIXTURE(Fixture, "expected_indexer_value_type_extra_2")
 {
-    ScopedFastFlag luauExpectedPropTypeFromIndexer{"LuauExpectedPropTypeFromIndexer", true};
-
     CheckResult result = check(R"(
         type X = {[any]: string | boolean}
 

tests/TypeInfer.test.cpp

@@ -13,8 +13,9 @@
 
 #include <algorithm>
 
-LUAU_FASTFLAG(LuauLowerBoundsCalculation)
+LUAU_FASTFLAG(LuauLowerBoundsCalculation);
-LUAU_FASTFLAG(LuauFixLocationSpanTableIndexExpr)
+LUAU_FASTFLAG(LuauFixLocationSpanTableIndexExpr);
+LUAU_FASTFLAG(DebugLuauDeferredConstraintResolution);
 
 using namespace Luau;
 
@@ -43,10 +44,7 @@ TEST_CASE_FIXTURE(Fixture, "tc_error")
     CheckResult result = check("local a = 7   local b = 'hi'   a = b");
     LUAU_REQUIRE_ERROR_COUNT(1, result);
 
-    CHECK_EQ(result.errors[0], (TypeError{Location{Position{0, 35}, Position{0, 36}}, TypeMismatch{
+    CHECK_EQ(result.errors[0], (TypeError{Location{Position{0, 35}, Position{0, 36}}, TypeMismatch{typeChecker.numberType, typeChecker.stringType}}));
-                                                                                          requireType("a"),
-                                                                                          requireType("b"),
-                                                                                      }}));
 }
 
 TEST_CASE_FIXTURE(Fixture, "tc_error_2")
@@ -86,6 +84,7 @@ TEST_CASE_FIXTURE(Fixture, "infer_locals_via_assignment_from_its_call_site")
 TEST_CASE_FIXTURE(Fixture, "infer_in_nocheck_mode")
 {
     ScopedFastFlag sff[]{
+        {"DebugLuauDeferredConstraintResolution", false},
         {"LuauReturnTypeInferenceInNonstrict", true},
         {"LuauLowerBoundsCalculation", true},
     };
@@ -236,10 +235,14 @@ TEST_CASE_FIXTURE(Fixture, "type_errors_infer_types")
     CHECK_EQ("boolean", toString(err->table));
     CHECK_EQ("x", err->key);
 
+    // TODO: Should we assert anything about these tests when DCR is being used?
+    if (!FFlag::DebugLuauDeferredConstraintResolution)
+    {
         CHECK_EQ("*unknown*", toString(requireType("c")));
         CHECK_EQ("*unknown*", toString(requireType("d")));
         CHECK_EQ("*unknown*", toString(requireType("e")));
         CHECK_EQ("*unknown*", toString(requireType("f")));
+    }
 }
 
 TEST_CASE_FIXTURE(Fixture, "should_be_able_to_infer_this_without_stack_overflowing")
@@ -352,40 +355,6 @@ TEST_CASE_FIXTURE(Fixture, "check_expr_recursion_limit")
     CHECK(nullptr != get<CodeTooComplex>(result.errors[0]));
 }
 
-TEST_CASE_FIXTURE(Fixture, "globals")
-{
-    CheckResult result = check(R"(
-        --!nonstrict
-        foo = true
-        foo = "now i'm a string!"
-    )");
-
-    LUAU_REQUIRE_NO_ERRORS(result);
-    CHECK_EQ("any", toString(requireType("foo")));
-}
-
-TEST_CASE_FIXTURE(Fixture, "globals2")
-{
-    ScopedFastFlag sff[]{
-        {"LuauReturnTypeInferenceInNonstrict", true},
-        {"LuauLowerBoundsCalculation", true},
-    };
-
-    CheckResult result = check(R"(
-        --!nonstrict
-        foo = function() return 1 end
-        foo = "now i'm a string!"
-    )");
-
-    LUAU_REQUIRE_ERROR_COUNT(1, result);
-
-    TypeMismatch* tm = get<TypeMismatch>(result.errors[0]);
-    REQUIRE(tm);
-    CHECK_EQ("() -> number", toString(tm->wantedType));
-    CHECK_EQ("string", toString(tm->givenType));
-    CHECK_EQ("() -> number", toString(requireType("foo")));
-}
-
 TEST_CASE_FIXTURE(Fixture, "globals_are_banned_in_strict_mode")
 {
     CheckResult result = check(R"(
@@ -400,23 +369,6 @@ TEST_CASE_FIXTURE(Fixture, "globals_are_banned_in_strict_mode")
     CHECK_EQ("foo", us->name);
 }
 
-TEST_CASE_FIXTURE(Fixture, "globals_everywhere")
-{
-    CheckResult result = check(R"(
-        --!nonstrict
-        foo = 1
-
-        if true then
-            bar = 2
-        end
-    )");
-
-    LUAU_REQUIRE_NO_ERRORS(result);
-
-    CHECK_EQ("any", toString(requireType("foo")));
-    CHECK_EQ("any", toString(requireType("bar")));
-}
-
 TEST_CASE_FIXTURE(BuiltinsFixture, "correctly_scope_locals_do")
 {
     CheckResult result = check(R"(
@@ -447,21 +399,6 @@ TEST_CASE_FIXTURE(Fixture, "checking_should_not_ice")
     CHECK_EQ("any", toString(requireType("value")));
 }
 
-// TEST_CASE_FIXTURE(Fixture, "infer_method_signature_of_argument")
-// {
-//     CheckResult result = check(R"(
-//         function f(a)
-//             if a.cond then
-//                 return a.method()
-//             end
-//         end
-//     )");
-
-//     LUAU_REQUIRE_NO_ERRORS(result);
-
-//     CHECK_EQ("A", toString(requireType("f")));
-// }
-
 TEST_CASE_FIXTURE(Fixture, "cyclic_follow")
 {
     check(R"(

tests/TypeInfer.typePacks.cpp

@@ -26,7 +26,7 @@ TEST_CASE_FIXTURE(Fixture, "infer_multi_return")
     const FunctionTypeVar* takeTwoType = get<FunctionTypeVar>(requireType("take_two"));
     REQUIRE(takeTwoType != nullptr);
 
-    const auto& [returns, tail] = flatten(takeTwoType->retType);
+    const auto& [returns, tail] = flatten(takeTwoType->retTypes);
 
     CHECK_EQ(2, returns.size());
     CHECK_EQ(typeChecker.numberType, follow(returns[0]));
@@ -73,7 +73,7 @@ TEST_CASE_FIXTURE(Fixture, "last_element_of_return_statement_can_itself_be_a_pac
     const FunctionTypeVar* takeOneMoreType = get<FunctionTypeVar>(requireType("take_three"));
     REQUIRE(takeOneMoreType != nullptr);
 
-    const auto& [rets, tail] = flatten(takeOneMoreType->retType);
+    const auto& [rets, tail] = flatten(takeOneMoreType->retTypes);
 
     REQUIRE_EQ(3, rets.size());
     CHECK_EQ(typeChecker.numberType, follow(rets[0]));
@@ -105,10 +105,10 @@ TEST_CASE_FIXTURE(Fixture, "return_type_should_be_empty_if_nothing_is_returned")
     LUAU_REQUIRE_NO_ERRORS(result);
     const FunctionTypeVar* fTy = get<FunctionTypeVar>(requireType("f"));
     REQUIRE(fTy != nullptr);
-    CHECK_EQ(0, size(fTy->retType));
+    CHECK_EQ(0, size(fTy->retTypes));
     const FunctionTypeVar* gTy = get<FunctionTypeVar>(requireType("g"));
     REQUIRE(gTy != nullptr);
-    CHECK_EQ(0, size(gTy->retType));
+    CHECK_EQ(0, size(gTy->retTypes));
 }
 
 TEST_CASE_FIXTURE(Fixture, "no_return_size_should_be_zero")
@@ -125,15 +125,15 @@ TEST_CASE_FIXTURE(Fixture, "no_return_size_should_be_zero")
 
     const FunctionTypeVar* fTy = get<FunctionTypeVar>(requireType("f"));
     REQUIRE(fTy != nullptr);
-    CHECK_EQ(1, size(follow(fTy->retType)));
+    CHECK_EQ(1, size(follow(fTy->retTypes)));
 
     const FunctionTypeVar* gTy = get<FunctionTypeVar>(requireType("g"));
     REQUIRE(gTy != nullptr);
-    CHECK_EQ(0, size(gTy->retType));
+    CHECK_EQ(0, size(gTy->retTypes));
 
     const FunctionTypeVar* hTy = get<FunctionTypeVar>(requireType("h"));
     REQUIRE(hTy != nullptr);
-    CHECK_EQ(0, size(hTy->retType));
+    CHECK_EQ(0, size(hTy->retTypes));
 }
 
 TEST_CASE_FIXTURE(Fixture, "varargs_inference_through_multiple_scopes")

tools/natvis/Analysis.natvis

@@ -6,40 +6,40 @@
   </Type>
 
   <Type Name="Luau::Variant&lt;*&gt;">
-    <DisplayString Condition="typeId == 0" Optional="true">{{ index=0, value={*($T1*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 0" Optional="true">{{ typeId=0, value={*($T1*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 1" Optional="true">{{ index=1, value={*($T2*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 1" Optional="true">{{ typeId=1, value={*($T2*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 2" Optional="true">{{ index=2, value={*($T3*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 2" Optional="true">{{ typeId=2, value={*($T3*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 3" Optional="true">{{ index=3, value={*($T4*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 3" Optional="true">{{ typeId=3, value={*($T4*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 4" Optional="true">{{ index=4, value={*($T5*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 4" Optional="true">{{ typeId=4, value={*($T5*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 5" Optional="true">{{ index=5, value={*($T6*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 5" Optional="true">{{ typeId=5, value={*($T6*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 6" Optional="true">{{ index=6, value={*($T7*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 6" Optional="true">{{ typeId=6, value={*($T7*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 7" Optional="true">{{ index=7, value={*($T8*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 7" Optional="true">{{ typeId=7, value={*($T8*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 8" Optional="true">{{ index=8, value={*($T9*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 8" Optional="true">{{ typeId=8, value={*($T9*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 9" Optional="true">{{ index=9, value={*($T10*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 9" Optional="true">{{ typeId=9, value={*($T10*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 10" Optional="true">{{ index=10, value={*($T11*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 10" Optional="true">{{ typeId=10, value={*($T11*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 11" Optional="true">{{ index=11, value={*($T12*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 11" Optional="true">{{ typeId=11, value={*($T12*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 12" Optional="true">{{ index=12, value={*($T13*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 12" Optional="true">{{ typeId=12, value={*($T13*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 13" Optional="true">{{ index=13, value={*($T14*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 13" Optional="true">{{ typeId=13, value={*($T14*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 14" Optional="true">{{ index=14, value={*($T15*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 14" Optional="true">{{ typeId=14, value={*($T15*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 15" Optional="true">{{ index=15, value={*($T16*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 15" Optional="true">{{ typeId=15, value={*($T16*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 16" Optional="true">{{ index=16, value={*($T17*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 16" Optional="true">{{ typeId=16, value={*($T17*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 17" Optional="true">{{ index=17, value={*($T18*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 17" Optional="true">{{ typeId=17, value={*($T18*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 18" Optional="true">{{ index=18, value={*($T19*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 18" Optional="true">{{ typeId=18, value={*($T19*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 19" Optional="true">{{ index=19, value={*($T20*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 19" Optional="true">{{ typeId=19, value={*($T20*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 20" Optional="true">{{ index=20, value={*($T21*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 20" Optional="true">{{ typeId=20, value={*($T21*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 21" Optional="true">{{ index=21, value={*($T22*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 21" Optional="true">{{ typeId=21, value={*($T22*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 22" Optional="true">{{ index=22, value={*($T23*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 22" Optional="true">{{ typeId=22, value={*($T23*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 23" Optional="true">{{ index=23, value={*($T24*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 23" Optional="true">{{ typeId=23, value={*($T24*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 24" Optional="true">{{ index=24, value={*($T25*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 24" Optional="true">{{ typeId=24, value={*($T25*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 25" Optional="true">{{ index=25, value={*($T26*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 25" Optional="true">{{ typeId=25, value={*($T26*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 26" Optional="true">{{ index=26, value={*($T27*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 26" Optional="true">{{ typeId=26, value={*($T27*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 27" Optional="true">{{ index=27, value={*($T28*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 27" Optional="true">{{ typeId=27, value={*($T28*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 28" Optional="true">{{ index=28, value={*($T29*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 28" Optional="true">{{ typeId=28, value={*($T29*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 29" Optional="true">{{ index=29, value={*($T30*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 29" Optional="true">{{ typeId=29, value={*($T30*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 30" Optional="true">{{ index=30, value={*($T31*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 30" Optional="true">{{ typeId=30, value={*($T31*)storage} }}</DisplayString>
-    <DisplayString Condition="typeId == 31" Optional="true">{{ index=31, value={*($T32*)storage} }}</DisplayString>
+    <DisplayString Condition="typeId == 31" Optional="true">{{ typeId=31, value={*($T32*)storage} }}</DisplayString>
     <Expand>
-      <Item Name="index">typeId</Item>
+      <Item Name="typeId">typeId</Item>
       <Item Name="[value]" Condition="typeId == 0" Optional="true">*($T1*)storage</Item>
       <Item Name="[value]" Condition="typeId == 1" Optional="true">*($T2*)storage</Item>
       <Item Name="[value]" Condition="typeId == 2" Optional="true">*($T3*)storage</Item>