Sync to upstream/release/578

Analysis/include/Luau/Constraint.h

@@ -81,6 +81,9 @@ struct IterableConstraint
 {
     TypePackId iterator;
     TypePackId variables;
+
+    const AstNode* nextAstFragment;
+    DenseHashMap<const AstNode*, TypeId>* astOverloadResolvedTypes;
 };
 
 // name(namedType) = name

Analysis/include/Luau/ConstraintSolver.h

@@ -201,7 +201,7 @@ struct ConstraintSolver
      * @param subType the sub-type to unify.
      * @param superType the super-type to unify.
      */
-    void unify(TypeId subType, TypeId superType, NotNull<Scope> scope);
+    ErrorVec unify(TypeId subType, TypeId superType, NotNull<Scope> scope);
 
     /**
      * Creates a new Unifier and performs a single unification operation. Commits
@@ -209,7 +209,7 @@ struct ConstraintSolver
      * @param subPack the sub-type pack to unify.
      * @param superPack the super-type pack to unify.
      */
-    void unify(TypePackId subPack, TypePackId superPack, NotNull<Scope> scope);
+    ErrorVec unify(TypePackId subPack, TypePackId superPack, NotNull<Scope> scope);
 
     /** Pushes a new solver constraint to the solver.
      * @param cv the body of the constraint.

Analysis/include/Luau/Error.h

@@ -343,13 +343,27 @@ struct UninhabitedTypePackFamily
     bool operator==(const UninhabitedTypePackFamily& rhs) const;
 };
 
+struct WhereClauseNeeded
+{
+    TypeId ty;
+
+    bool operator==(const WhereClauseNeeded& rhs) const;
+};
+
+struct PackWhereClauseNeeded
+{
+    TypePackId tp;
+
+    bool operator==(const PackWhereClauseNeeded& rhs) const;
+};
+
 using TypeErrorData =
     Variant<TypeMismatch, UnknownSymbol, UnknownProperty, NotATable, CannotExtendTable, OnlyTablesCanHaveMethods, DuplicateTypeDefinition,
         CountMismatch, FunctionDoesNotTakeSelf, FunctionRequiresSelf, OccursCheckFailed, UnknownRequire, IncorrectGenericParameterCount, SyntaxError,
         CodeTooComplex, UnificationTooComplex, UnknownPropButFoundLikeProp, GenericError, InternalError, CannotCallNonFunction, ExtraInformation,
         DeprecatedApiUsed, ModuleHasCyclicDependency, IllegalRequire, FunctionExitsWithoutReturning, DuplicateGenericParameter,
         CannotInferBinaryOperation, MissingProperties, SwappedGenericTypeParameter, OptionalValueAccess, MissingUnionProperty, TypesAreUnrelated,
-        NormalizationTooComplex, TypePackMismatch, DynamicPropertyLookupOnClassesUnsafe, UninhabitedTypeFamily, UninhabitedTypePackFamily>;
+        NormalizationTooComplex, TypePackMismatch, DynamicPropertyLookupOnClassesUnsafe, UninhabitedTypeFamily, UninhabitedTypePackFamily, WhereClauseNeeded, PackWhereClauseNeeded>;
 
 struct TypeErrorSummary
 {

Analysis/include/Luau/Frontend.h

@@ -182,8 +182,6 @@ struct Frontend
     std::optional<CheckResult> getCheckResult(const ModuleName& name, bool accumulateNested, bool forAutocomplete = false);
 
 private:
-    CheckResult check_DEPRECATED(const ModuleName& name, std::optional<FrontendOptions> optionOverride = {});
-
     struct TypeCheckLimits
     {
         std::optional<double> finishTime;

Analysis/include/Luau/Normalize.h

@@ -285,14 +285,19 @@ class Normalizer
     std::unordered_map<const TypeIds*, TypeId> cachedIntersections;
     std::unordered_map<const TypeIds*, TypeId> cachedUnions;
     std::unordered_map<const TypeIds*, std::unique_ptr<TypeIds>> cachedTypeIds;
+
+    DenseHashMap<TypeId, bool> cachedIsInhabited{nullptr};
+    DenseHashMap<std::pair<TypeId, TypeId>, bool, TypeIdPairHash> cachedIsInhabitedIntersection{{nullptr, nullptr}};
+
     bool withinResourceLimits();
 
 public:
     TypeArena* arena;
     NotNull<BuiltinTypes> builtinTypes;
     NotNull<UnifierSharedState> sharedState;
+    bool cacheInhabitance = false;
 
-    Normalizer(TypeArena* arena, NotNull<BuiltinTypes> builtinTypes, NotNull<UnifierSharedState> sharedState);
+    Normalizer(TypeArena* arena, NotNull<BuiltinTypes> builtinTypes, NotNull<UnifierSharedState> sharedState, bool cacheInhabitance = false);
     Normalizer(const Normalizer&) = delete;
     Normalizer(Normalizer&&) = delete;
     Normalizer() = delete;
@@ -355,8 +360,10 @@ public:
     bool normalizeIntersections(const std::vector<TypeId>& intersections, NormalizedType& outType);
 
     // Check for inhabitance
-    bool isInhabited(TypeId ty, std::unordered_set<TypeId> seen = {});
+    bool isInhabited(TypeId ty);
+    bool isInhabited(TypeId ty, std::unordered_set<TypeId> seen);
     bool isInhabited(const NormalizedType* norm, std::unordered_set<TypeId> seen = {});
+
     // Check for intersections being inhabited
     bool isIntersectionInhabited(TypeId left, TypeId right);
 

Analysis/include/Luau/Substitution.h

@@ -113,6 +113,13 @@ struct Tarjan
     void visitChild(TypeId ty);
     void visitChild(TypePackId ty);
 
+    template<typename Ty>
+    void visitChild(std::optional<Ty> ty)
+    {
+        if (ty)
+            visitChild(*ty);
+    }
+
     // Visit the root vertex.
     TarjanResult visitRoot(TypeId ty);
     TarjanResult visitRoot(TypePackId ty);
@@ -127,10 +134,22 @@ struct Tarjan
     {
         return false;
     }
+
     virtual bool ignoreChildren(TypePackId ty)
     {
         return false;
     }
+
+    // Some subclasses might ignore children visit, but not other actions like replacing the children
+    virtual bool ignoreChildrenVisit(TypeId ty)
+    {
+        return ignoreChildren(ty);
+    }
+
+    virtual bool ignoreChildrenVisit(TypePackId ty)
+    {
+        return ignoreChildren(ty);
+    }
 };
 
 // We use Tarjan to calculate dirty bits. We set `dirty[i]` true
@@ -186,8 +205,10 @@ public:
 
     TypeId replace(TypeId ty);
     TypePackId replace(TypePackId tp);
+
     void replaceChildren(TypeId ty);
     void replaceChildren(TypePackId tp);
+
     TypeId clone(TypeId ty);
     TypePackId clone(TypePackId tp);
 
@@ -211,6 +232,16 @@ public:
     {
         return arena->addTypePack(TypePackVar{tp});
     }
+
+private:
+    template<typename Ty>
+    std::optional<Ty> replace(std::optional<Ty> ty)
+    {
+        if (ty)
+            return replace(*ty);
+        else
+            return std::nullopt;
+    }
 };
 
 } // namespace Luau

Analysis/include/Luau/Type.h

@@ -388,7 +388,13 @@ struct Property
     static Property writeonly(TypeId ty);
     static Property rw(TypeId ty);                 // Shared read-write type.
     static Property rw(TypeId read, TypeId write); // Separate read-write type.
-    static std::optional<Property> create(std::optional<TypeId> read, std::optional<TypeId> write);
+
+    // Invariant: at least one of the two optionals are not nullopt!
+    // If the read type is not nullopt, but the write type is, then the property is readonly.
+    // If the read type is nullopt, but the write type is not, then the property is writeonly.
+    // If the read and write types are not nullopt, then the property is read and write.
+    // Otherwise, an assertion where read and write types are both nullopt will be tripped.
+    static Property create(std::optional<TypeId> read, std::optional<TypeId> write);
 
     bool deprecated = false;
     std::string deprecatedSuggestion;
@@ -414,6 +420,8 @@ struct Property
     std::optional<TypeId> readType() const;
     std::optional<TypeId> writeType() const;
 
+    bool isShared() const;
+
 private:
     std::optional<TypeId> readTy;
     std::optional<TypeId> writeTy;
@@ -614,30 +622,26 @@ struct IntersectionType
 struct LazyType
 {
     LazyType() = default;
-    LazyType(std::function<TypeId()> thunk_DEPRECATED, std::function<void(LazyType&)> unwrap)
+    LazyType(std::function<void(LazyType&)> unwrap)
-        : thunk_DEPRECATED(thunk_DEPRECATED)
+        : unwrap(unwrap)
-        , unwrap(unwrap)
     {
     }
 
     // std::atomic is sad and requires a manual copy
     LazyType(const LazyType& rhs)
-        : thunk_DEPRECATED(rhs.thunk_DEPRECATED)
+        : unwrap(rhs.unwrap)
-        , unwrap(rhs.unwrap)
         , unwrapped(rhs.unwrapped.load())
     {
     }
 
     LazyType(LazyType&& rhs) noexcept
-        : thunk_DEPRECATED(std::move(rhs.thunk_DEPRECATED))
+        : unwrap(std::move(rhs.unwrap))
-        , unwrap(std::move(rhs.unwrap))
         , unwrapped(rhs.unwrapped.load())
     {
     }
 
     LazyType& operator=(const LazyType& rhs)
     {
-        thunk_DEPRECATED = rhs.thunk_DEPRECATED;
         unwrap = rhs.unwrap;
         unwrapped = rhs.unwrapped.load();
 
@@ -646,15 +650,12 @@ struct LazyType
 
     LazyType& operator=(LazyType&& rhs) noexcept
     {
-        thunk_DEPRECATED = std::move(rhs.thunk_DEPRECATED);
         unwrap = std::move(rhs.unwrap);
         unwrapped = rhs.unwrapped.load();
 
         return *this;
     }
 
-    std::function<TypeId()> thunk_DEPRECATED;
-
     std::function<void(LazyType&)> unwrap;
     std::atomic<TypeId> unwrapped = nullptr;
 };

Analysis/include/Luau/TypeUtils.h

@@ -55,4 +55,13 @@ std::vector<TypeId> reduceUnion(const std::vector<TypeId>& types);
  */
 TypeId stripNil(NotNull<BuiltinTypes> builtinTypes, TypeArena& arena, TypeId ty);
 
+template<typename T, typename Ty>
+const T* get(std::optional<Ty> ty)
+{
+    if (ty)
+        return get<T>(*ty);
+    else
+        return nullptr;
+}
+
 } // namespace Luau

Analysis/include/Luau/Unifier.h

@@ -54,7 +54,6 @@ struct Unifier
     TypeArena* const types;
     NotNull<BuiltinTypes> builtinTypes;
     NotNull<Normalizer> normalizer;
-    Mode mode;
 
     NotNull<Scope> scope; // const Scope maybe
     TxnLog log;
@@ -78,7 +77,7 @@ struct Unifier
     std::vector<TypePackId> blockedTypePacks;
 
     Unifier(
-        NotNull<Normalizer> normalizer, Mode mode, NotNull<Scope> scope, const Location& location, Variance variance, TxnLog* parentLog = nullptr);
+        NotNull<Normalizer> normalizer, NotNull<Scope> scope, const Location& location, Variance variance, TxnLog* parentLog = nullptr);
 
     // Configure the Unifier to test for scope subsumption via embedded Scope
     // pointers rather than TypeLevels.
@@ -154,7 +153,6 @@ public:
     LUAU_NOINLINE void reportError(Location location, TypeErrorData data);
 
 private:
-    bool isNonstrictMode() const;
     TypeMismatch::Context mismatchContext();
 
     void checkChildUnifierTypeMismatch(const ErrorVec& innerErrors, TypeId wantedType, TypeId givenType);

Analysis/include/Luau/VisitType.h

@@ -10,6 +10,7 @@
 
 LUAU_FASTINT(LuauVisitRecursionLimit)
 LUAU_FASTFLAG(LuauBoundLazyTypes2)
+LUAU_FASTFLAG(DebugLuauReadWriteProperties)
 
 namespace Luau
 {
@@ -250,7 +251,18 @@ struct GenericTypeVisitor
                 else
                 {
                     for (auto& [_name, prop] : ttv->props)
-                        traverse(prop.type());
+                    {
+                        if (FFlag::DebugLuauReadWriteProperties)
+                        {
+                            if (auto ty = prop.readType())
+                                traverse(*ty);
+
+                            if (auto ty = prop.writeType())
+                                traverse(*ty);
+                        }
+                        else
+                            traverse(prop.type());
+                    }
 
                     if (ttv->indexer)
                     {
@@ -273,7 +285,18 @@ struct GenericTypeVisitor
             if (visit(ty, *ctv))
             {
                 for (const auto& [name, prop] : ctv->props)
-                    traverse(prop.type());
+                {
+                    if (FFlag::DebugLuauReadWriteProperties)
+                    {
+                        if (auto ty = prop.readType())
+                            traverse(*ty);
+
+                        if (auto ty = prop.writeType())
+                            traverse(*ty);
+                    }
+                    else
+                        traverse(prop.type());
+                }
 
                 if (ctv->parent)
                     traverse(*ctv->parent);
@@ -311,11 +334,9 @@ struct GenericTypeVisitor
         }
         else if (auto ltv = get<LazyType>(ty))
         {
-            if (FFlag::LuauBoundLazyTypes2)
+            if (TypeId unwrapped = ltv->unwrapped)
-            {
+                traverse(unwrapped);
-                if (TypeId unwrapped = ltv->unwrapped)
+
-                    traverse(unwrapped);
-            }
             // Visiting into LazyType that hasn't been unwrapped may necessarily cause infinite expansion, so we don't do that on purpose.
             // Asserting also makes no sense, because the type _will_ happen here, most likely as a property of some ClassType
             // that doesn't need to be expanded.

Analysis/src/AstQuery.cpp

@@ -11,6 +11,8 @@
 
 #include <algorithm>
 
+LUAU_FASTFLAG(DebugLuauReadWriteProperties)
+
 namespace Luau
 {
 
@@ -501,12 +503,28 @@ std::optional<DocumentationSymbol> getDocumentationSymbolAtPosition(const Source
                 if (const TableType* ttv = get<TableType>(parentTy))
                 {
                     if (auto propIt = ttv->props.find(indexName->index.value); propIt != ttv->props.end())
-                        return checkOverloadedDocumentationSymbol(module, propIt->second.type(), parentExpr, propIt->second.documentationSymbol);
+                    {
+                        if (FFlag::DebugLuauReadWriteProperties)
+                        {
+                            if (auto ty = propIt->second.readType())
+                                return checkOverloadedDocumentationSymbol(module, *ty, parentExpr, propIt->second.documentationSymbol);
+                        }
+                        else
+                            return checkOverloadedDocumentationSymbol(module, propIt->second.type(), parentExpr, propIt->second.documentationSymbol);
+                    }
                 }
                 else if (const ClassType* ctv = get<ClassType>(parentTy))
                 {
                     if (auto propIt = ctv->props.find(indexName->index.value); propIt != ctv->props.end())
-                        return checkOverloadedDocumentationSymbol(module, propIt->second.type(), parentExpr, propIt->second.documentationSymbol);
+                    {
+                        if (FFlag::DebugLuauReadWriteProperties)
+                        {
+                            if (auto ty = propIt->second.readType())
+                                return checkOverloadedDocumentationSymbol(module, *ty, parentExpr, propIt->second.documentationSymbol);
+                        }
+                        else
+                            return checkOverloadedDocumentationSymbol(module, propIt->second.type(), parentExpr, propIt->second.documentationSymbol);
+                    }
                 }
             }
         }

Analysis/src/Autocomplete.cpp

@@ -12,6 +12,8 @@
 #include <unordered_set>
 #include <utility>
 
+LUAU_FASTFLAG(DebugLuauReadWriteProperties)
+
 static const std::unordered_set<std::string> kStatementStartingKeywords = {
     "while", "if", "local", "repeat", "function", "do", "for", "return", "break", "continue", "type", "export"};
 
@@ -138,7 +140,7 @@ static bool checkTypeMatch(TypeId subTy, TypeId superTy, NotNull<Scope> scope, T
     InternalErrorReporter iceReporter;
     UnifierSharedState unifierState(&iceReporter);
     Normalizer normalizer{typeArena, builtinTypes, NotNull{&unifierState}};
-    Unifier unifier(NotNull<Normalizer>{&normalizer}, Mode::Strict, scope, Location(), Variance::Covariant);
+    Unifier unifier(NotNull<Normalizer>{&normalizer}, scope, Location(), Variance::Covariant);
 
     // Cost of normalization can be too high for autocomplete response time requirements
     unifier.normalize = false;
@@ -259,10 +261,22 @@ static void autocompleteProps(const Module& module, TypeArena* typeArena, NotNul
             // already populated, it takes precedence over the property we found just now.
             if (result.count(name) == 0 && name != kParseNameError)
             {
-                Luau::TypeId type = Luau::follow(prop.type());
+                Luau::TypeId type;
+
+                if (FFlag::DebugLuauReadWriteProperties)
+                {
+                    if (auto ty = prop.readType())
+                        type = follow(*ty);
+                    else
+                        continue;
+                }
+                else
+                    type = follow(prop.type());
+
                 TypeCorrectKind typeCorrect = indexType == PropIndexType::Key
                                                   ? TypeCorrectKind::Correct
                                                   : checkTypeCorrectKind(module, typeArena, builtinTypes, nodes.back(), {{}, {}}, type);
+
                 ParenthesesRecommendation parens =
                     indexType == PropIndexType::Key ? ParenthesesRecommendation::None : getParenRecommendation(type, nodes, typeCorrect);
 

Analysis/src/ConstraintGraphBuilder.cpp

@@ -755,8 +755,8 @@ ControlFlow ConstraintGraphBuilder::visit(const ScopePtr& scope, AstStatForIn* f
     // It is always ok to provide too few variables, so we give this pack a free tail.
     TypePackId variablePack = arena->addTypePack(std::move(variableTypes), arena->addTypePack(FreeTypePack{loopScope.get()}));
 
-    addConstraint(loopScope, getLocation(forIn->values), IterableConstraint{iterator, variablePack});
+    addConstraint(
-
+        loopScope, getLocation(forIn->values), IterableConstraint{iterator, variablePack, forIn->values.data[0], &module->astOverloadResolvedTypes});
     visit(loopScope, forIn->body);
 
     return ControlFlow::None;

Analysis/src/ConstraintSolver.cpp

@@ -20,7 +20,6 @@
 #include "Luau/VisitType.h"
 
 LUAU_FASTFLAGVARIABLE(DebugLuauLogSolver, false);
-LUAU_FASTFLAG(LuauRequirePathTrueModuleName)
 
 namespace Luau
 {
@@ -252,6 +251,11 @@ struct InstantiationQueuer : TypeOnceVisitor
         solver->pushConstraint(scope, location, ReduceConstraint{ty});
         return true;
     }
+
+    bool visit(TypeId ty, const ClassType& ctv) override
+    {
+        return false;
+    }
 };
 
 ConstraintSolver::ConstraintSolver(NotNull<Normalizer> normalizer, NotNull<Scope> rootScope, std::vector<NotNull<Constraint>> constraints,
@@ -749,7 +753,10 @@ bool ConstraintSolver::tryDispatch(const BinaryConstraint& c, NotNull<const Cons
                     mmResult = builtinTypes->booleanType;
                     break;
                 default:
-                    mmResult = first(ftv->retTypes).value_or(errorRecoveryType());
+                    if (get<NeverType>(leftType) || get<NeverType>(rightType))
+                        mmResult = builtinTypes->neverType;
+                    else
+                        mmResult = first(ftv->retTypes).value_or(errorRecoveryType());
                 }
 
                 asMutable(resultType)->ty.emplace<BoundType>(mmResult);
@@ -785,6 +792,13 @@ bool ConstraintSolver::tryDispatch(const BinaryConstraint& c, NotNull<const Cons
             unblock(resultType);
             return true;
         }
+        else if (get<NeverType>(leftType) || get<NeverType>(rightType))
+        {
+            unify(leftType, rightType, constraint->scope);
+            asMutable(resultType)->ty.emplace<BoundType>(builtinTypes->neverType);
+            unblock(resultType);
+            return true;
+        }
 
         break;
     }
@@ -800,6 +814,13 @@ bool ConstraintSolver::tryDispatch(const BinaryConstraint& c, NotNull<const Cons
             unblock(resultType);
             return true;
         }
+        else if (get<NeverType>(leftType) || get<NeverType>(rightType))
+        {
+            unify(leftType, rightType, constraint->scope);
+            asMutable(resultType)->ty.emplace<BoundType>(builtinTypes->neverType);
+            unblock(resultType);
+            return true;
+        }
 
         break;
     // Inexact comparisons require that the types be both numbers or both
@@ -808,7 +829,8 @@ bool ConstraintSolver::tryDispatch(const BinaryConstraint& c, NotNull<const Cons
     case AstExprBinary::Op::CompareGt:
     case AstExprBinary::Op::CompareLe:
     case AstExprBinary::Op::CompareLt:
-        if ((isNumber(leftType) && isNumber(rightType)) || (isString(leftType) && isString(rightType)))
+        if ((isNumber(leftType) && isNumber(rightType)) || (isString(leftType) && isString(rightType)) || get<NeverType>(leftType) ||
+            get<NeverType>(rightType))
         {
             asMutable(resultType)->ty.emplace<BoundType>(builtinTypes->booleanType);
             unblock(resultType);
@@ -1291,7 +1313,7 @@ bool ConstraintSolver::tryDispatch(const FunctionCallConstraint& c, NotNull<cons
             return true;
         }
 
-        Unifier u{normalizer, Mode::Strict, constraint->scope, Location{}, Covariant};
+        Unifier u{normalizer, constraint->scope, Location{}, Covariant};
         u.enableScopeTests();
 
         u.tryUnify(*instantiated, inferredTy, /* isFunctionCall */ true);
@@ -1344,7 +1366,7 @@ bool ConstraintSolver::tryDispatch(const FunctionCallConstraint& c, NotNull<cons
     }
 
     // We found no matching overloads.
-    Unifier u{normalizer, Mode::Strict, constraint->scope, Location{}, Covariant};
+    Unifier u{normalizer, constraint->scope, Location{}, Covariant};
     u.enableScopeTests();
 
     u.tryUnify(inferredTy, builtinTypes->anyType);
@@ -1746,6 +1768,11 @@ struct FindRefineConstraintBlockers : TypeOnceVisitor
         found.insert(ty);
         return false;
     }
+
+    bool visit(TypeId ty, const ClassType&) override
+    {
+        return false;
+    }
 };
 
 }
@@ -1932,6 +1959,15 @@ bool ConstraintSolver::tryDispatchIterableTable(TypeId iteratorTy, const Iterabl
             unify(*errorified, ty, constraint->scope);
     };
 
+    auto neverify = [&](auto ty) {
+        Anyification anyify{arena, constraint->scope, builtinTypes, &iceReporter, builtinTypes->neverType, builtinTypes->neverTypePack};
+        std::optional neverified = anyify.substitute(ty);
+        if (!neverified)
+            reportError(CodeTooComplex{}, constraint->location);
+        else
+            unify(*neverified, ty, constraint->scope);
+    };
+
     if (get<AnyType>(iteratorTy))
     {
         anyify(c.variables);
@@ -1944,6 +1980,12 @@ bool ConstraintSolver::tryDispatchIterableTable(TypeId iteratorTy, const Iterabl
         return true;
     }
 
+    if (get<NeverType>(iteratorTy))
+    {
+        neverify(c.variables);
+        return true;
+    }
+
     // Irksome: I don't think we have any way to guarantee that this table
     // type never has a metatable.
 
@@ -2072,7 +2114,11 @@ bool ConstraintSolver::tryDispatchIterableFunction(
     const TypePackId nextRetPack = arena->addTypePack(TypePack{{retIndex}, valueTailTy});
 
     const TypeId expectedNextTy = arena->addType(FunctionType{TypeLevel{}, constraint->scope, nextArgPack, nextRetPack});
-    unify(nextTy, expectedNextTy, constraint->scope);
+    ErrorVec errors = unify(nextTy, expectedNextTy, constraint->scope);
+
+    // if there are no errors from unifying the two, we can pass forward the expected type as our selected resolution.
+    if (errors.empty())
+        (*c.astOverloadResolvedTypes)[c.nextAstFragment] = expectedNextTy;
 
     auto it = begin(nextRetPack);
     std::vector<TypeId> modifiedNextRetHead;
@@ -2122,7 +2168,7 @@ std::pair<std::vector<TypeId>, std::optional<TypeId>> ConstraintSolver::lookupTa
     else if (auto ttv = getMutable<TableType>(subjectType))
     {
         if (auto prop = ttv->props.find(propName); prop != ttv->props.end())
-            return {{}, prop->second.type()};
+            return {{}, FFlag::DebugLuauReadWriteProperties ? prop->second.readType() : prop->second.type()};
         else if (ttv->indexer && maybeString(ttv->indexer->indexType))
             return {{}, ttv->indexer->indexResultType};
         else if (ttv->state == TableState::Free)
@@ -2275,7 +2321,7 @@ static TypePackId getErrorType(NotNull<BuiltinTypes> builtinTypes, TypePackId)
 template <typename TID>
 bool ConstraintSolver::tryUnify(NotNull<const Constraint> constraint, TID subTy, TID superTy)
 {
-    Unifier u{normalizer, Mode::Strict, constraint->scope, constraint->location, Covariant};
+    Unifier u{normalizer, constraint->scope, constraint->location, Covariant};
     u.enableScopeTests();
 
     u.tryUnify(subTy, superTy);
@@ -2379,12 +2425,17 @@ struct Blocker : TypeOnceVisitor
     {
     }
 
-    bool visit(TypeId ty, const PendingExpansionType&)
+    bool visit(TypeId ty, const PendingExpansionType&) override
     {
         blocked = true;
         solver->block(ty, constraint);
         return false;
     }
+
+    bool visit(TypeId ty, const ClassType&) override
+    {
+        return false;
+    }
 };
 
 bool ConstraintSolver::blockOnPendingTypes(TypeId target, NotNull<const Constraint> constraint)
@@ -2492,9 +2543,9 @@ bool ConstraintSolver::isBlocked(NotNull<const Constraint> constraint)
     return blockedIt != blockedConstraints.end() && blockedIt->second > 0;
 }
 
-void ConstraintSolver::unify(TypeId subType, TypeId superType, NotNull<Scope> scope)
+ErrorVec ConstraintSolver::unify(TypeId subType, TypeId superType, NotNull<Scope> scope)
 {
-    Unifier u{normalizer, Mode::Strict, scope, Location{}, Covariant};
+    Unifier u{normalizer, scope, Location{}, Covariant};
     u.enableScopeTests();
 
     u.tryUnify(subType, superType);
@@ -2512,12 +2563,14 @@ void ConstraintSolver::unify(TypeId subType, TypeId superType, NotNull<Scope> sc
 
     unblock(changedTypes);
     unblock(changedPacks);
+
+    return std::move(u.errors);
 }
 
-void ConstraintSolver::unify(TypePackId subPack, TypePackId superPack, NotNull<Scope> scope)
+ErrorVec ConstraintSolver::unify(TypePackId subPack, TypePackId superPack, NotNull<Scope> scope)
 {
     UnifierSharedState sharedState{&iceReporter};
-    Unifier u{normalizer, Mode::Strict, scope, Location{}, Covariant};
+    Unifier u{normalizer, scope, Location{}, Covariant};
     u.enableScopeTests();
 
     u.tryUnify(subPack, superPack);
@@ -2528,6 +2581,8 @@ void ConstraintSolver::unify(TypePackId subPack, TypePackId superPack, NotNull<S
 
     unblock(changedTypes);
     unblock(changedPacks);
+
+    return std::move(u.errors);
 }
 
 NotNull<Constraint> ConstraintSolver::pushConstraint(NotNull<Scope> scope, const Location& location, ConstraintV cv)
@@ -2550,7 +2605,7 @@ TypeId ConstraintSolver::resolveModule(const ModuleInfo& info, const Location& l
 
     for (const auto& [location, path] : requireCycles)
     {
-        if (!path.empty() && path.front() == (FFlag::LuauRequirePathTrueModuleName ? info.name : moduleResolver->getHumanReadableModuleName(info.name)))
+        if (!path.empty() && path.front() == info.name)
             return builtinTypes->anyType;
     }
 
@@ -2612,7 +2667,7 @@ TypeId ConstraintSolver::unionOfTypes(TypeId a, TypeId b, NotNull<Scope> scope,
 
     if (unifyFreeTypes && (get<FreeType>(a) || get<FreeType>(b)))
     {
-        Unifier u{normalizer, Mode::Strict, scope, Location{}, Covariant};
+        Unifier u{normalizer, scope, Location{}, Covariant};
         u.enableScopeTests();
         u.tryUnify(b, a);
 

Analysis/src/Error.cpp

@@ -11,7 +11,6 @@
 #include <type_traits>
 
 LUAU_FASTFLAGVARIABLE(LuauTypeMismatchInvarianceInError, false)
-LUAU_FASTFLAGVARIABLE(LuauRequirePathTrueModuleName, false)
 
 static std::string wrongNumberOfArgsString(
     size_t expectedCount, std::optional<size_t> maximumCount, size_t actualCount, const char* argPrefix = nullptr, bool isVariadic = false)
@@ -350,7 +349,7 @@ struct ErrorConverter
             else
                 s += " -> ";
 
-            if (FFlag::LuauRequirePathTrueModuleName && fileResolver != nullptr)
+            if (fileResolver != nullptr)
                 s += fileResolver->getHumanReadableModuleName(name);
             else
                 s += name;
@@ -494,6 +493,16 @@ struct ErrorConverter
     {
         return "Type pack family instance " + Luau::toString(e.tp) + " is uninhabited";
     }
+
+    std::string operator()(const WhereClauseNeeded& e) const
+    {
+        return "Type family instance " + Luau::toString(e.ty) + " depends on generic function parameters but does not appear in the function signature; this construct cannot be type-checked at this time";
+    }
+
+    std::string operator()(const PackWhereClauseNeeded& e) const
+    {
+        return "Type pack family instance " + Luau::toString(e.tp) + " depends on generic function parameters but does not appear in the function signature; this construct cannot be type-checked at this time";
+    }
 };
 
 struct InvalidNameChecker
@@ -806,6 +815,16 @@ bool UninhabitedTypePackFamily::operator==(const UninhabitedTypePackFamily& rhs)
     return tp == rhs.tp;
 }
 
+bool WhereClauseNeeded::operator==(const WhereClauseNeeded& rhs) const
+{
+    return ty == rhs.ty;
+}
+
+bool PackWhereClauseNeeded::operator==(const PackWhereClauseNeeded& rhs) const
+{
+    return tp == rhs.tp;
+}
+
 std::string toString(const TypeError& error)
 {
     return toString(error, TypeErrorToStringOptions{});
@@ -968,6 +987,10 @@ void copyError(T& e, TypeArena& destArena, CloneState cloneState)
         e.ty = clone(e.ty);
     else if constexpr (std::is_same_v<T, UninhabitedTypePackFamily>)
         e.tp = clone(e.tp);
+    else if constexpr (std::is_same_v<T, WhereClauseNeeded>)
+        e.ty = clone(e.ty);
+    else if constexpr (std::is_same_v<T, PackWhereClauseNeeded>)
+        e.tp = clone(e.tp);
     else
         static_assert(always_false_v<T>, "Non-exhaustive type switch");
 }

Analysis/src/Frontend.cpp

@@ -34,9 +34,7 @@ LUAU_FASTFLAGVARIABLE(LuauKnowsTheDataModel3, false)
 LUAU_FASTINTVARIABLE(LuauAutocompleteCheckTimeoutMs, 100)
 LUAU_FASTFLAGVARIABLE(DebugLuauDeferredConstraintResolution, false)
 LUAU_FASTFLAGVARIABLE(DebugLuauLogSolverToJson, false)
-LUAU_FASTFLAG(LuauRequirePathTrueModuleName)
 LUAU_FASTFLAGVARIABLE(DebugLuauReadWriteProperties, false)
-LUAU_FASTFLAGVARIABLE(LuauSplitFrontendProcessing, false)
 LUAU_FASTFLAGVARIABLE(LuauTypeCheckerUseCorrectScope, false)
 
 namespace Luau
@@ -349,9 +347,9 @@ std::vector<RequireCycle> getRequireCycles(const FileResolver* resolver,
                 if (top == start)
                 {
                     for (const SourceNode* node : path)
-                        cycle.push_back(FFlag::LuauRequirePathTrueModuleName ? node->name : node->humanReadableName);
+                        cycle.push_back(node->name);
 
-                    cycle.push_back(FFlag::LuauRequirePathTrueModuleName ? top->name : top->humanReadableName);
+                    cycle.push_back(top->name);
                     break;
                 }
             }
@@ -419,9 +417,6 @@ Frontend::Frontend(FileResolver* fileResolver, ConfigResolver* configResolver, c
 
 CheckResult Frontend::check(const ModuleName& name, std::optional<FrontendOptions> optionOverride)
 {
-    if (!FFlag::LuauSplitFrontendProcessing)
-        return check_DEPRECATED(name, optionOverride);
-
     LUAU_TIMETRACE_SCOPE("Frontend::check", "Frontend");
     LUAU_TIMETRACE_ARGUMENT("name", name.c_str());
 
@@ -463,200 +458,6 @@ CheckResult Frontend::check(const ModuleName& name, std::optional<FrontendOption
     return checkResult;
 }
 
-CheckResult Frontend::check_DEPRECATED(const ModuleName& name, std::optional<FrontendOptions> optionOverride)
-{
-    LUAU_TIMETRACE_SCOPE("Frontend::check", "Frontend");
-    LUAU_TIMETRACE_ARGUMENT("name", name.c_str());
-
-    FrontendOptions frontendOptions = optionOverride.value_or(options);
-    CheckResult checkResult;
-
-    FrontendModuleResolver& resolver = frontendOptions.forAutocomplete ? moduleResolverForAutocomplete : moduleResolver;
-
-    auto it = sourceNodes.find(name);
-    if (it != sourceNodes.end() && !it->second->hasDirtyModule(frontendOptions.forAutocomplete))
-    {
-        // No recheck required.
-        ModulePtr module = resolver.getModule(name);
-
-        if (!module)
-            throw InternalCompilerError("Frontend::modules does not have data for " + name, name);
-
-        checkResult.errors = accumulateErrors(sourceNodes, resolver, name);
-
-        // Get lint result only for top checked module
-        checkResult.lintResult = module->lintResult;
-
-        return checkResult;
-    }
-
-    std::vector<ModuleName> buildQueue;
-    bool cycleDetected = parseGraph(buildQueue, name, frontendOptions.forAutocomplete);
-
-    for (const ModuleName& moduleName : buildQueue)
-    {
-        LUAU_ASSERT(sourceNodes.count(moduleName));
-        SourceNode& sourceNode = *sourceNodes[moduleName];
-
-        if (!sourceNode.hasDirtyModule(frontendOptions.forAutocomplete))
-            continue;
-
-        LUAU_ASSERT(sourceModules.count(moduleName));
-        SourceModule& sourceModule = *sourceModules[moduleName];
-
-        const Config& config = configResolver->getConfig(moduleName);
-
-        Mode mode = sourceModule.mode.value_or(config.mode);
-
-        ScopePtr environmentScope = getModuleEnvironment(sourceModule, config, frontendOptions.forAutocomplete);
-
-        double timestamp = getTimestamp();
-
-        std::vector<RequireCycle> requireCycles;
-
-        // in NoCheck mode we only need to compute the value of .cyclic for typeck
-        // in the future we could replace toposort with an algorithm that can flag cyclic nodes by itself
-        // however, for now getRequireCycles isn't expensive in practice on the cases we care about, and long term
-        // all correct programs must be acyclic so this code triggers rarely
-        if (cycleDetected)
-            requireCycles = getRequireCycles(fileResolver, sourceNodes, &sourceNode, mode == Mode::NoCheck);
-
-        // This is used by the type checker to replace the resulting type of cyclic modules with any
-        sourceModule.cyclic = !requireCycles.empty();
-
-        if (frontendOptions.forAutocomplete)
-        {
-            double autocompleteTimeLimit = FInt::LuauAutocompleteCheckTimeoutMs / 1000.0;
-
-            // The autocomplete typecheck is always in strict mode with DM awareness
-            // to provide better type information for IDE features
-            TypeCheckLimits typeCheckLimits;
-
-            if (autocompleteTimeLimit != 0.0)
-                typeCheckLimits.finishTime = TimeTrace::getClock() + autocompleteTimeLimit;
-            else
-                typeCheckLimits.finishTime = std::nullopt;
-
-            // TODO: This is a dirty ad hoc solution for autocomplete timeouts
-            // We are trying to dynamically adjust our existing limits to lower total typechecking time under the limit
-            // so that we'll have type information for the whole file at lower quality instead of a full abort in the middle
-            if (FInt::LuauTarjanChildLimit > 0)
-                typeCheckLimits.instantiationChildLimit = std::max(1, int(FInt::LuauTarjanChildLimit * sourceNode.autocompleteLimitsMult));
-            else
-                typeCheckLimits.instantiationChildLimit = std::nullopt;
-
-            if (FInt::LuauTypeInferIterationLimit > 0)
-                typeCheckLimits.unifierIterationLimit = std::max(1, int(FInt::LuauTypeInferIterationLimit * sourceNode.autocompleteLimitsMult));
-            else
-                typeCheckLimits.unifierIterationLimit = std::nullopt;
-
-            ModulePtr moduleForAutocomplete = check(sourceModule, Mode::Strict, requireCycles, environmentScope, /*forAutocomplete*/ true,
-                /*recordJsonLog*/ false, typeCheckLimits);
-
-            resolver.setModule(moduleName, moduleForAutocomplete);
-
-            double duration = getTimestamp() - timestamp;
-
-            if (moduleForAutocomplete->timeout)
-            {
-                checkResult.timeoutHits.push_back(moduleName);
-
-                sourceNode.autocompleteLimitsMult = sourceNode.autocompleteLimitsMult / 2.0;
-            }
-            else if (duration < autocompleteTimeLimit / 2.0)
-            {
-                sourceNode.autocompleteLimitsMult = std::min(sourceNode.autocompleteLimitsMult * 2.0, 1.0);
-            }
-
-            stats.timeCheck += duration;
-            stats.filesStrict += 1;
-
-            sourceNode.dirtyModuleForAutocomplete = false;
-            continue;
-        }
-
-        const bool recordJsonLog = FFlag::DebugLuauLogSolverToJson && moduleName == name;
-        ModulePtr module = check(sourceModule, mode, requireCycles, environmentScope, /*forAutocomplete*/ false, recordJsonLog, {});
-
-        stats.timeCheck += getTimestamp() - timestamp;
-        stats.filesStrict += mode == Mode::Strict;
-        stats.filesNonstrict += mode == Mode::Nonstrict;
-
-        if (module == nullptr)
-            throw InternalCompilerError("Frontend::check produced a nullptr module for " + moduleName, moduleName);
-
-        if (FFlag::DebugLuauDeferredConstraintResolution && mode == Mode::NoCheck)
-            module->errors.clear();
-
-        if (frontendOptions.runLintChecks)
-        {
-            LUAU_TIMETRACE_SCOPE("lint", "Frontend");
-
-            LintOptions lintOptions = frontendOptions.enabledLintWarnings.value_or(config.enabledLint);
-            filterLintOptions(lintOptions, sourceModule.hotcomments, mode);
-
-            double timestamp = getTimestamp();
-
-            std::vector<LintWarning> warnings =
-                Luau::lint(sourceModule.root, *sourceModule.names, environmentScope, module.get(), sourceModule.hotcomments, lintOptions);
-
-            stats.timeLint += getTimestamp() - timestamp;
-
-            module->lintResult = classifyLints(warnings, config);
-        }
-
-        if (!frontendOptions.retainFullTypeGraphs)
-        {
-            // copyErrors needs to allocate into interfaceTypes as it copies
-            // types out of internalTypes, so we unfreeze it here.
-            unfreeze(module->interfaceTypes);
-            copyErrors(module->errors, module->interfaceTypes);
-            freeze(module->interfaceTypes);
-
-            module->internalTypes.clear();
-
-            module->astTypes.clear();
-            module->astTypePacks.clear();
-            module->astExpectedTypes.clear();
-            module->astOriginalCallTypes.clear();
-            module->astOverloadResolvedTypes.clear();
-            module->astResolvedTypes.clear();
-            module->astResolvedTypePacks.clear();
-            module->astScopes.clear();
-
-            module->scopes.clear();
-        }
-
-        if (mode != Mode::NoCheck)
-        {
-            for (const RequireCycle& cyc : requireCycles)
-            {
-                TypeError te{cyc.location, moduleName, ModuleHasCyclicDependency{cyc.path}};
-
-                module->errors.push_back(te);
-            }
-        }
-
-        ErrorVec parseErrors;
-
-        for (const ParseError& pe : sourceModule.parseErrors)
-            parseErrors.push_back(TypeError{pe.getLocation(), moduleName, SyntaxError{pe.what()}});
-
-        module->errors.insert(module->errors.begin(), parseErrors.begin(), parseErrors.end());
-
-        checkResult.errors.insert(checkResult.errors.end(), module->errors.begin(), module->errors.end());
-
-        resolver.setModule(moduleName, std::move(module));
-        sourceNode.dirtyModule = false;
-    }
-
-    // Get lint result only for top checked module
-    if (ModulePtr module = resolver.getModule(name))
-        checkResult.lintResult = module->lintResult;
-
-    return checkResult;
-}
-
 void Frontend::queueModuleCheck(const std::vector<ModuleName>& names)
 {
     moduleQueue.insert(moduleQueue.end(), names.begin(), names.end());
@@ -996,8 +797,6 @@ bool Frontend::parseGraph(
 void Frontend::addBuildQueueItems(std::vector<BuildQueueItem>& items, std::vector<ModuleName>& buildQueue, bool cycleDetected,
     std::unordered_set<Luau::ModuleName>& seen, const FrontendOptions& frontendOptions)
 {
-    LUAU_ASSERT(FFlag::LuauSplitFrontendProcessing);
-
     for (const ModuleName& moduleName : buildQueue)
     {
         if (seen.count(moduleName))
@@ -1038,8 +837,6 @@ void Frontend::addBuildQueueItems(std::vector<BuildQueueItem>& items, std::vecto
 
 void Frontend::checkBuildQueueItem(BuildQueueItem& item)
 {
-    LUAU_ASSERT(FFlag::LuauSplitFrontendProcessing);
-
     SourceNode& sourceNode = *item.sourceNode;
     const SourceModule& sourceModule = *item.sourceModule;
     const Config& config = item.config;
@@ -1139,7 +936,8 @@ void Frontend::checkBuildQueueItem(BuildQueueItem& item)
         module->astResolvedTypePacks.clear();
         module->astScopes.clear();
 
-        module->scopes.clear();
+        if (!FFlag::DebugLuauDeferredConstraintResolution)
+            module->scopes.clear();
     }
 
     if (mode != Mode::NoCheck)
@@ -1164,8 +962,6 @@ void Frontend::checkBuildQueueItem(BuildQueueItem& item)
 
 void Frontend::checkBuildQueueItems(std::vector<BuildQueueItem>& items)
 {
-    LUAU_ASSERT(FFlag::LuauSplitFrontendProcessing);
-
     for (BuildQueueItem& item : items)
     {
         checkBuildQueueItem(item);

Analysis/src/IostreamHelpers.cpp

@@ -196,6 +196,10 @@ static void errorToString(std::ostream& stream, const T& err)
         stream << "UninhabitedTypeFamily { " << toString(err.ty) << " }";
     else if constexpr (std::is_same_v<T, UninhabitedTypePackFamily>)
         stream << "UninhabitedTypePackFamily { " << toString(err.tp) << " }";
+    else if constexpr (std::is_same_v<T, WhereClauseNeeded>)
+        stream << "WhereClauseNeeded { " << toString(err.ty) << " }";
+    else if constexpr (std::is_same_v<T, PackWhereClauseNeeded>)
+        stream << "PackWhereClauseNeeded { " << toString(err.tp) << " }";
     else
         static_assert(always_false_v<T>, "Non-exhaustive type switch");
 }

Analysis/src/Module.cpp

@@ -19,6 +19,7 @@ LUAU_FASTFLAGVARIABLE(LuauClonePublicInterfaceLess2, false);
 LUAU_FASTFLAG(LuauSubstitutionReentrant);
 LUAU_FASTFLAG(LuauClassTypeVarsInSubstitution);
 LUAU_FASTFLAG(LuauSubstitutionFixMissingFields);
+LUAU_FASTFLAGVARIABLE(LuauCloneSkipNonInternalVisit, false);
 
 namespace Luau
 {
@@ -98,6 +99,22 @@ struct ClonePublicInterface : Substitution
         return tp->owningArena == &module->internalTypes;
     }
 
+    bool ignoreChildrenVisit(TypeId ty) override
+    {
+        if (FFlag::LuauCloneSkipNonInternalVisit && ty->owningArena != &module->internalTypes)
+            return true;
+
+        return false;
+    }
+
+    bool ignoreChildrenVisit(TypePackId tp) override
+    {
+        if (FFlag::LuauCloneSkipNonInternalVisit && tp->owningArena != &module->internalTypes)
+            return true;
+
+        return false;
+    }
+
     TypeId clean(TypeId ty) override
     {
         TypeId result = clone(ty);

Analysis/src/Normalize.cpp

@@ -21,6 +21,7 @@ LUAU_FASTFLAGVARIABLE(LuauNormalizeBlockedTypes, false);
 LUAU_FASTFLAG(DebugLuauDeferredConstraintResolution)
 LUAU_FASTFLAG(LuauUninhabitedSubAnything2)
 LUAU_FASTFLAG(LuauTransitiveSubtyping)
+LUAU_FASTFLAG(DebugLuauReadWriteProperties)
 
 namespace Luau
 {
@@ -277,6 +278,22 @@ bool Normalizer::isInhabited(const NormalizedType* norm, std::unordered_set<Type
     return false;
 }
 
+bool Normalizer::isInhabited(TypeId ty)
+{
+    if (cacheInhabitance)
+    {
+        if (bool* result = cachedIsInhabited.find(ty))
+            return *result;
+    }
+
+    bool result = isInhabited(ty, {});
+
+    if (cacheInhabitance)
+        cachedIsInhabited[ty] = result;
+
+    return result;
+}
+
 bool Normalizer::isInhabited(TypeId ty, std::unordered_set<TypeId> seen)
 {
     // TODO: use log.follow(ty), CLI-64291
@@ -297,8 +314,18 @@ bool Normalizer::isInhabited(TypeId ty, std::unordered_set<TypeId> seen)
     {
         for (const auto& [_, prop] : ttv->props)
         {
-            if (!isInhabited(prop.type(), seen))
+            if (FFlag::DebugLuauReadWriteProperties)
-                return false;
+            {
+                // A table enclosing a read property whose type is uninhabitable is also itself uninhabitable,
+                // but not its write property. That just means the write property doesn't exist, and so is readonly.
+                if (auto ty = prop.readType(); ty && !isInhabited(*ty, seen))
+                    return false;
+            }
+            else
+            {
+                if (!isInhabited(prop.type(), seen))
+                    return false;
+            }
         }
         return true;
     }
@@ -314,14 +341,32 @@ bool Normalizer::isIntersectionInhabited(TypeId left, TypeId right)
 {
     left = follow(left);
     right = follow(right);
+
+    if (cacheInhabitance)
+    {
+        if (bool* result = cachedIsInhabitedIntersection.find({left, right}))
+            return *result;
+    }
+
     std::unordered_set<TypeId> seen = {};
     seen.insert(left);
     seen.insert(right);
 
     NormalizedType norm{builtinTypes};
     if (!normalizeIntersections({left, right}, norm))
+    {
+        if (cacheInhabitance)
+            cachedIsInhabitedIntersection[{left, right}] = false;
+
         return false;
-    return isInhabited(&norm, seen);
+    }
+
+    bool result = isInhabited(&norm, seen);
+
+    if (cacheInhabitance)
+        cachedIsInhabitedIntersection[{left, right}] = result;
+
+    return result;
 }
 
 static int tyvarIndex(TypeId ty)
@@ -568,10 +613,11 @@ static void assertInvariant(const NormalizedType& norm)
 #endif
 }
 
-Normalizer::Normalizer(TypeArena* arena, NotNull<BuiltinTypes> builtinTypes, NotNull<UnifierSharedState> sharedState)
+Normalizer::Normalizer(TypeArena* arena, NotNull<BuiltinTypes> builtinTypes, NotNull<UnifierSharedState> sharedState, bool cacheInhabitance)
     : arena(arena)
     , builtinTypes(builtinTypes)
     , sharedState(sharedState)
+    , cacheInhabitance(cacheInhabitance)
 {
 }
 
@@ -1315,7 +1361,8 @@ bool Normalizer::withinResourceLimits()
     // If cache is too large, clear it
     if (FInt::LuauNormalizeCacheLimit > 0)
     {
-        size_t cacheUsage = cachedNormals.size() + cachedIntersections.size() + cachedUnions.size() + cachedTypeIds.size();
+        size_t cacheUsage = cachedNormals.size() + cachedIntersections.size() + cachedUnions.size() + cachedTypeIds.size() +
+                            cachedIsInhabited.size() + cachedIsInhabitedIntersection.size();
         if (cacheUsage > size_t(FInt::LuauNormalizeCacheLimit))
         {
             clearCaches();
@@ -2726,7 +2773,7 @@ bool isSubtype(TypeId subTy, TypeId superTy, NotNull<Scope> scope, NotNull<Built
     UnifierSharedState sharedState{&ice};
     TypeArena arena;
     Normalizer normalizer{&arena, builtinTypes, NotNull{&sharedState}};
-    Unifier u{NotNull{&normalizer}, Mode::Strict, scope, Location{}, Covariant};
+    Unifier u{NotNull{&normalizer}, scope, Location{}, Covariant};
 
     u.tryUnify(subTy, superTy);
     return !u.failure;
@@ -2739,7 +2786,7 @@ bool isSubtype(TypePackId subPack, TypePackId superPack, NotNull<Scope> scope, N
     UnifierSharedState sharedState{&ice};
     TypeArena arena;
     Normalizer normalizer{&arena, builtinTypes, NotNull{&sharedState}};
-    Unifier u{NotNull{&normalizer}, Mode::Strict, scope, Location{}, Covariant};
+    Unifier u{NotNull{&normalizer}, scope, Location{}, Covariant};
 
     u.tryUnify(subPack, superPack);
     return !u.failure;
@@ -2750,7 +2797,7 @@ bool isConsistentSubtype(TypeId subTy, TypeId superTy, NotNull<Scope> scope, Not
     UnifierSharedState sharedState{&ice};
     TypeArena arena;
     Normalizer normalizer{&arena, builtinTypes, NotNull{&sharedState}};
-    Unifier u{NotNull{&normalizer}, Mode::Strict, scope, Location{}, Covariant};
+    Unifier u{NotNull{&normalizer}, scope, Location{}, Covariant};
 
     u.tryUnify(subTy, superTy);
     const bool ok = u.errors.empty() && u.log.empty();
@@ -2763,7 +2810,7 @@ bool isConsistentSubtype(
     UnifierSharedState sharedState{&ice};
     TypeArena arena;
     Normalizer normalizer{&arena, builtinTypes, NotNull{&sharedState}};
-    Unifier u{NotNull{&normalizer}, Mode::Strict, scope, Location{}, Covariant};
+    Unifier u{NotNull{&normalizer}, scope, Location{}, Covariant};
 
     u.tryUnify(subPack, superPack);
     const bool ok = u.errors.empty() && u.log.empty();

Analysis/src/Substitution.cpp

@@ -13,6 +13,8 @@ LUAU_FASTFLAG(LuauClonePublicInterfaceLess2)
 LUAU_FASTINTVARIABLE(LuauTarjanChildLimit, 10000)
 LUAU_FASTFLAGVARIABLE(LuauClassTypeVarsInSubstitution, false)
 LUAU_FASTFLAGVARIABLE(LuauSubstitutionReentrant, false)
+LUAU_FASTFLAG(DebugLuauReadWriteProperties)
+LUAU_FASTFLAG(LuauCloneSkipNonInternalVisit)
 
 namespace Luau
 {
@@ -214,7 +216,7 @@ void Tarjan::visitChildren(TypeId ty, int index)
 {
     LUAU_ASSERT(ty == log->follow(ty));
 
-    if (ignoreChildren(ty))
+    if (FFlag::LuauCloneSkipNonInternalVisit ? ignoreChildrenVisit(ty) : ignoreChildren(ty))
         return;
 
     if (auto pty = log->pending(ty))
@@ -237,7 +239,16 @@ void Tarjan::visitChildren(TypeId ty, int index)
     {
         LUAU_ASSERT(!ttv->boundTo);
         for (const auto& [name, prop] : ttv->props)
-            visitChild(prop.type());
+        {
+            if (FFlag::DebugLuauReadWriteProperties)
+            {
+                visitChild(prop.readType());
+                visitChild(prop.writeType());
+            }
+            else
+                visitChild(prop.type());
+        }
+
         if (ttv->indexer)
         {
             visitChild(ttv->indexer->indexType);
@@ -311,7 +322,7 @@ void Tarjan::visitChildren(TypePackId tp, int index)
 {
     LUAU_ASSERT(tp == log->follow(tp));
 
-    if (ignoreChildren(tp))
+    if (FFlag::LuauCloneSkipNonInternalVisit ? ignoreChildrenVisit(tp) : ignoreChildren(tp))
         return;
 
     if (auto ptp = log->pending(tp))
@@ -793,7 +804,13 @@ void Substitution::replaceChildren(TypeId ty)
     {
         LUAU_ASSERT(!ttv->boundTo);
         for (auto& [name, prop] : ttv->props)
-            prop.setType(replace(prop.type()));
+        {
+            if (FFlag::DebugLuauReadWriteProperties)
+                prop = Property::create(replace(prop.readType()), replace(prop.writeType()));
+            else
+                prop.setType(replace(prop.type()));
+        }
+
         if (ttv->indexer)
         {
             ttv->indexer->indexType = replace(ttv->indexer->indexType);

Analysis/src/ToString.cpp

@@ -335,6 +335,44 @@ struct TypeStringifier
             tv->ty);
     }
 
+    void stringify(const std::string& name, const Property& prop)
+    {
+        if (isIdentifier(name))
+            state.emit(name);
+        else
+        {
+            state.emit("[\"");
+            state.emit(escape(name));
+            state.emit("\"]");
+        }
+        state.emit(": ");
+
+        if (FFlag::DebugLuauReadWriteProperties)
+        {
+            // We special case the stringification if the property's read and write types are shared.
+            if (prop.isShared())
+                return stringify(*prop.readType());
+
+            // Otherwise emit them separately.
+            if (auto ty = prop.readType())
+            {
+                state.emit("read ");
+                stringify(*ty);
+            }
+
+            if (prop.readType() && prop.writeType())
+                state.emit(" + ");
+
+            if (auto ty = prop.writeType())
+            {
+                state.emit("write ");
+                stringify(*ty);
+            }
+        }
+        else
+            stringify(prop.type());
+    }
+
     void stringify(TypePackId tp);
     void stringify(TypePackId tpid, const std::vector<std::optional<FunctionArgument>>& names);
 
@@ -672,16 +710,8 @@ struct TypeStringifier
                 break;
             }
 
-            if (isIdentifier(name))
+            stringify(name, prop);
-                state.emit(name);
+
-            else
-            {
-                state.emit("[\"");
-                state.emit(escape(name));
-                state.emit("\"]");
-            }
-            state.emit(": ");
-            stringify(prop.type());
             comma = true;
             ++index;
         }

Analysis/src/Type.cpp

@@ -27,7 +27,6 @@ LUAU_FASTINT(LuauTypeInferRecursionLimit)
 LUAU_FASTFLAG(LuauInstantiateInSubtyping)
 LUAU_FASTFLAG(LuauNormalizeBlockedTypes)
 LUAU_FASTFLAG(DebugLuauReadWriteProperties)
-LUAU_FASTFLAGVARIABLE(LuauBoundLazyTypes2, false)
 
 namespace Luau
 {
@@ -78,65 +77,31 @@ TypeId follow(TypeId t)
 TypeId follow(TypeId t, const void* context, TypeId (*mapper)(const void*, TypeId))
 {
     auto advance = [context, mapper](TypeId ty) -> std::optional<TypeId> {
-        if (FFlag::LuauBoundLazyTypes2)
-        {
-            TypeId mapped = mapper(context, ty);
-
-            if (auto btv = get<Unifiable::Bound<TypeId>>(mapped))
-                return btv->boundTo;
-
-            if (auto ttv = get<TableType>(mapped))
-                return ttv->boundTo;
-
-            if (auto ltv = getMutable<LazyType>(mapped))
-                return unwrapLazy(ltv);
-
-            return std::nullopt;
-        }
-        else
-        {
-            if (auto btv = get<Unifiable::Bound<TypeId>>(mapper(context, ty)))
-                return btv->boundTo;
-            else if (auto ttv = get<TableType>(mapper(context, ty)))
-                return ttv->boundTo;
-            else
-                return std::nullopt;
-        }
-    };
-
-    auto force = [context, mapper](TypeId ty) {
         TypeId mapped = mapper(context, ty);
 
-        if (auto ltv = get_if<LazyType>(&mapped->ty))
+        if (auto btv = get<Unifiable::Bound<TypeId>>(mapped))
-        {
+            return btv->boundTo;
-            TypeId res = ltv->thunk_DEPRECATED();
-            if (get<LazyType>(res))
-                throw InternalCompilerError("Lazy Type cannot resolve to another Lazy Type");
 
-            *asMutable(ty) = BoundType(res);
+        if (auto ttv = get<TableType>(mapped))
-        }
+            return ttv->boundTo;
+
+        if (auto ltv = getMutable<LazyType>(mapped))
+            return unwrapLazy(ltv);
+
+        return std::nullopt;
     };
 
-    if (!FFlag::LuauBoundLazyTypes2)
-        force(t);
-
     TypeId cycleTester = t; // Null once we've determined that there is no cycle
     if (auto a = advance(cycleTester))
         cycleTester = *a;
     else
         return t;
 
-    if (FFlag::LuauBoundLazyTypes2)
+    if (!advance(cycleTester)) // Short circuit traversal for the rather common case when advance(advance(t)) == null
-    {
+        return cycleTester;
-        if (!advance(cycleTester)) // Short circuit traversal for the rather common case when advance(advance(t)) == null
-            return cycleTester;
-    }
 
     while (true)
     {
-        if (!FFlag::LuauBoundLazyTypes2)
-            force(t);
-
         auto a1 = advance(t);
         if (a1)
             t = *a1;
@@ -684,16 +649,17 @@ Property Property::rw(TypeId read, TypeId write)
     return p;
 }
 
-std::optional<Property> Property::create(std::optional<TypeId> read, std::optional<TypeId> write)
+Property Property::create(std::optional<TypeId> read, std::optional<TypeId> write)
 {
     if (read && !write)
         return Property::readonly(*read);
     else if (!read && write)
         return Property::writeonly(*write);
-    else if (read && write)
-        return Property::rw(*read, *write);
     else
-        return std::nullopt;
+    {
+        LUAU_ASSERT(read && write);
+        return Property::rw(*read, *write);
+    }
 }
 
 TypeId Property::type() const
@@ -705,6 +671,7 @@ TypeId Property::type() const
 
 void Property::setType(TypeId ty)
 {
+    LUAU_ASSERT(!FFlag::DebugLuauReadWriteProperties);
     readTy = ty;
 }
 
@@ -722,6 +689,11 @@ std::optional<TypeId> Property::writeType() const
     return writeTy;
 }
 
+bool Property::isShared() const
+{
+    return readTy && writeTy && readTy == writeTy;
+}
+
 TableType::TableType(TableState state, TypeLevel level, Scope* scope)
     : state(state)
     , level(level)

Analysis/src/TypeChecker2.cpp

@@ -13,9 +13,11 @@
 #include "Luau/ToString.h"
 #include "Luau/TxnLog.h"
 #include "Luau/Type.h"
+#include "Luau/TypePack.h"
 #include "Luau/TypeUtils.h"
 #include "Luau/Unifier.h"
 #include "Luau/TypeFamily.h"
+#include "Luau/VisitType.h"
 
 #include <algorithm>
 
@@ -81,6 +83,146 @@ static std::optional<std::string> getIdentifierOfBaseVar(AstExpr* node)
     return std::nullopt;
 }
 
+template<typename T>
+bool areEquivalent(const T& a, const T& b)
+{
+    if (a.family != b.family)
+        return false;
+
+    if (a.typeArguments.size() != b.typeArguments.size() || a.packArguments.size() != b.packArguments.size())
+        return false;
+
+    for (size_t i = 0; i < a.typeArguments.size(); ++i)
+    {
+        if (follow(a.typeArguments[i]) != follow(b.typeArguments[i]))
+            return false;
+    }
+
+    for (size_t i = 0; i < a.packArguments.size(); ++i)
+    {
+        if (follow(a.packArguments[i]) != follow(b.packArguments[i]))
+            return false;
+    }
+
+    return true;
+}
+
+struct FamilyFinder : TypeOnceVisitor
+{
+    DenseHashSet<TypeId> mentionedFamilies{nullptr};
+    DenseHashSet<TypePackId> mentionedFamilyPacks{nullptr};
+
+    bool visit(TypeId ty, const TypeFamilyInstanceType&) override
+    {
+        mentionedFamilies.insert(ty);
+        return true;
+    }
+
+    bool visit(TypePackId tp, const TypeFamilyInstanceTypePack&) override
+    {
+        mentionedFamilyPacks.insert(tp);
+        return true;
+    }
+};
+
+struct InternalFamilyFinder : TypeOnceVisitor
+{
+    DenseHashSet<TypeId> internalFamilies{nullptr};
+    DenseHashSet<TypePackId> internalPackFamilies{nullptr};
+    DenseHashSet<TypeId> mentionedFamilies{nullptr};
+    DenseHashSet<TypePackId> mentionedFamilyPacks{nullptr};
+
+    InternalFamilyFinder(std::vector<TypeId>& declStack)
+    {
+        FamilyFinder f;
+        for (TypeId fn : declStack)
+            f.traverse(fn);
+
+        mentionedFamilies = std::move(f.mentionedFamilies);
+        mentionedFamilyPacks = std::move(f.mentionedFamilyPacks);
+    }
+
+    bool visit(TypeId ty, const TypeFamilyInstanceType& tfit) override
+    {
+        bool hasGeneric = false;
+
+        for (TypeId p : tfit.typeArguments)
+        {
+            if (get<GenericType>(follow(p)))
+            {
+                hasGeneric = true;
+                break;
+            }
+        }
+
+        for (TypePackId p : tfit.packArguments)
+        {
+            if (get<GenericTypePack>(follow(p)))
+            {
+                hasGeneric = true;
+                break;
+            }
+        }
+
+        if (hasGeneric)
+        {
+            for (TypeId mentioned : mentionedFamilies)
+            {
+                const TypeFamilyInstanceType* mentionedTfit = get<TypeFamilyInstanceType>(mentioned);
+                LUAU_ASSERT(mentionedTfit);
+                if (areEquivalent(tfit, *mentionedTfit))
+                {
+                    return true;
+                }
+            }
+
+            internalFamilies.insert(ty);
+        }
+
+        return true;
+    }
+
+    bool visit(TypePackId tp, const TypeFamilyInstanceTypePack& tfitp) override
+    {
+        bool hasGeneric = false;
+
+        for (TypeId p : tfitp.typeArguments)
+        {
+            if (get<GenericType>(follow(p)))
+            {
+                hasGeneric = true;
+                break;
+            }
+        }
+
+        for (TypePackId p : tfitp.packArguments)
+        {
+            if (get<GenericTypePack>(follow(p)))
+            {
+                hasGeneric = true;
+                break;
+            }
+        }
+
+        if (hasGeneric)
+        {
+            for (TypePackId mentioned : mentionedFamilyPacks)
+            {
+                const TypeFamilyInstanceTypePack* mentionedTfitp = get<TypeFamilyInstanceTypePack>(mentioned);
+                LUAU_ASSERT(mentionedTfitp);
+                if (areEquivalent(tfitp, *mentionedTfitp))
+                {
+                    return true;
+                }
+            }
+
+            internalPackFamilies.insert(tp);
+        }
+
+        return true;
+    }
+};
+
 struct TypeChecker2
 {
     NotNull<BuiltinTypes> builtinTypes;
@@ -91,16 +233,20 @@ struct TypeChecker2
     TypeArena testArena;
 
     std::vector<NotNull<Scope>> stack;
+    std::vector<TypeId> functionDeclStack;
+
+    DenseHashSet<TypeId> noTypeFamilyErrors{nullptr};
 
     Normalizer normalizer;
 
-    TypeChecker2(NotNull<BuiltinTypes> builtinTypes, NotNull<UnifierSharedState> unifierState, DcrLogger* logger, const SourceModule* sourceModule, Module* module)
+    TypeChecker2(NotNull<BuiltinTypes> builtinTypes, NotNull<UnifierSharedState> unifierState, DcrLogger* logger, const SourceModule* sourceModule,
+        Module* module)
         : builtinTypes(builtinTypes)
         , logger(logger)
         , ice(unifierState->iceHandler)
         , sourceModule(sourceModule)
         , module(module)
-        , normalizer{&testArena, builtinTypes, unifierState}
+        , normalizer{&testArena, builtinTypes, unifierState, /* cacheInhabitance */ true}
     {
     }
 
@@ -112,10 +258,31 @@ struct TypeChecker2
             return std::nullopt;
     }
 
+    void checkForInternalFamily(TypeId ty, Location location)
+    {
+        InternalFamilyFinder finder(functionDeclStack);
+        finder.traverse(ty);
+
+        for (TypeId internal : finder.internalFamilies)
+            reportError(WhereClauseNeeded{internal}, location);
+
+        for (TypePackId internal : finder.internalPackFamilies)
+            reportError(PackWhereClauseNeeded{internal}, location);
+    }
+
     TypeId checkForFamilyInhabitance(TypeId instance, Location location)
     {
+        if (noTypeFamilyErrors.find(instance))
+            return instance;
+
         TxnLog fake{};
-        reportErrors(reduceFamilies(instance, location, NotNull{&testArena}, builtinTypes, stack.back(), NotNull{&normalizer}, &fake, true).errors);
+        ErrorVec errors =
+            reduceFamilies(instance, location, NotNull{&testArena}, builtinTypes, stack.back(), NotNull{&normalizer}, &fake, true).errors;
+
+        if (errors.empty())
+            noTypeFamilyErrors.insert(instance);
+
+        reportErrors(std::move(errors));
         return instance;
     }
 
@@ -316,7 +483,7 @@ struct TypeChecker2
         TypeArena* arena = &testArena;
         TypePackId actualRetType = reconstructPack(ret->list, *arena);
 
-        Unifier u{NotNull{&normalizer}, Mode::Strict, stack.back(), ret->location, Covariant};
+        Unifier u{NotNull{&normalizer}, stack.back(), ret->location, Covariant};
         u.hideousFixMeGenericsAreActuallyFree = true;
 
         u.tryUnify(actualRetType, expectedRetType);
@@ -466,12 +633,47 @@ struct TypeChecker2
             variableTypes.emplace_back(*ty);
         }
 
-        // ugh.  There's nothing in the AST to hang a whole type pack on for the
+        AstExpr* firstValue = forInStatement->values.data[0];
-        // set of iteratees, so we have to piece it back together by hand.
+
+        // we need to build up a typepack for the iterators/values portion of the for-in statement.
         std::vector<TypeId> valueTypes;
-        for (size_t i = 0; i < forInStatement->values.size - 1; ++i)
+        std::optional<TypePackId> iteratorTail;
+
+        // since the first value may be the only iterator (e.g. if it is a call), we want to
+        // look to see if it has a resulting typepack as our iterators.
+        TypePackId* retPack = module->astTypePacks.find(firstValue);
+        if (retPack)
+        {
+            auto [head, tail] = flatten(*retPack);
+            valueTypes = head;
+            iteratorTail = tail;
+        }
+        else
+        {
+            valueTypes.emplace_back(lookupType(firstValue));
+        }
+
+        // if the initial and expected types from the iterator unified during constraint solving,
+        // we'll have a resolved type to use here, but we'll only use it if either the iterator is
+        // directly present in the for-in statement or if we have an iterator state constraining us
+        TypeId* resolvedTy = module->astOverloadResolvedTypes.find(firstValue);
+        if (resolvedTy && (!retPack || valueTypes.size() > 1))
+            valueTypes[0] = *resolvedTy;
+
+        for (size_t i = 1; i < forInStatement->values.size - 1; ++i)
+        {
             valueTypes.emplace_back(lookupType(forInStatement->values.data[i]));
-        TypePackId iteratorTail = lookupPack(forInStatement->values.data[forInStatement->values.size - 1]);
+        }
+
+        // if we had more than one value, the tail from the first value is no longer appropriate to use.
+        if (forInStatement->values.size > 1)
+        {
+            auto [head, tail] = flatten(lookupPack(forInStatement->values.data[forInStatement->values.size - 1]));
+            valueTypes.insert(valueTypes.end(), head.begin(), head.end());
+            iteratorTail = tail;
+        }
+
+        // and now we can put everything together to get the actual typepack of the iterators.
         TypePackId iteratorPack = arena.addTypePack(valueTypes, iteratorTail);
 
         // ... and then expand it out to 3 values (if possible)
@@ -518,26 +720,16 @@ struct TypeChecker2
             // This depends on the types in iterateePack and therefore
             // iteratorTypes.
 
+            // If the iteratee is an error type, then we can't really say anything else about iteration over it.
+            // After all, it _could've_ been a table.
+            if (get<ErrorType>(follow(flattenPack(iterFtv->argTypes))))
+                return;
+
             // If iteratorTypes is too short to be a valid call to nextFn, we have to report a count mismatch error.
             // If 2 is too short to be a valid call to nextFn, we have to report a count mismatch error.
             // If 2 is too long to be a valid call to nextFn, we have to report a count mismatch error.
             auto [minCount, maxCount] = getParameterExtents(TxnLog::empty(), iterFtv->argTypes, /*includeHiddenVariadics*/ true);
 
-            if (minCount > 2)
-            {
-                if (isMm)
-                    reportError(GenericError{"__iter metamethod must return (next[, table[, state]])"}, getLocation(forInStatement->values));
-                else
-                    reportError(GenericError{"for..in loops must be passed (next[, table[, state]])"}, getLocation(forInStatement->values));
-            }
-            if (maxCount && *maxCount < 2)
-            {
-                if (isMm)
-                    reportError(GenericError{"__iter metamethod must return (next[, table[, state]])"}, getLocation(forInStatement->values));
-                else
-                    reportError(GenericError{"for..in loops must be passed (next[, table[, state]])"}, getLocation(forInStatement->values));
-            }
-
             TypePack flattenedArgTypes = extendTypePack(arena, builtinTypes, iterFtv->argTypes, 2);
             size_t firstIterationArgCount = iterTys.empty() ? 0 : iterTys.size() - 1;
             size_t actualArgCount = expectedVariableTypes.head.size();
@@ -546,7 +738,7 @@ struct TypeChecker2
                 if (isMm)
                     reportError(GenericError{"__iter metamethod must return (next[, table[, state]])"}, getLocation(forInStatement->values));
                 else
-                    reportError(CountMismatch{2, std::nullopt, firstIterationArgCount, CountMismatch::Arg}, forInStatement->vars.data[0]->location);
+                    reportError(CountMismatch{2, std::nullopt, firstIterationArgCount, CountMismatch::Arg}, forInStatement->values.data[0]->location);
             }
 
             else if (actualArgCount < minCount)
@@ -554,7 +746,7 @@ struct TypeChecker2
                 if (isMm)
                     reportError(GenericError{"__iter metamethod must return (next[, table[, state]])"}, getLocation(forInStatement->values));
                 else
-                    reportError(CountMismatch{2, std::nullopt, firstIterationArgCount, CountMismatch::Arg}, forInStatement->vars.data[0]->location);
+                    reportError(CountMismatch{2, std::nullopt, firstIterationArgCount, CountMismatch::Arg}, forInStatement->values.data[0]->location);
             }
 
 
@@ -1211,6 +1403,7 @@ struct TypeChecker2
         visitGenerics(fn->generics, fn->genericPacks);
 
         TypeId inferredFnTy = lookupType(fn);
+        functionDeclStack.push_back(inferredFnTy);
 
         const NormalizedType* normalizedFnTy = normalizer.normalize(inferredFnTy);
         if (!normalizedFnTy)
@@ -1260,6 +1453,8 @@ struct TypeChecker2
         }
 
         visit(fn->body);
+
+        functionDeclStack.pop_back();
     }
 
     void visit(AstExprTable* expr)
@@ -1370,7 +1565,10 @@ struct TypeChecker2
         TypeId expectedResult = lookupType(expr);
 
         if (get<TypeFamilyInstanceType>(expectedResult))
+        {
+            checkForInternalFamily(expectedResult, expr->location);
             return expectedResult;
+        }
 
         if (expr->op == AstExprBinary::Op::Or)
         {
@@ -1379,9 +1577,9 @@ struct TypeChecker2
 
         bool isStringOperation = isString(leftType) && isString(rightType);
 
-        if (get<AnyType>(leftType) || get<ErrorType>(leftType))
+        if (get<AnyType>(leftType) || get<ErrorType>(leftType) || get<NeverType>(leftType))
             return leftType;
-        else if (get<AnyType>(rightType) || get<ErrorType>(rightType))
+        else if (get<AnyType>(rightType) || get<ErrorType>(rightType) || get<NeverType>(rightType))
             return rightType;
 
         if ((get<BlockedType>(leftType) || get<FreeType>(leftType) || get<GenericType>(leftType)) && !isEquality && !isLogical)
@@ -1982,7 +2180,7 @@ struct TypeChecker2
     bool isSubtype(TID subTy, TID superTy, NotNull<Scope> scope, bool genericsOkay = false)
     {
         TypeArena arena;
-        Unifier u{NotNull{&normalizer}, Mode::Strict, scope, Location{}, Covariant};
+        Unifier u{NotNull{&normalizer}, scope, Location{}, Covariant};
         u.hideousFixMeGenericsAreActuallyFree = genericsOkay;
         u.enableScopeTests();
 
@@ -1995,7 +2193,7 @@ struct TypeChecker2
     ErrorVec tryUnify(NotNull<Scope> scope, const Location& location, TID subTy, TID superTy, CountMismatch::Context context = CountMismatch::Arg,
         bool genericsOkay = false)
     {
-        Unifier u{NotNull{&normalizer}, Mode::Strict, scope, location, Covariant};
+        Unifier u{NotNull{&normalizer}, scope, location, Covariant};
         u.ctx = context;
         u.hideousFixMeGenericsAreActuallyFree = genericsOkay;
         u.enableScopeTests();

Analysis/src/TypeFamily.cpp

@@ -301,6 +301,9 @@ FamilyGraphReductionResult reduceFamilies(TypeId entrypoint, Location location,
         return FamilyGraphReductionResult{};
     }
 
+    if (collector.tys.empty() && collector.tps.empty())
+        return {};
+
     return reduceFamiliesInternal(std::move(collector.tys), std::move(collector.tps), location, arena, builtins, scope, normalizer, log, force);
 }
 
@@ -318,6 +321,9 @@ FamilyGraphReductionResult reduceFamilies(TypePackId entrypoint, Location locati
         return FamilyGraphReductionResult{};
     }
 
+    if (collector.tys.empty() && collector.tps.empty())
+        return {};
+
     return reduceFamiliesInternal(std::move(collector.tys), std::move(collector.tps), location, arena, builtins, scope, normalizer, log, force);
 }
 
@@ -338,8 +344,10 @@ TypeFamilyReductionResult<TypeId> addFamilyFn(std::vector<TypeId> typeParams, st
 
     TypeId lhsTy = log->follow(typeParams.at(0));
     TypeId rhsTy = log->follow(typeParams.at(1));
+    const NormalizedType* normLhsTy = normalizer->normalize(lhsTy);
+    const NormalizedType* normRhsTy = normalizer->normalize(rhsTy);
 
-    if (isNumber(lhsTy) && isNumber(rhsTy))
+    if (normLhsTy && normRhsTy && normLhsTy->isNumber() && normRhsTy->isNumber())
     {
         return {builtins->numberType, false, {}, {}};
     }
@@ -398,7 +406,7 @@ TypeFamilyReductionResult<TypeId> addFamilyFn(std::vector<TypeId> typeParams, st
                 inferredArgs = {rhsTy, lhsTy};
 
             TypePackId inferredArgPack = arena->addTypePack(std::move(inferredArgs));
-            Unifier u{normalizer, Mode::Strict, scope, Location{}, Variance::Covariant, log.get()};
+            Unifier u{normalizer, scope, Location{}, Variance::Covariant, log.get()};
             u.tryUnify(inferredArgPack, instantiatedMmFtv->argTypes);
 
             if (std::optional<TypeId> ret = first(instantiatedMmFtv->retTypes); ret && u.errors.empty())

Analysis/src/TypeInfer.cpp

@@ -39,7 +39,6 @@ LUAU_FASTFLAG(LuauUninhabitedSubAnything2)
 LUAU_FASTFLAG(LuauOccursIsntAlwaysFailure)
 LUAU_FASTFLAGVARIABLE(LuauTypecheckTypeguards, false)
 LUAU_FASTFLAGVARIABLE(LuauTinyControlFlowAnalysis, false)
-LUAU_FASTFLAG(LuauRequirePathTrueModuleName)
 LUAU_FASTFLAGVARIABLE(LuauTypecheckClassTypeIndexers, false)
 
 namespace Luau
@@ -2769,8 +2768,9 @@ TypeId TypeChecker::checkRelationalOperation(
 
         std::string metamethodName = opToMetaTableEntry(expr.op);
 
-        std::optional<TypeId> leftMetatable = isString(lhsType) ? std::nullopt : getMetatable(follow(lhsType), builtinTypes);
+        std::optional<TypeId> stringNoMT = std::nullopt; // works around gcc false positive "maybe uninitialized" warnings
-        std::optional<TypeId> rightMetatable = isString(rhsType) ? std::nullopt : getMetatable(follow(rhsType), builtinTypes);
+        std::optional<TypeId> leftMetatable = isString(lhsType) ? stringNoMT : getMetatable(follow(lhsType), builtinTypes);
+        std::optional<TypeId> rightMetatable = isString(rhsType) ? stringNoMT : getMetatable(follow(rhsType), builtinTypes);
 
         if (leftMetatable != rightMetatable)
         {
@@ -4676,7 +4676,7 @@ TypeId TypeChecker::checkRequire(const ScopePtr& scope, const ModuleInfo& module
     // Types of requires that transitively refer to current module have to be replaced with 'any'
     for (const auto& [location, path] : requireCycles)
     {
-        if (!path.empty() && path.front() == (FFlag::LuauRequirePathTrueModuleName ? moduleInfo.name : resolver->getHumanReadableModuleName(moduleInfo.name)))
+        if (!path.empty() && path.front() == moduleInfo.name)
             return anyType;
     }
 
@@ -5043,7 +5043,7 @@ void TypeChecker::merge(RefinementMap& l, const RefinementMap& r)
 
 Unifier TypeChecker::mkUnifier(const ScopePtr& scope, const Location& location)
 {
-    return Unifier{NotNull{&normalizer}, currentModule->mode, NotNull{scope.get()}, location, Variance::Covariant};
+    return Unifier{NotNull{&normalizer}, NotNull{scope.get()}, location, Variance::Covariant};
 }
 
 TypeId TypeChecker::freshType(const ScopePtr& scope)

Analysis/src/Unifier.cpp

@@ -396,11 +396,10 @@ TypeMismatch::Context Unifier::mismatchContext()
     }
 }
 
-Unifier::Unifier(NotNull<Normalizer> normalizer, Mode mode, NotNull<Scope> scope, const Location& location, Variance variance, TxnLog* parentLog)
+Unifier::Unifier(NotNull<Normalizer> normalizer, NotNull<Scope> scope, const Location& location, Variance variance, TxnLog* parentLog)
     : types(normalizer->arena)
     , builtinTypes(normalizer->builtinTypes)
     , normalizer(normalizer)
-    , mode(mode)
     , scope(scope)
     , log(parentLog)
     , location(location)
@@ -423,6 +422,12 @@ static bool isBlocked(const TxnLog& log, TypeId ty)
     return get<BlockedType>(ty) || get<PendingExpansionType>(ty);
 }
 
+static bool isBlocked(const TxnLog& log, TypePackId tp)
+{
+    tp = log.follow(tp);
+    return get<BlockedTypePack>(tp);
+}
+
 void Unifier::tryUnify_(TypeId subTy, TypeId superTy, bool isFunctionCall, bool isIntersection)
 {
     RecursionLimiter _ra(&sharedState.counters.recursionCount, sharedState.counters.recursionLimit);
@@ -1761,6 +1766,19 @@ void Unifier::tryUnify_(TypePackId subTp, TypePackId superTp, bool isFunctionCal
     if (log.haveSeen(superTp, subTp))
         return;
 
+    if (isBlocked(log, subTp) && isBlocked(log, superTp))
+    {
+        blockedTypePacks.push_back(subTp);
+        blockedTypePacks.push_back(superTp);
+    }
+    else if (isBlocked(log, subTp))
+    {
+        blockedTypePacks.push_back(subTp);
+    }
+    else if (isBlocked(log, superTp))
+    {
+        blockedTypePacks.push_back(superTp);
+    }
     if (log.getMutable<FreeTypePack>(superTp))
     {
         if (!occursCheck(superTp, subTp, /* reversed = */ true))
@@ -2795,7 +2813,12 @@ void Unifier::tryUnifyVariadics(TypePackId subTp, TypePackId superTp, bool rever
         if (std::optional<TypePackId> maybeTail = subIter.tail())
         {
             TypePackId tail = follow(*maybeTail);
-            if (get<FreeTypePack>(tail))
+
+            if (isBlocked(log, tail))
+            {
+                blockedTypePacks.push_back(tail);
+            }
+            else if (get<FreeTypePack>(tail))
             {
                 log.replace(tail, BoundTypePack(superTp));
             }
@@ -3094,7 +3117,7 @@ bool Unifier::occursCheck(DenseHashSet<TypePackId>& seen, TypePackId needle, Typ
 
 Unifier Unifier::makeChildUnifier()
 {
-    Unifier u = Unifier{normalizer, mode, scope, location, variance, &log};
+    Unifier u = Unifier{normalizer, scope, location, variance, &log};
     u.normalize = normalize;
     u.checkInhabited = checkInhabited;
 
@@ -3125,12 +3148,6 @@ void Unifier::reportError(TypeError err)
     failure = true;
 }
 
-
-bool Unifier::isNonstrictMode() const
-{
-    return (mode == Mode::Nonstrict) || (mode == Mode::NoCheck);
-}
-
 void Unifier::checkChildUnifierTypeMismatch(const ErrorVec& innerErrors, TypeId wantedType, TypeId givenType)
 {
     if (auto e = hasUnificationTooComplex(innerErrors))

CodeGen/include/Luau/AssemblyBuilderA64.h

@@ -157,6 +157,8 @@ public:
     void fcmpz(RegisterA64 src);
     void fcsel(RegisterA64 dst, RegisterA64 src1, RegisterA64 src2, ConditionA64 cond);
 
+    void udf();
+
     // Run final checks
     bool finalize();
 

CodeGen/include/Luau/AssemblyBuilderX64.h

@@ -99,6 +99,7 @@ public:
     void call(OperandX64 op);
 
     void int3();
+    void ud2();
 
     void bsr(RegisterX64 dst, OperandX64 src);
     void bsf(RegisterX64 dst, OperandX64 src);

CodeGen/include/Luau/IrBuilder.h

@@ -38,7 +38,6 @@ struct IrBuilder
 
     IrOp undef();
 
-    IrOp constBool(bool value);
     IrOp constInt(int value);
     IrOp constUint(unsigned value);
     IrOp constDouble(double value);

CodeGen/include/Luau/IrData.h

@@ -283,7 +283,7 @@ enum class IrCmd : uint8_t
     // A: builtin
     // B: Rn (result start)
     // C: Rn (argument start)
-    // D: Rn or Kn or a boolean that's false (optional second argument)
+    // D: Rn or Kn or undef (optional second argument)
     // E: int (argument count)
     // F: int (result count)
     FASTCALL,
@@ -292,7 +292,7 @@ enum class IrCmd : uint8_t
     // A: builtin
     // B: Rn (result start)
     // C: Rn (argument start)
-    // D: Rn or Kn or a boolean that's false (optional second argument)
+    // D: Rn or Kn or undef (optional second argument)
     // E: int (argument count or -1 to use all arguments up to stack top)
     // F: int (result count or -1 to preserve all results and adjust stack top)
     INVOKE_FASTCALL,
@@ -360,39 +360,46 @@ enum class IrCmd : uint8_t
 
     // Guard against tag mismatch
     // A, B: tag
-    // C: block
+    // C: block/undef
     // In final x64 lowering, A can also be Rn
+    // When undef is specified instead of a block, execution is aborted on check failure
     CHECK_TAG,
 
     // Guard against readonly table
     // A: pointer (Table)
-    // B: block
+    // B: block/undef
+    // When undef is specified instead of a block, execution is aborted on check failure
     CHECK_READONLY,
 
     // Guard against table having a metatable
     // A: pointer (Table)
-    // B: block
+    // B: block/undef
+    // When undef is specified instead of a block, execution is aborted on check failure
     CHECK_NO_METATABLE,
 
     // Guard against executing in unsafe environment
-    // A: block
+    // A: block/undef
+    // When undef is specified instead of a block, execution is aborted on check failure
     CHECK_SAFE_ENV,
 
     // Guard against index overflowing the table array size
     // A: pointer (Table)
     // B: int (index)
-    // C: block
+    // C: block/undef
+    // When undef is specified instead of a block, execution is aborted on check failure
     CHECK_ARRAY_SIZE,
 
     // Guard against cached table node slot not matching the actual table node slot for a key
     // A: pointer (LuaNode)
     // B: Kn
-    // C: block
+    // C: block/undef
+    // When undef is specified instead of a block, execution is aborted on check failure
     CHECK_SLOT_MATCH,
 
     // Guard against table node with a linked next node to ensure that our lookup hits the main position of the key
     // A: pointer (LuaNode)
-    // B: block
+    // B: block/undef
+    // When undef is specified instead of a block, execution is aborted on check failure
     CHECK_NODE_NO_NEXT,
 
     // Special operations
@@ -428,7 +435,7 @@ enum class IrCmd : uint8_t
 
     // While capture is a no-op right now, it might be useful to track register/upvalue lifetimes
     // A: Rn or UPn
-    // B: boolean (true for reference capture, false for value capture)
+    // B: unsigned int (1 for reference capture, 0 for value capture)
     CAPTURE,
 
     // Operations that don't have an IR representation yet
@@ -581,7 +588,6 @@ enum class IrCmd : uint8_t
 
 enum class IrConstKind : uint8_t
 {
-    Bool,
     Int,
     Uint,
     Double,
@@ -867,27 +873,6 @@ struct IrFunction
         return value.valueTag;
     }
 
-    bool boolOp(IrOp op)
-    {
-        IrConst& value = constOp(op);
-
-        LUAU_ASSERT(value.kind == IrConstKind::Bool);
-        return value.valueBool;
-    }
-
-    std::optional<bool> asBoolOp(IrOp op)
-    {
-        if (op.kind != IrOpKind::Constant)
-            return std::nullopt;
-
-        IrConst& value = constOp(op);
-
-        if (value.kind != IrConstKind::Bool)
-            return std::nullopt;
-
-        return value.valueBool;
-    }
-
     int intOp(IrOp op)
     {
         IrConst& value = constOp(op);

CodeGen/src/AssemblyBuilderA64.cpp

@@ -687,6 +687,11 @@ void AssemblyBuilderA64::fcsel(RegisterA64 dst, RegisterA64 src1, RegisterA64 sr
     placeCS("fcsel", dst, src1, src2, cond, 0b11110'01'1, 0b11);
 }
 
+void AssemblyBuilderA64::udf()
+{
+    place0("udf", 0);
+}
+
 bool AssemblyBuilderA64::finalize()
 {
     code.resize(codePos - code.data());

CodeGen/src/AssemblyBuilderX64.cpp

@@ -472,6 +472,15 @@ void AssemblyBuilderX64::int3()
     commit();
 }
 
+void AssemblyBuilderX64::ud2()
+{
+    if (logText)
+        log("ud2");
+
+    place(0x0f);
+    place(0x0b);
+}
+
 void AssemblyBuilderX64::bsr(RegisterX64 dst, OperandX64 src)
 {
     if (logText)

CodeGen/src/CodeAllocator.cpp

@@ -51,13 +51,13 @@ static void makePagesExecutable(uint8_t* mem, size_t size)
 
     DWORD oldProtect;
     if (VirtualProtect(mem, size, PAGE_EXECUTE_READ, &oldProtect) == 0)
-        LUAU_ASSERT(!"failed to change page protection");
+        LUAU_ASSERT(!"Failed to change page protection");
 }
 
 static void flushInstructionCache(uint8_t* mem, size_t size)
 {
     if (FlushInstructionCache(GetCurrentProcess(), mem, size) == 0)
-        LUAU_ASSERT(!"failed to flush instruction cache");
+        LUAU_ASSERT(!"Failed to flush instruction cache");
 }
 #else
 static uint8_t* allocatePages(size_t size)
@@ -68,7 +68,7 @@ static uint8_t* allocatePages(size_t size)
 static void freePages(uint8_t* mem, size_t size)
 {
     if (munmap(mem, alignToPageSize(size)) != 0)
-        LUAU_ASSERT(!"failed to deallocate block memory");
+        LUAU_ASSERT(!"Failed to deallocate block memory");
 }
 
 static void makePagesExecutable(uint8_t* mem, size_t size)
@@ -77,7 +77,7 @@ static void makePagesExecutable(uint8_t* mem, size_t size)
     LUAU_ASSERT(size == alignToPageSize(size));
 
     if (mprotect(mem, size, PROT_READ | PROT_EXEC) != 0)
-        LUAU_ASSERT(!"failed to change page protection");
+        LUAU_ASSERT(!"Failed to change page protection");
 }
 
 static void flushInstructionCache(uint8_t* mem, size_t size)

CodeGen/src/CodeBlockUnwind.cpp

@@ -79,7 +79,7 @@ void* createBlockUnwindInfo(void* context, uint8_t* block, size_t blockSize, siz
 #if defined(_WIN32) && defined(_M_X64)
     if (!RtlAddFunctionTable((RUNTIME_FUNCTION*)block, uint32_t(unwind->getFunctionCount()), uintptr_t(block)))
     {
-        LUAU_ASSERT(!"failed to allocate function table");
+        LUAU_ASSERT(!"Failed to allocate function table");
         return nullptr;
     }
 #elif defined(__linux__) || defined(__APPLE__)
@@ -94,7 +94,7 @@ void destroyBlockUnwindInfo(void* context, void* unwindData)
 {
 #if defined(_WIN32) && defined(_M_X64)
     if (!RtlDeleteFunctionTable((RUNTIME_FUNCTION*)unwindData))
-        LUAU_ASSERT(!"failed to deallocate function table");
+        LUAU_ASSERT(!"Failed to deallocate function table");
 #elif defined(__linux__) || defined(__APPLE__)
     visitFdeEntries((char*)unwindData, __deregister_frame);
 #endif

CodeGen/src/CodeGen.cpp

@@ -18,7 +18,6 @@
 #include "Luau/AssemblyBuilderA64.h"
 #include "Luau/AssemblyBuilderX64.h"
 
-#include "CustomExecUtils.h"
 #include "NativeState.h"
 
 #include "CodeGenA64.h"
@@ -59,6 +58,8 @@ namespace Luau
 namespace CodeGen
 {
 
+static const Instruction kCodeEntryInsn = LOP_NATIVECALL;
+
 static void* gPerfLogContext = nullptr;
 static PerfLogFn gPerfLogFn = nullptr;
 
@@ -332,9 +333,15 @@ static std::optional<NativeProto> assembleFunction(AssemblyBuilder& build, Nativ
     return createNativeProto(proto, ir);
 }
 
+static NativeState* getNativeState(lua_State* L)
+{
+    return static_cast<NativeState*>(L->global->ecb.context);
+}
+
 static void onCloseState(lua_State* L)
 {
-    destroyNativeState(L);
+    delete getNativeState(L);
+    L->global->ecb = lua_ExecutionCallbacks();
 }
 
 static void onDestroyFunction(lua_State* L, Proto* proto)
@@ -342,6 +349,7 @@ static void onDestroyFunction(lua_State* L, Proto* proto)
     destroyExecData(proto->execdata);
     proto->execdata = nullptr;
     proto->exectarget = 0;
+    proto->codeentry = proto->code;
 }
 
 static int onEnter(lua_State* L, Proto* proto)
@@ -362,7 +370,7 @@ static void onSetBreakpoint(lua_State* L, Proto* proto, int instruction)
     if (!proto->execdata)
         return;
 
-    LUAU_ASSERT(!"native breakpoints are not implemented");
+    LUAU_ASSERT(!"Native breakpoints are not implemented");
 }
 
 #if defined(__aarch64__)
@@ -430,39 +438,34 @@ void create(lua_State* L)
 {
     LUAU_ASSERT(isSupported());
 
-    NativeState& data = *createNativeState(L);
+    std::unique_ptr<NativeState> data = std::make_unique<NativeState>();
 
 #if defined(_WIN32)
-    data.unwindBuilder = std::make_unique<UnwindBuilderWin>();
+    data->unwindBuilder = std::make_unique<UnwindBuilderWin>();
 #else
-    data.unwindBuilder = std::make_unique<UnwindBuilderDwarf2>();
+    data->unwindBuilder = std::make_unique<UnwindBuilderDwarf2>();
 #endif
 
-    data.codeAllocator.context = data.unwindBuilder.get();
+    data->codeAllocator.context = data->unwindBuilder.get();
-    data.codeAllocator.createBlockUnwindInfo = createBlockUnwindInfo;
+    data->codeAllocator.createBlockUnwindInfo = createBlockUnwindInfo;
-    data.codeAllocator.destroyBlockUnwindInfo = destroyBlockUnwindInfo;
+    data->codeAllocator.destroyBlockUnwindInfo = destroyBlockUnwindInfo;
 
-    initFunctions(data);
+    initFunctions(*data);
 
 #if defined(__x86_64__) || defined(_M_X64)
-    if (!X64::initHeaderFunctions(data))
+    if (!X64::initHeaderFunctions(*data))
-    {
-        destroyNativeState(L);
         return;
-    }
 #elif defined(__aarch64__)
-    if (!A64::initHeaderFunctions(data))
+    if (!A64::initHeaderFunctions(*data))
-    {
-        destroyNativeState(L);
         return;
-    }
 #endif
 
     if (gPerfLogFn)
-        gPerfLogFn(gPerfLogContext, uintptr_t(data.context.gateEntry), 4096, "<luau gate>");
+        gPerfLogFn(gPerfLogContext, uintptr_t(data->context.gateEntry), 4096, "<luau gate>");
 
-    lua_ExecutionCallbacks* ecb = getExecutionCallbacks(L);
+    lua_ExecutionCallbacks* ecb = &L->global->ecb;
 
+    ecb->context = data.release();
     ecb->close = onCloseState;
     ecb->destroy = onDestroyFunction;
     ecb->enter = onEnter;
@@ -490,7 +493,8 @@ void compile(lua_State* L, int idx)
     const TValue* func = luaA_toobject(L, idx);
 
     // If initialization has failed, do not compile any functions
-    if (!getNativeState(L))
+    NativeState* data = getNativeState(L);
+    if (!data)
         return;
 
 #if defined(__aarch64__)
@@ -499,8 +503,6 @@ void compile(lua_State* L, int idx)
     X64::AssemblyBuilderX64 build(/* logText= */ false);
 #endif
 
-    NativeState* data = getNativeState(L);
-
     std::vector<Proto*> protos;
     gatherFunctions(protos, clvalue(func)->l.p);
 
@@ -564,6 +566,7 @@ void compile(lua_State* L, int idx)
         // the memory is now managed by VM and will be freed via onDestroyFunction
         result.p->execdata = result.execdata;
         result.p->exectarget = uintptr_t(codeStart) + result.exectarget;
+        result.p->codeentry = &kCodeEntryInsn;
     }
 }
 

CodeGen/src/CodeGenA64.cpp

@@ -5,7 +5,6 @@
 #include "Luau/UnwindBuilder.h"
 
 #include "BitUtils.h"
-#include "CustomExecUtils.h"
 #include "NativeState.h"
 #include "EmitCommonA64.h"
 
@@ -95,13 +94,14 @@ static void emitReentry(AssemblyBuilderA64& build, ModuleHelpers& helpers)
     build.ldr(x2, mem(rState, offsetof(lua_State, ci))); // L->ci
 
     // We need to check if the new frame can be executed natively
-    // TOOD: .flags and .savedpc load below can be fused with ldp
+    // TODO: .flags and .savedpc load below can be fused with ldp
     build.ldr(w3, mem(x2, offsetof(CallInfo, flags)));
-    build.tbz(x3, countrz(LUA_CALLINFO_CUSTOM), helpers.exitContinueVm);
+    build.tbz(x3, countrz(LUA_CALLINFO_NATIVE), helpers.exitContinueVm);
 
     build.mov(rClosure, x0);
-    build.ldr(rConstants, mem(x1, offsetof(Proto, k))); // proto->k
+
-    build.ldr(rCode, mem(x1, offsetof(Proto, code)));   // proto->code
+    LUAU_ASSERT(offsetof(Proto, code) == offsetof(Proto, k) + 8);
+    build.ldp(rConstants, rCode, mem(x1, offsetof(Proto, k))); // proto->k, proto->code
 
     // Get instruction index from instruction pointer
     // To get instruction index from instruction pointer, we need to divide byte offset by 4
@@ -145,8 +145,9 @@ static EntryLocations buildEntryFunction(AssemblyBuilderA64& build, UnwindBuilde
     build.mov(rNativeContext, x3);
 
     build.ldr(rBase, mem(x0, offsetof(lua_State, base))); // L->base
-    build.ldr(rConstants, mem(x1, offsetof(Proto, k)));   // proto->k
+
-    build.ldr(rCode, mem(x1, offsetof(Proto, code)));     // proto->code
+    LUAU_ASSERT(offsetof(Proto, code) == offsetof(Proto, k) + 8);
+    build.ldp(rConstants, rCode, mem(x1, offsetof(Proto, k))); // proto->k, proto->code
 
     build.ldr(x9, mem(x0, offsetof(lua_State, ci)));          // L->ci
     build.ldr(x9, mem(x9, offsetof(CallInfo, func)));         // L->ci->func
@@ -194,7 +195,7 @@ bool initHeaderFunctions(NativeState& data)
     if (!data.codeAllocator.allocate(build.data.data(), int(build.data.size()), reinterpret_cast<const uint8_t*>(build.code.data()),
             int(build.code.size() * sizeof(build.code[0])), data.gateData, data.gateDataSize, codeStart))
     {
-        LUAU_ASSERT(!"failed to create entry function");
+        LUAU_ASSERT(!"Failed to create entry function");
         return false;
     }
 

CodeGen/src/CodeGenUtils.cpp

@@ -1,8 +1,6 @@
 // This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
 #include "CodeGenUtils.h"
 
-#include "CustomExecUtils.h"
-
 #include "lvm.h"
 
 #include "lbuiltins.h"
@@ -268,7 +266,7 @@ Closure* callFallback(lua_State* L, StkId ra, StkId argtop, int nresults)
         ci->savedpc = p->code;
 
         if (LUAU_LIKELY(p->execdata != NULL))
-            ci->flags = LUA_CALLINFO_CUSTOM;
+            ci->flags = LUA_CALLINFO_NATIVE;
 
         return ccl;
     }

CodeGen/src/CodeGenX64.cpp

@@ -4,7 +4,6 @@
 #include "Luau/AssemblyBuilderX64.h"
 #include "Luau/UnwindBuilder.h"
 
-#include "CustomExecUtils.h"
 #include "NativeState.h"
 #include "EmitCommonX64.h"
 
@@ -160,7 +159,7 @@ bool initHeaderFunctions(NativeState& data)
     if (!data.codeAllocator.allocate(
             build.data.data(), int(build.data.size()), build.code.data(), int(build.code.size()), data.gateData, data.gateDataSize, codeStart))
     {
-        LUAU_ASSERT(!"failed to create entry function");
+        LUAU_ASSERT(!"Failed to create entry function");
         return false;
     }
 

CodeGen/src/CustomExecUtils.h

@@ -1,106 +0,0 @@
-// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
-#pragma once
-
-#include "NativeState.h"
-
-#include "lobject.h"
-#include "lstate.h"
-
-namespace Luau
-{
-namespace CodeGen
-{
-
-// Here we define helper functions to wrap interaction with Luau custom execution API so that it works with or without LUA_CUSTOM_EXECUTION
-
-#if LUA_CUSTOM_EXECUTION
-
-inline lua_ExecutionCallbacks* getExecutionCallbacks(lua_State* L)
-{
-    return &L->global->ecb;
-}
-
-inline NativeState* getNativeState(lua_State* L)
-{
-    lua_ExecutionCallbacks* ecb = getExecutionCallbacks(L);
-    return (NativeState*)ecb->context;
-}
-
-inline void setNativeState(lua_State* L, NativeState* nativeState)
-{
-    lua_ExecutionCallbacks* ecb = getExecutionCallbacks(L);
-    ecb->context = nativeState;
-}
-
-inline NativeState* createNativeState(lua_State* L)
-{
-    NativeState* state = new NativeState();
-    setNativeState(L, state);
-    return state;
-}
-
-inline void destroyNativeState(lua_State* L)
-{
-    NativeState* state = getNativeState(L);
-    setNativeState(L, nullptr);
-    delete state;
-}
-
-#else
-
-inline lua_ExecutionCallbacks* getExecutionCallbacks(lua_State* L)
-{
-    return nullptr;
-}
-
-inline NativeState* getNativeState(lua_State* L)
-{
-    return nullptr;
-}
-
-inline void setNativeState(lua_State* L, NativeState* nativeState) {}
-
-inline NativeState* createNativeState(lua_State* L)
-{
-    return nullptr;
-}
-
-inline void destroyNativeState(lua_State* L) {}
-
-#endif
-
-inline int getOpLength(LuauOpcode op)
-{
-    switch (op)
-    {
-    case LOP_GETGLOBAL:
-    case LOP_SETGLOBAL:
-    case LOP_GETIMPORT:
-    case LOP_GETTABLEKS:
-    case LOP_SETTABLEKS:
-    case LOP_NAMECALL:
-    case LOP_JUMPIFEQ:
-    case LOP_JUMPIFLE:
-    case LOP_JUMPIFLT:
-    case LOP_JUMPIFNOTEQ:
-    case LOP_JUMPIFNOTLE:
-    case LOP_JUMPIFNOTLT:
-    case LOP_NEWTABLE:
-    case LOP_SETLIST:
-    case LOP_FORGLOOP:
-    case LOP_LOADKX:
-    case LOP_FASTCALL2:
-    case LOP_FASTCALL2K:
-    case LOP_JUMPXEQKNIL:
-    case LOP_JUMPXEQKB:
-    case LOP_JUMPXEQKN:
-    case LOP_JUMPXEQKS:
-        return 2;
-
-    default:
-        return 1;
-    }
-}
-
-} // namespace CodeGen
-} // namespace Luau

CodeGen/src/EmitBuiltinsX64.cpp

@@ -119,7 +119,6 @@ void emitBuiltin(IrRegAllocX64& regs, AssemblyBuilderX64& build, int bfid, int r
         return emitBuiltinTypeof(regs, build, ra, arg);
     default:
         LUAU_ASSERT(!"Missing x64 lowering");
-        break;
     }
 }
 

CodeGen/src/EmitCommonX64.cpp

@@ -6,7 +6,6 @@
 #include "Luau/IrData.h"
 #include "Luau/IrRegAllocX64.h"
 
-#include "CustomExecUtils.h"
 #include "NativeState.h"
 
 #include "lgc.h"

CodeGen/src/EmitCommonX64.h

@@ -184,33 +184,6 @@ inline void jumpIfTruthy(AssemblyBuilderX64& build, int ri, Label& target, Label
     build.jcc(ConditionX64::NotEqual, target); // true if boolean value is 'true'
 }
 
-inline void jumpIfNodeKeyTagIsNot(AssemblyBuilderX64& build, RegisterX64 tmp, RegisterX64 node, lua_Type tag, Label& label)
-{
-    tmp.size = SizeX64::dword;
-
-    build.mov(tmp, luauNodeKeyTag(node));
-    build.and_(tmp, kTKeyTagMask);
-    build.cmp(tmp, tag);
-    build.jcc(ConditionX64::NotEqual, label);
-}
-
-inline void jumpIfNodeValueTagIs(AssemblyBuilderX64& build, RegisterX64 node, lua_Type tag, Label& label)
-{
-    build.cmp(dword[node + offsetof(LuaNode, val) + offsetof(TValue, tt)], tag);
-    build.jcc(ConditionX64::Equal, label);
-}
-
-inline void jumpIfNodeKeyNotInExpectedSlot(AssemblyBuilderX64& build, RegisterX64 tmp, RegisterX64 node, OperandX64 expectedKey, Label& label)
-{
-    jumpIfNodeKeyTagIsNot(build, tmp, node, LUA_TSTRING, label);
-
-    build.mov(tmp, expectedKey);
-    build.cmp(tmp, luauNodeKeyValue(node));
-    build.jcc(ConditionX64::NotEqual, label);
-
-    jumpIfNodeValueTagIs(build, node, LUA_TNIL, label);
-}
-
 void jumpOnNumberCmp(AssemblyBuilderX64& build, RegisterX64 tmp, OperandX64 lhs, OperandX64 rhs, IrCondition cond, Label& label);
 void jumpOnAnyCmpFallback(IrRegAllocX64& regs, AssemblyBuilderX64& build, int ra, int rb, IrCondition cond, Label& label);
 

CodeGen/src/EmitInstructionX64.cpp

@@ -4,8 +4,10 @@
 #include "Luau/AssemblyBuilderX64.h"
 #include "Luau/IrRegAllocX64.h"
 
-#include "CustomExecUtils.h"
 #include "EmitCommonX64.h"
+#include "NativeState.h"
+
+#include "lstate.h"
 
 namespace Luau
 {
@@ -87,8 +89,8 @@ void emitInstCall(AssemblyBuilderX64& build, ModuleHelpers& helpers, int ra, int
         build.test(rax, rax);
         build.jcc(ConditionX64::Zero, helpers.continueCallInVm);
 
-        // Mark call frame as custom
+        // Mark call frame as native
-        build.mov(dword[ci + offsetof(CallInfo, flags)], LUA_CALLINFO_CUSTOM);
+        build.mov(dword[ci + offsetof(CallInfo, flags)], LUA_CALLINFO_NATIVE);
 
         // Switch current constants
         build.mov(rConstants, qword[proto + offsetof(Proto, k)]);
@@ -298,7 +300,7 @@ void emitInstReturn(AssemblyBuilderX64& build, ModuleHelpers& helpers, int ra, i
 
     build.mov(execdata, qword[proto + offsetof(Proto, execdata)]);
 
-    build.test(byte[cip + offsetof(CallInfo, flags)], LUA_CALLINFO_CUSTOM);
+    build.test(byte[cip + offsetof(CallInfo, flags)], LUA_CALLINFO_NATIVE);
     build.jcc(ConditionX64::Zero, helpers.exitContinueVm); // Continue in interpreter if function has no native data
 
     // Change constants

CodeGen/src/IrAnalysis.cpp

@@ -113,7 +113,7 @@ uint32_t getNextInstUse(IrFunction& function, uint32_t targetInstIdx, uint32_t s
     }
 
     // There must be a next use since there is the last use location
-    LUAU_ASSERT(!"failed to find next use");
+    LUAU_ASSERT(!"Failed to find next use");
     return targetInst.lastUse;
 }
 
@@ -338,7 +338,7 @@ static RegisterSet computeBlockLiveInRegSet(IrFunction& function, const IrBlock&
         case IrCmd::CAPTURE:
             maybeUse(inst.a);
 
-            if (function.boolOp(inst.b))
+            if (function.uintOp(inst.b) == 1)
                 capturedRegs.set(vmRegOp(inst.a), true);
             break;
         case IrCmd::SETLIST:

CodeGen/src/IrBuilder.cpp

@@ -4,7 +4,6 @@
 #include "Luau/IrAnalysis.h"
 #include "Luau/IrUtils.h"
 
-#include "CustomExecUtils.h"
 #include "IrTranslation.h"
 
 #include "lapi.h"
@@ -19,7 +18,7 @@ namespace CodeGen
 constexpr unsigned kNoAssociatedBlockIndex = ~0u;
 
 IrBuilder::IrBuilder()
-    : constantMap({IrConstKind::Bool, ~0ull})
+    : constantMap({IrConstKind::Tag, ~0ull})
 {
 }
 
@@ -410,8 +409,7 @@ void IrBuilder::translateInst(LuauOpcode op, const Instruction* pc, int i)
         break;
     }
     default:
-        LUAU_ASSERT(!"unknown instruction");
+        LUAU_ASSERT(!"Unknown instruction");
-        break;
     }
 }
 
@@ -449,7 +447,7 @@ void IrBuilder::clone(const IrBlock& source, bool removeCurrentTerminator)
             if (const uint32_t* newIndex = instRedir.find(op.index))
                 op.index = *newIndex;
             else
-                LUAU_ASSERT(!"values can only be used if they are defined in the same block");
+                LUAU_ASSERT(!"Values can only be used if they are defined in the same block");
         }
     };
 
@@ -501,14 +499,6 @@ IrOp IrBuilder::undef()
     return {IrOpKind::Undef, 0};
 }
 
-IrOp IrBuilder::constBool(bool value)
-{
-    IrConst constant;
-    constant.kind = IrConstKind::Bool;
-    constant.valueBool = value;
-    return constAny(constant, uint64_t(value));
-}
-
 IrOp IrBuilder::constInt(int value)
 {
     IrConst constant;

CodeGen/src/IrDump.cpp

@@ -390,9 +390,6 @@ void toString(std::string& result, IrConst constant)
 {
     switch (constant.kind)
     {
-    case IrConstKind::Bool:
-        append(result, constant.valueBool ? "true" : "false");
-        break;
     case IrConstKind::Int:
         append(result, "%di", constant.valueInt);
         break;

CodeGen/src/IrLoweringA64.cpp

@@ -96,6 +96,15 @@ static void emitAddOffset(AssemblyBuilderA64& build, RegisterA64 dst, RegisterA6
     }
 }
 
+static void emitAbort(AssemblyBuilderA64& build, Label& abort)
+{
+    Label skip;
+    build.b(skip);
+    build.setLabel(abort);
+    build.udf();
+    build.setLabel(skip);
+}
+
 static void emitFallback(AssemblyBuilderA64& build, int offset, int pcpos)
 {
     // fallback(L, instruction, base, k)
@@ -256,7 +265,11 @@ void IrLoweringA64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
         }
         else if (inst.b.kind == IrOpKind::Constant)
         {
-            if (intOp(inst.b) * sizeof(TValue) <= AssemblyBuilderA64::kMaxImmediate)
+            if (intOp(inst.b) == 0)
+            {
+                // no offset required
+            }
+            else if (intOp(inst.b) * sizeof(TValue) <= AssemblyBuilderA64::kMaxImmediate)
             {
                 build.add(inst.regA64, inst.regA64, uint16_t(intOp(inst.b) * sizeof(TValue)));
             }
@@ -562,7 +575,14 @@ void IrLoweringA64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
         break;
     }
     case IrCmd::JUMP_EQ_TAG:
-        if (inst.a.kind == IrOpKind::Inst && inst.b.kind == IrOpKind::Constant)
+    {
+        RegisterA64 zr = noreg;
+
+        if (inst.a.kind == IrOpKind::Constant && tagOp(inst.a) == 0)
+            zr = regOp(inst.b);
+        else if (inst.b.kind == IrOpKind::Constant && tagOp(inst.b) == 0)
+            zr = regOp(inst.a);
+        else if (inst.a.kind == IrOpKind::Inst && inst.b.kind == IrOpKind::Constant)
             build.cmp(regOp(inst.a), tagOp(inst.b));
         else if (inst.a.kind == IrOpKind::Inst && inst.b.kind == IrOpKind::Inst)
             build.cmp(regOp(inst.a), regOp(inst.b));
@@ -573,19 +593,33 @@ void IrLoweringA64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
 
         if (isFallthroughBlock(blockOp(inst.d), next))
         {
-            build.b(ConditionA64::Equal, labelOp(inst.c));
+            if (zr != noreg)
+                build.cbz(zr, labelOp(inst.c));
+            else
+                build.b(ConditionA64::Equal, labelOp(inst.c));
             jumpOrFallthrough(blockOp(inst.d), next);
         }
         else
         {
-            build.b(ConditionA64::NotEqual, labelOp(inst.d));
+            if (zr != noreg)
+                build.cbnz(zr, labelOp(inst.d));
+            else
+                build.b(ConditionA64::NotEqual, labelOp(inst.d));
             jumpOrFallthrough(blockOp(inst.c), next);
         }
         break;
+    }
     case IrCmd::JUMP_EQ_INT:
-        LUAU_ASSERT(unsigned(intOp(inst.b)) <= AssemblyBuilderA64::kMaxImmediate);
+        if (intOp(inst.b) == 0)
-        build.cmp(regOp(inst.a), uint16_t(intOp(inst.b)));
+        {
-        build.b(ConditionA64::Equal, labelOp(inst.c));
+            build.cbz(regOp(inst.a), labelOp(inst.c));
+        }
+        else
+        {
+            LUAU_ASSERT(unsigned(intOp(inst.b)) <= AssemblyBuilderA64::kMaxImmediate);
+            build.cmp(regOp(inst.a), uint16_t(intOp(inst.b)));
+            build.b(ConditionA64::Equal, labelOp(inst.c));
+        }
         jumpOrFallthrough(blockOp(inst.d), next);
         break;
     case IrCmd::JUMP_LT_INT:
@@ -871,7 +905,7 @@ void IrLoweringA64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
             build.add(x2, rBase, uint16_t(vmRegOp(inst.c) * sizeof(TValue)));
         else if (inst.c.kind == IrOpKind::Constant)
         {
-            TValue n;
+            TValue n = {};
             setnvalue(&n, uintOp(inst.c));
             build.adr(x2, &n, sizeof(n));
         }
@@ -893,7 +927,7 @@ void IrLoweringA64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
             build.add(x2, rBase, uint16_t(vmRegOp(inst.c) * sizeof(TValue)));
         else if (inst.c.kind == IrOpKind::Constant)
         {
-            TValue n;
+            TValue n = {};
             setnvalue(&n, uintOp(inst.c));
             build.adr(x2, &n, sizeof(n));
         }
@@ -908,25 +942,17 @@ void IrLoweringA64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
         break;
     case IrCmd::GET_IMPORT:
         regs.spill(build, index);
-        // luaV_getimport(L, cl->env, k, aux, /* propagatenil= */ false)
+        // luaV_getimport(L, cl->env, k, ra, aux, /* propagatenil= */ false)
         build.mov(x0, rState);
         build.ldr(x1, mem(rClosure, offsetof(Closure, env)));
         build.mov(x2, rConstants);
-        build.mov(w3, uintOp(inst.b));
+        build.add(x3, rBase, uint16_t(vmRegOp(inst.a) * sizeof(TValue)));
-        build.mov(w4, 0);
+        build.mov(w4, uintOp(inst.b));
-        build.ldr(x5, mem(rNativeContext, offsetof(NativeContext, luaV_getimport)));
+        build.mov(w5, 0);
-        build.blr(x5);
+        build.ldr(x6, mem(rNativeContext, offsetof(NativeContext, luaV_getimport)));
+        build.blr(x6);
 
         emitUpdateBase(build);
-
-        // setobj2s(L, ra, L->top - 1)
-        build.ldr(x0, mem(rState, offsetof(lua_State, top)));
-        build.sub(x0, x0, sizeof(TValue));
-        build.ldr(q0, x0);
-        build.str(q0, mem(rBase, vmRegOp(inst.a) * sizeof(TValue)));
-
-        // L->top--
-        build.str(x0, mem(rState, offsetof(lua_State, top)));
         break;
     case IrCmd::CONCAT:
         regs.spill(build, index);
@@ -1003,62 +1029,99 @@ void IrLoweringA64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
         // note: no emitUpdateBase necessary because prepareFORN does not reallocate stack
         break;
     case IrCmd::CHECK_TAG:
-        build.cmp(regOp(inst.a), tagOp(inst.b));
+    {
-        build.b(ConditionA64::NotEqual, labelOp(inst.c));
+        Label abort; // used when guard aborts execution
+        Label& fail = inst.c.kind == IrOpKind::Undef ? abort : labelOp(inst.c);
+        if (tagOp(inst.b) == 0)
+        {
+            build.cbnz(regOp(inst.a), fail);
+        }
+        else
+        {
+            build.cmp(regOp(inst.a), tagOp(inst.b));
+            build.b(ConditionA64::NotEqual, fail);
+        }
+        if (abort.id)
+            emitAbort(build, abort);
         break;
+    }
     case IrCmd::CHECK_READONLY:
     {
+        Label abort; // used when guard aborts execution
         RegisterA64 temp = regs.allocTemp(KindA64::w);
         build.ldrb(temp, mem(regOp(inst.a), offsetof(Table, readonly)));
-        build.cbnz(temp, labelOp(inst.b));
+        build.cbnz(temp, inst.b.kind == IrOpKind::Undef ? abort : labelOp(inst.b));
+        if (abort.id)
+            emitAbort(build, abort);
         break;
     }
     case IrCmd::CHECK_NO_METATABLE:
     {
+        Label abort; // used when guard aborts execution
         RegisterA64 temp = regs.allocTemp(KindA64::x);
         build.ldr(temp, mem(regOp(inst.a), offsetof(Table, metatable)));
-        build.cbnz(temp, labelOp(inst.b));
+        build.cbnz(temp, inst.b.kind == IrOpKind::Undef ? abort : labelOp(inst.b));
+        if (abort.id)
+            emitAbort(build, abort);
         break;
     }
     case IrCmd::CHECK_SAFE_ENV:
     {
+        Label abort; // used when guard aborts execution
         RegisterA64 temp = regs.allocTemp(KindA64::x);
         RegisterA64 tempw = castReg(KindA64::w, temp);
         build.ldr(temp, mem(rClosure, offsetof(Closure, env)));
         build.ldrb(tempw, mem(temp, offsetof(Table, safeenv)));
-        build.cbz(tempw, labelOp(inst.a));
+        build.cbz(tempw, inst.a.kind == IrOpKind::Undef ? abort : labelOp(inst.a));
+        if (abort.id)
+            emitAbort(build, abort);
         break;
     }
     case IrCmd::CHECK_ARRAY_SIZE:
     {
+        Label abort; // used when guard aborts execution
+        Label& fail = inst.c.kind == IrOpKind::Undef ? abort : labelOp(inst.c);
+
         RegisterA64 temp = regs.allocTemp(KindA64::w);
         build.ldr(temp, mem(regOp(inst.a), offsetof(Table, sizearray)));
 
         if (inst.b.kind == IrOpKind::Inst)
+        {
             build.cmp(temp, regOp(inst.b));
+            build.b(ConditionA64::UnsignedLessEqual, fail);
+        }
         else if (inst.b.kind == IrOpKind::Constant)
         {
-            if (size_t(intOp(inst.b)) <= AssemblyBuilderA64::kMaxImmediate)
+            if (intOp(inst.b) == 0)
+            {
+                build.cbz(temp, fail);
+            }
+            else if (size_t(intOp(inst.b)) <= AssemblyBuilderA64::kMaxImmediate)
             {
                 build.cmp(temp, uint16_t(intOp(inst.b)));
+                build.b(ConditionA64::UnsignedLessEqual, fail);
             }
             else
             {
                 RegisterA64 temp2 = regs.allocTemp(KindA64::w);
                 build.mov(temp2, intOp(inst.b));
                 build.cmp(temp, temp2);
+                build.b(ConditionA64::UnsignedLessEqual, fail);
             }
         }
         else
             LUAU_ASSERT(!"Unsupported instruction form");
 
-        build.b(ConditionA64::UnsignedLessEqual, labelOp(inst.c));
+        if (abort.id)
+            emitAbort(build, abort);
         break;
     }
     case IrCmd::JUMP_SLOT_MATCH:
     case IrCmd::CHECK_SLOT_MATCH:
     {
-        Label& mismatch = inst.cmd == IrCmd::JUMP_SLOT_MATCH ? labelOp(inst.d) : labelOp(inst.c);
+        Label abort; // used when guard aborts execution
+        const IrOp& mismatchOp = inst.cmd == IrCmd::JUMP_SLOT_MATCH ? inst.d : inst.c;
+        Label& mismatch = mismatchOp.kind == IrOpKind::Undef ? abort : labelOp(mismatchOp);
 
         RegisterA64 temp1 = regs.allocTemp(KindA64::x);
         RegisterA64 temp1w = castReg(KindA64::w, temp1);
@@ -1081,15 +1144,21 @@ void IrLoweringA64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
 
         if (inst.cmd == IrCmd::JUMP_SLOT_MATCH)
             jumpOrFallthrough(blockOp(inst.c), next);
+        else if (abort.id)
+            emitAbort(build, abort);
         break;
     }
     case IrCmd::CHECK_NODE_NO_NEXT:
     {
+        Label abort; // used when guard aborts execution
         RegisterA64 temp = regs.allocTemp(KindA64::w);
 
         build.ldr(temp, mem(regOp(inst.a), offsetof(LuaNode, key) + kOffsetOfTKeyTagNext));
         build.lsr(temp, temp, kTKeyTagBits);
-        build.cbnz(temp, labelOp(inst.b));
+        build.cbnz(temp, inst.b.kind == IrOpKind::Undef ? abort : labelOp(inst.b));
+
+        if (abort.id)
+            emitAbort(build, abort);
         break;
     }
     case IrCmd::INTERRUPT:
@@ -1762,11 +1831,6 @@ uint8_t IrLoweringA64::tagOp(IrOp op) const
     return function.tagOp(op);
 }
 
-bool IrLoweringA64::boolOp(IrOp op) const
-{
-    return function.boolOp(op);
-}
-
 int IrLoweringA64::intOp(IrOp op) const
 {
     return function.intOp(op);

CodeGen/src/IrLoweringA64.h

@@ -48,7 +48,6 @@ struct IrLoweringA64
     // Operand data lookup helpers
     IrConst constOp(IrOp op) const;
     uint8_t tagOp(IrOp op) const;
-    bool boolOp(IrOp op) const;
     int intOp(IrOp op) const;
     unsigned uintOp(IrOp op) const;
     double doubleOp(IrOp op) const;

CodeGen/src/IrLoweringX64.cpp

@@ -575,14 +575,6 @@ void IrLoweringX64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
         jumpOnAnyCmpFallback(regs, build, vmRegOp(inst.a), vmRegOp(inst.b), conditionOp(inst.c), labelOp(inst.d));
         jumpOrFallthrough(blockOp(inst.e), next);
         break;
-    case IrCmd::JUMP_SLOT_MATCH:
-    {
-        ScopedRegX64 tmp{regs, SizeX64::qword};
-
-        jumpIfNodeKeyNotInExpectedSlot(build, tmp.reg, regOp(inst.a), luauConstantValue(vmConstOp(inst.b)), labelOp(inst.d));
-        jumpOrFallthrough(blockOp(inst.c), next);
-        break;
-    }
     case IrCmd::TABLE_LEN:
     {
         IrCallWrapperX64 callWrap(regs, build, index);
@@ -782,7 +774,7 @@ void IrLoweringX64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
         }
         else if (inst.c.kind == IrOpKind::Constant)
         {
-            TValue n;
+            TValue n = {};
             setnvalue(&n, uintOp(inst.c));
             callGetTable(regs, build, vmRegOp(inst.b), build.bytes(&n, sizeof(n)), vmRegOp(inst.a));
         }
@@ -798,7 +790,7 @@ void IrLoweringX64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
         }
         else if (inst.c.kind == IrOpKind::Constant)
         {
-            TValue n;
+            TValue n = {};
             setnvalue(&n, uintOp(inst.c));
             callSetTable(regs, build, vmRegOp(inst.b), build.bytes(&n, sizeof(n)), vmRegOp(inst.a));
         }
@@ -817,24 +809,12 @@ void IrLoweringX64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
         callWrap.addArgument(SizeX64::qword, rState);
         callWrap.addArgument(SizeX64::qword, qword[tmp1.release() + offsetof(Closure, env)]);
         callWrap.addArgument(SizeX64::qword, rConstants);
+        callWrap.addArgument(SizeX64::qword, luauRegAddress(vmRegOp(inst.a)));
         callWrap.addArgument(SizeX64::dword, uintOp(inst.b));
         callWrap.addArgument(SizeX64::dword, 0);
         callWrap.call(qword[rNativeContext + offsetof(NativeContext, luaV_getimport)]);
 
         emitUpdateBase(build);
-
-        ScopedRegX64 tmp2{regs, SizeX64::qword};
-
-        // setobj2s(L, ra, L->top - 1)
-        build.mov(tmp2.reg, qword[rState + offsetof(lua_State, top)]);
-        build.sub(tmp2.reg, sizeof(TValue));
-
-        ScopedRegX64 tmp3{regs, SizeX64::xmmword};
-        build.vmovups(tmp3.reg, xmmword[tmp2.reg]);
-        build.vmovups(luauReg(vmRegOp(inst.a)), tmp3.reg);
-
-        // L->top--
-        build.mov(qword[rState + offsetof(lua_State, top)], tmp2.reg);
         break;
     }
     case IrCmd::CONCAT:
@@ -897,15 +877,15 @@ void IrLoweringX64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
         break;
     case IrCmd::CHECK_TAG:
         build.cmp(memRegTagOp(inst.a), tagOp(inst.b));
-        build.jcc(ConditionX64::NotEqual, labelOp(inst.c));
+        jumpOrAbortOnUndef(ConditionX64::NotEqual, ConditionX64::Equal, inst.c);
         break;
     case IrCmd::CHECK_READONLY:
         build.cmp(byte[regOp(inst.a) + offsetof(Table, readonly)], 0);
-        build.jcc(ConditionX64::NotEqual, labelOp(inst.b));
+        jumpOrAbortOnUndef(ConditionX64::NotEqual, ConditionX64::Equal, inst.b);
         break;
     case IrCmd::CHECK_NO_METATABLE:
         build.cmp(qword[regOp(inst.a) + offsetof(Table, metatable)], 0);
-        build.jcc(ConditionX64::NotEqual, labelOp(inst.b));
+        jumpOrAbortOnUndef(ConditionX64::NotEqual, ConditionX64::Equal, inst.b);
         break;
     case IrCmd::CHECK_SAFE_ENV:
     {
@@ -914,7 +894,7 @@ void IrLoweringX64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
         build.mov(tmp.reg, sClosure);
         build.mov(tmp.reg, qword[tmp.reg + offsetof(Closure, env)]);
         build.cmp(byte[tmp.reg + offsetof(Table, safeenv)], 0);
-        build.jcc(ConditionX64::Equal, labelOp(inst.a));
+        jumpOrAbortOnUndef(ConditionX64::Equal, ConditionX64::NotEqual, inst.a);
         break;
     }
     case IrCmd::CHECK_ARRAY_SIZE:
@@ -925,13 +905,44 @@ void IrLoweringX64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
         else
             LUAU_ASSERT(!"Unsupported instruction form");
 
-        build.jcc(ConditionX64::BelowEqual, labelOp(inst.c));
+        jumpOrAbortOnUndef(ConditionX64::BelowEqual, ConditionX64::NotBelowEqual, inst.c);
         break;
+    case IrCmd::JUMP_SLOT_MATCH:
     case IrCmd::CHECK_SLOT_MATCH:
     {
+        Label abort; // Used when guard aborts execution
+        const IrOp& mismatchOp = inst.cmd == IrCmd::JUMP_SLOT_MATCH ? inst.d : inst.c;
+        Label& mismatch = mismatchOp.kind == IrOpKind::Undef ? abort : labelOp(mismatchOp);
+
         ScopedRegX64 tmp{regs, SizeX64::qword};
 
-        jumpIfNodeKeyNotInExpectedSlot(build, tmp.reg, regOp(inst.a), luauConstantValue(vmConstOp(inst.b)), labelOp(inst.c));
+        // Check if node key tag is a string
+        build.mov(dwordReg(tmp.reg), luauNodeKeyTag(regOp(inst.a)));
+        build.and_(dwordReg(tmp.reg), kTKeyTagMask);
+        build.cmp(dwordReg(tmp.reg), LUA_TSTRING);
+        build.jcc(ConditionX64::NotEqual, mismatch);
+
+        // Check that node key value matches the expected one
+        build.mov(tmp.reg, luauConstantValue(vmConstOp(inst.b)));
+        build.cmp(tmp.reg, luauNodeKeyValue(regOp(inst.a)));
+        build.jcc(ConditionX64::NotEqual, mismatch);
+
+        // Check that node value is not nil
+        build.cmp(dword[regOp(inst.a) + offsetof(LuaNode, val) + offsetof(TValue, tt)], LUA_TNIL);
+        build.jcc(ConditionX64::Equal, mismatch);
+
+        if (inst.cmd == IrCmd::JUMP_SLOT_MATCH)
+        {
+            jumpOrFallthrough(blockOp(inst.c), next);
+        }
+        else if (mismatchOp.kind == IrOpKind::Undef)
+        {
+            Label skip;
+            build.jmp(skip);
+            build.setLabel(abort);
+            build.ud2();
+            build.setLabel(skip);
+        }
         break;
     }
     case IrCmd::CHECK_NODE_NO_NEXT:
@@ -940,7 +951,7 @@ void IrLoweringX64::lowerInst(IrInst& inst, uint32_t index, IrBlock& next)
 
         build.mov(tmp.reg, dword[regOp(inst.a) + offsetof(LuaNode, key) + kOffsetOfTKeyTagNext]);
         build.shr(tmp.reg, kTKeyTagBits);
-        build.jcc(ConditionX64::NotZero, labelOp(inst.b));
+        jumpOrAbortOnUndef(ConditionX64::NotZero, ConditionX64::Zero, inst.b);
         break;
     }
     case IrCmd::INTERRUPT:
@@ -1356,6 +1367,21 @@ void IrLoweringX64::jumpOrFallthrough(IrBlock& target, IrBlock& next)
         build.jmp(target.label);
 }
 
+void IrLoweringX64::jumpOrAbortOnUndef(ConditionX64 cond, ConditionX64 condInverse, IrOp targetOrUndef)
+{
+    if (targetOrUndef.kind == IrOpKind::Undef)
+    {
+        Label skip;
+        build.jcc(condInverse, skip);
+        build.ud2();
+        build.setLabel(skip);
+    }
+    else
+    {
+        build.jcc(cond, labelOp(targetOrUndef));
+    }
+}
+
 OperandX64 IrLoweringX64::memRegDoubleOp(IrOp op)
 {
     switch (op.kind)
@@ -1428,11 +1454,6 @@ uint8_t IrLoweringX64::tagOp(IrOp op) const
     return function.tagOp(op);
 }
 
-bool IrLoweringX64::boolOp(IrOp op) const
-{
-    return function.boolOp(op);
-}
-
 int IrLoweringX64::intOp(IrOp op) const
 {
     return function.intOp(op);

CodeGen/src/IrLoweringX64.h

@@ -34,6 +34,7 @@ struct IrLoweringX64
 
     bool isFallthroughBlock(IrBlock target, IrBlock next);
     void jumpOrFallthrough(IrBlock& target, IrBlock& next);
+    void jumpOrAbortOnUndef(ConditionX64 cond, ConditionX64 condInverse, IrOp targetOrUndef);
 
     void storeDoubleAsFloat(OperandX64 dst, IrOp src);
 
@@ -45,7 +46,6 @@ struct IrLoweringX64
 
     IrConst constOp(IrOp op) const;
     uint8_t tagOp(IrOp op) const;
-    bool boolOp(IrOp op) const;
     int intOp(IrOp op) const;
     unsigned uintOp(IrOp op) const;
     double doubleOp(IrOp op) const;

CodeGen/src/IrRegAllocX64.cpp

@@ -194,7 +194,7 @@ void IrRegAllocX64::preserve(IrInst& inst)
         else if (spill.valueKind == IrValueKind::Tag || spill.valueKind == IrValueKind::Int)
             build.mov(dword[sSpillArea + i * 8], inst.regX64);
         else
-            LUAU_ASSERT(!"unsupported value kind");
+            LUAU_ASSERT(!"Unsupported value kind");
 
         usedSpillSlots.set(i);
 
@@ -318,7 +318,7 @@ unsigned IrRegAllocX64::findSpillStackSlot(IrValueKind valueKind)
         return i;
     }
 
-    LUAU_ASSERT(!"nowhere to spill");
+    LUAU_ASSERT(!"Nowhere to spill");
     return ~0u;
 }
 

CodeGen/src/IrTranslation.cpp

@@ -5,10 +5,10 @@
 #include "Luau/IrBuilder.h"
 #include "Luau/IrUtils.h"
 
-#include "CustomExecUtils.h"
 #include "IrTranslateBuiltins.h"
 
 #include "lobject.h"
+#include "lstate.h"
 #include "ltm.h"
 
 namespace Luau
@@ -366,7 +366,7 @@ static void translateInstBinaryNumeric(IrBuilder& build, int ra, int rb, int rc,
             result = build.inst(IrCmd::INVOKE_LIBM, build.constUint(LBF_MATH_POW), vb, vc);
             break;
         default:
-            LUAU_ASSERT(!"unsupported binary op");
+            LUAU_ASSERT(!"Unsupported binary op");
         }
     }
 
@@ -1068,13 +1068,13 @@ void translateInstCapture(IrBuilder& build, const Instruction* pc, int pcpos)
     switch (type)
     {
     case LCT_VAL:
-        build.inst(IrCmd::CAPTURE, build.vmReg(index), build.constBool(false));
+        build.inst(IrCmd::CAPTURE, build.vmReg(index), build.constUint(0));
         break;
     case LCT_REF:
-        build.inst(IrCmd::CAPTURE, build.vmReg(index), build.constBool(true));
+        build.inst(IrCmd::CAPTURE, build.vmReg(index), build.constUint(1));
         break;
     case LCT_UPVAL:
-        build.inst(IrCmd::CAPTURE, build.vmUpvalue(index), build.constBool(false));
+        build.inst(IrCmd::CAPTURE, build.vmUpvalue(index), build.constUint(0));
         break;
     default:
         LUAU_ASSERT(!"Unknown upvalue capture type");

CodeGen/src/IrTranslation.h

@@ -1,6 +1,8 @@
 // This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
 #pragma once
 
+#include "Luau/Bytecode.h"
+
 #include <stdint.h>
 
 #include "ltm.h"
@@ -64,5 +66,38 @@ void translateInstNamecall(IrBuilder& build, const Instruction* pc, int pcpos);
 void translateInstAndX(IrBuilder& build, const Instruction* pc, int pcpos, IrOp c);
 void translateInstOrX(IrBuilder& build, const Instruction* pc, int pcpos, IrOp c);
 
+inline int getOpLength(LuauOpcode op)
+{
+    switch (op)
+    {
+    case LOP_GETGLOBAL:
+    case LOP_SETGLOBAL:
+    case LOP_GETIMPORT:
+    case LOP_GETTABLEKS:
+    case LOP_SETTABLEKS:
+    case LOP_NAMECALL:
+    case LOP_JUMPIFEQ:
+    case LOP_JUMPIFLE:
+    case LOP_JUMPIFLT:
+    case LOP_JUMPIFNOTEQ:
+    case LOP_JUMPIFNOTLE:
+    case LOP_JUMPIFNOTLT:
+    case LOP_NEWTABLE:
+    case LOP_SETLIST:
+    case LOP_FORGLOOP:
+    case LOP_LOADKX:
+    case LOP_FASTCALL2:
+    case LOP_FASTCALL2K:
+    case LOP_JUMPXEQKNIL:
+    case LOP_JUMPXEQKB:
+    case LOP_JUMPXEQKN:
+    case LOP_JUMPXEQKS:
+        return 2;
+
+    default:
+        return 1;
+    }
+}
+
 } // namespace CodeGen
 } // namespace Luau

CodeGen/src/IrUtils.cpp

@@ -356,7 +356,11 @@ void applySubstitutions(IrFunction& function, IrOp& op)
             src.useCount--;
 
             if (src.useCount == 0)
+            {
+                src.cmd = IrCmd::NOP;
                 removeUse(function, src.a);
+                src.a = {};
+            }
         }
     }
 }
@@ -396,7 +400,7 @@ bool compare(double a, double b, IrCondition cond)
     case IrCondition::NotGreaterEqual:
         return !(a >= b);
     default:
-        LUAU_ASSERT(!"unsupported conidtion");
+        LUAU_ASSERT(!"Unsupported condition");
     }
 
     return false;

CodeGen/src/NativeState.cpp

@@ -4,7 +4,6 @@
 #include "Luau/UnwindBuilder.h"
 
 #include "CodeGenUtils.h"
-#include "CustomExecUtils.h"
 
 #include "lbuiltins.h"
 #include "lgc.h"

CodeGen/src/NativeState.h

@@ -38,7 +38,7 @@ struct NativeContext
     void (*luaV_prepareFORN)(lua_State* L, StkId plimit, StkId pstep, StkId pinit) = nullptr;
     void (*luaV_gettable)(lua_State* L, const TValue* t, TValue* key, StkId val) = nullptr;
     void (*luaV_settable)(lua_State* L, const TValue* t, TValue* key, StkId val) = nullptr;
-    void (*luaV_getimport)(lua_State* L, Table* env, TValue* k, uint32_t id, bool propagatenil) = nullptr;
+    void (*luaV_getimport)(lua_State* L, Table* env, TValue* k, StkId res, uint32_t id, bool propagatenil) = nullptr;
     void (*luaV_concat)(lua_State* L, int total, int last) = nullptr;
 
     int (*luaH_getn)(Table* t) = nullptr;

CodeGen/src/OptimizeConstProp.cpp

@@ -12,6 +12,7 @@
 #include <vector>
 
 LUAU_FASTINTVARIABLE(LuauCodeGenMinLinearBlockPath, 3)
+LUAU_FASTFLAGVARIABLE(DebugLuauAbortingChecks, false)
 
 namespace Luau
 {
@@ -57,6 +58,12 @@ struct ConstPropState
         return 0xff;
     }
 
+    void updateTag(IrOp op, uint8_t tag)
+    {
+        if (RegisterInfo* info = tryGetRegisterInfo(op))
+            info->tag = tag;
+    }
+
     void saveTag(IrOp op, uint8_t tag)
     {
         if (RegisterInfo* info = tryGetRegisterInfo(op))
@@ -202,7 +209,7 @@ struct ConstPropState
         if (RegisterLink* link = instLink.find(instOp.index))
         {
             // Check that the target register hasn't changed the value
-            if (link->version > regs[link->reg].version)
+            if (link->version < regs[link->reg].version)
                 return nullptr;
 
             return link;
@@ -619,13 +626,20 @@ static void constPropInInst(ConstPropState& state, IrBuilder& build, IrFunction&
         if (uint8_t tag = state.tryGetTag(inst.a); tag != 0xff)
         {
             if (tag == b)
-                kill(function, inst);
+            {
+                if (FFlag::DebugLuauAbortingChecks)
+                    replace(function, inst.c, build.undef());
+                else
+                    kill(function, inst);
+            }
             else
+            {
                 replace(function, block, index, {IrCmd::JUMP, inst.c}); // Shows a conflict in assumptions on this path
+            }
         }
         else
         {
-            state.saveTag(inst.a, b); // We can assume the tag value going forward
+            state.updateTag(inst.a, b); // We can assume the tag value going forward
         }
         break;
     }
@@ -633,25 +647,46 @@ static void constPropInInst(ConstPropState& state, IrBuilder& build, IrFunction&
         if (RegisterInfo* info = state.tryGetRegisterInfo(inst.a))
         {
             if (info->knownNotReadonly)
-                kill(function, inst);
+            {
+                if (FFlag::DebugLuauAbortingChecks)
+                    replace(function, inst.b, build.undef());
+                else
+                    kill(function, inst);
+            }
             else
+            {
                 info->knownNotReadonly = true;
+            }
         }
         break;
     case IrCmd::CHECK_NO_METATABLE:
         if (RegisterInfo* info = state.tryGetRegisterInfo(inst.a))
         {
             if (info->knownNoMetatable)
-                kill(function, inst);
+            {
+                if (FFlag::DebugLuauAbortingChecks)
+                    replace(function, inst.b, build.undef());
+                else
+                    kill(function, inst);
+            }
             else
+            {
                 info->knownNoMetatable = true;
+            }
         }
         break;
     case IrCmd::CHECK_SAFE_ENV:
         if (state.inSafeEnv)
-            kill(function, inst);
+        {
+            if (FFlag::DebugLuauAbortingChecks)
+                replace(function, inst.a, build.undef());
+            else
+                kill(function, inst);
+        }
         else
+        {
             state.inSafeEnv = true;
+        }
         break;
     case IrCmd::CHECK_GC:
         // It is enough to perform a GC check once in a block

Common/include/Luau/Bytecode.h

@@ -300,8 +300,9 @@ enum LuauOpcode
     // A: target register (see FORGLOOP for register layout)
     LOP_FORGPREP_NEXT,
 
-    // removed in v3
+    // NATIVECALL: start executing new function in native code
-    LOP_DEP_FORGLOOP_NEXT,
+    // this is a pseudo-instruction that is never emitted by bytecode compiler, but can be constructed at runtime to accelerate native code dispatch
+    LOP_NATIVECALL,
 
     // GETVARARGS: copy variables into the target register from vararg storage for current function
     // A: target register

Compiler/src/Builtins.cpp

@@ -252,8 +252,7 @@ BuiltinInfo getBuiltinInfo(int bfid)
         return {-1, -1};
 
     case LBF_ASSERT:
-        return {-1, -1};
+        return {-1, -1}; // assert() returns all values when first value is truthy
-        ; // assert() returns all values when first value is truthy
 
     case LBF_MATH_ABS:
     case LBF_MATH_ACOS:

Compiler/src/Compiler.cpp

@@ -25,7 +25,7 @@ LUAU_FASTINTVARIABLE(LuauCompileInlineThreshold, 25)
 LUAU_FASTINTVARIABLE(LuauCompileInlineThresholdMaxBoost, 300)
 LUAU_FASTINTVARIABLE(LuauCompileInlineDepth, 5)
 
-LUAU_FASTFLAGVARIABLE(LuauCompileLimitInsns, false)
+LUAU_FASTFLAGVARIABLE(LuauCompileInlineDefer, false)
 
 namespace Luau
 {
@@ -250,7 +250,7 @@ struct Compiler
 
         popLocals(0);
 
-        if (FFlag::LuauCompileLimitInsns && bytecode.getInstructionCount() > kMaxInstructionCount)
+        if (bytecode.getInstructionCount() > kMaxInstructionCount)
             CompileError::raise(func->location, "Exceeded function instruction limit; split the function into parts to compile");
 
         bytecode.endFunction(uint8_t(stackSize), uint8_t(upvals.size()));
@@ -559,10 +559,19 @@ struct Compiler
 
         size_t oldLocals = localStack.size();
 
-        // note that we push the frame early; this is needed to block recursive inline attempts
+        std::vector<InlineArg> args;
-        inlineFrames.push_back({func, oldLocals, target, targetCount});
+        if (FFlag::LuauCompileInlineDefer)
+        {
+            args.reserve(func->args.size);
+        }
+        else
+        {
+            // note that we push the frame early; this is needed to block recursive inline attempts
+            inlineFrames.push_back({func, oldLocals, target, targetCount});
+        }
 
         // evaluate all arguments; note that we don't emit code for constant arguments (relying on constant folding)
+        // note that compiler state (variable registers/values) does not change here - we defer that to a separate loop below to handle nested calls
         for (size_t i = 0; i < func->args.size; ++i)
         {
             AstLocal* var = func->args.data[i];
@@ -581,8 +590,16 @@ struct Compiler
                 else
                     LUAU_ASSERT(!"Unexpected expression type");
 
-                for (size_t j = i; j < func->args.size; ++j)
+                if (FFlag::LuauCompileInlineDefer)
-                    pushLocal(func->args.data[j], uint8_t(reg + (j - i)));
+                {
+                    for (size_t j = i; j < func->args.size; ++j)
+                        args.push_back({func->args.data[j], uint8_t(reg + (j - i))});
+                }
+                else
+                {
+                    for (size_t j = i; j < func->args.size; ++j)
+                        pushLocal(func->args.data[j], uint8_t(reg + (j - i)));
+                }
 
                 // all remaining function arguments have been allocated and assigned to
                 break;
@@ -597,17 +614,26 @@ struct Compiler
                 else
                     bytecode.emitABC(LOP_LOADNIL, reg, 0, 0);
 
-                pushLocal(var, reg);
+                if (FFlag::LuauCompileInlineDefer)
+                    args.push_back({var, reg});
+                else
+                    pushLocal(var, reg);
             }
             else if (arg == nullptr)
             {
                 // since the argument is not mutated, we can simply fold the value into the expressions that need it
-                locstants[var] = {Constant::Type_Nil};
+                if (FFlag::LuauCompileInlineDefer)
+                    args.push_back({var, kInvalidReg, {Constant::Type_Nil}});
+                else
+                    locstants[var] = {Constant::Type_Nil};
             }
             else if (const Constant* cv = constants.find(arg); cv && cv->type != Constant::Type_Unknown)
             {
                 // since the argument is not mutated, we can simply fold the value into the expressions that need it
-                locstants[var] = *cv;
+                if (FFlag::LuauCompileInlineDefer)
+                    args.push_back({var, kInvalidReg, *cv});
+                else
+                    locstants[var] = *cv;
             }
             else
             {
@@ -617,13 +643,20 @@ struct Compiler
                 // if the argument is a local that isn't mutated, we will simply reuse the existing register
                 if (int reg = le ? getExprLocalReg(le) : -1; reg >= 0 && (!lv || !lv->written))
                 {
-                    pushLocal(var, uint8_t(reg));
+                    if (FFlag::LuauCompileInlineDefer)
+                        args.push_back({var, uint8_t(reg)});
+                    else
+                        pushLocal(var, uint8_t(reg));
                 }
                 else
                 {
                     uint8_t temp = allocReg(arg, 1);
                     compileExprTemp(arg, temp);
-                    pushLocal(var, temp);
+
+                    if (FFlag::LuauCompileInlineDefer)
+                        args.push_back({var, temp});
+                    else
+                        pushLocal(var, temp);
                 }
             }
         }
@@ -635,6 +668,20 @@ struct Compiler
             compileExprAuto(expr->args.data[i], rsi);
         }
 
+        if (FFlag::LuauCompileInlineDefer)
+        {
+            // apply all evaluated arguments to the compiler state
+            // note: locals use current startpc for debug info, although some of them have been computed earlier; this is similar to compileStatLocal
+            for (InlineArg& arg : args)
+                if (arg.value.type == Constant::Type_Unknown)
+                    pushLocal(arg.local, arg.reg);
+                else
+                    locstants[arg.local] = arg.value;
+
+            // the inline frame will be used to compile return statements as well as to reject recursive inlining attempts
+            inlineFrames.push_back({func, oldLocals, target, targetCount});
+        }
+
         // fold constant values updated above into expressions in the function body
         foldConstants(constants, variables, locstants, builtinsFold, func->body);
 
@@ -3747,6 +3794,14 @@ struct Compiler
         AstExpr* untilCondition;
     };
 
+    struct InlineArg
+    {
+        AstLocal* local;
+
+        uint8_t reg;
+        Constant value;
+    };
+
     struct InlineFrame
     {
         AstExprFunction* func;

Sources.cmake

@@ -113,7 +113,6 @@ target_sources(Luau.CodeGen PRIVATE
 
     CodeGen/src/BitUtils.h
     CodeGen/src/ByteUtils.h
-    CodeGen/src/CustomExecUtils.h
     CodeGen/src/CodeGenUtils.h
     CodeGen/src/CodeGenA64.h
     CodeGen/src/CodeGenX64.h
@@ -404,6 +403,7 @@ if(TARGET Luau.UnitTest)
         tests/TypeInfer.primitives.test.cpp
         tests/TypeInfer.provisional.test.cpp
         tests/TypeInfer.refinements.test.cpp
+        tests/TypeInfer.rwprops.test.cpp
         tests/TypeInfer.singletons.test.cpp
         tests/TypeInfer.tables.test.cpp
         tests/TypeInfer.test.cpp

VM/src/lfunc.cpp

@@ -31,7 +31,7 @@ Proto* luaF_newproto(lua_State* L)
     f->source = NULL;
     f->debugname = NULL;
     f->debuginsn = NULL;
-
+    f->codeentry = NULL;
     f->execdata = NULL;
     f->exectarget = 0;
 

VM/src/lobject.h

@@ -275,6 +275,7 @@ typedef struct Proto
     TString* debugname;
     uint8_t* debuginsn; // a copy of code[] array with just opcodes
 
+    const Instruction* codeentry;
     void* execdata;
     uintptr_t exectarget;
 

VM/src/lstate.cpp

@@ -219,9 +219,7 @@ lua_State* lua_newstate(lua_Alloc f, void* ud)
 
     g->cb = lua_Callbacks();
 
-#if LUA_CUSTOM_EXECUTION
     g->ecb = lua_ExecutionCallbacks();
-#endif
 
     g->gcstats = GCStats();
 

VM/src/lstate.h

@@ -69,7 +69,7 @@ typedef struct CallInfo
 
 #define LUA_CALLINFO_RETURN (1 << 0) // should the interpreter return after returning from this callinfo? first frame must have this set
 #define LUA_CALLINFO_HANDLE (1 << 1) // should the error thrown during execution get handled by continuation from this callinfo? func must be C
-#define LUA_CALLINFO_CUSTOM (1 << 2) // should this function be executed using custom execution callback
+#define LUA_CALLINFO_NATIVE (1 << 2) // should this function be executed using execution callback for native code
 
 #define curr_func(L) (clvalue(L->ci->func))
 #define ci_func(ci) (clvalue((ci)->func))
@@ -211,9 +211,7 @@ typedef struct global_State
 
     lua_Callbacks cb;
 
-#if LUA_CUSTOM_EXECUTION
     lua_ExecutionCallbacks ecb;
-#endif
 
     void (*udatagc[LUA_UTAG_LIMIT])(lua_State*, void*); // for each userdata tag, a gc callback to be called immediately before freeing memory
 

VM/src/lvm.h

@@ -23,7 +23,8 @@ LUAI_FUNC int luaV_tostring(lua_State* L, StkId obj);
 LUAI_FUNC void luaV_gettable(lua_State* L, const TValue* t, TValue* key, StkId val);
 LUAI_FUNC void luaV_settable(lua_State* L, const TValue* t, TValue* key, StkId val);
 LUAI_FUNC void luaV_concat(lua_State* L, int total, int last);
-LUAI_FUNC void luaV_getimport(lua_State* L, Table* env, TValue* k, uint32_t id, bool propagatenil);
+LUAI_FUNC void luaV_getimport(lua_State* L, Table* env, TValue* k, StkId res, uint32_t id, bool propagatenil);
+LUAI_FUNC void luaV_getimport_dep(lua_State* L, Table* env, TValue* k, uint32_t id, bool propagatenil);
 LUAI_FUNC void luaV_prepareFORN(lua_State* L, StkId plimit, StkId pstep, StkId pinit);
 LUAI_FUNC void luaV_callTM(lua_State* L, int nparams, int res);
 LUAI_FUNC void luaV_tryfuncTM(lua_State* L, StkId func);

VM/src/lvmexecute.cpp

@@ -17,6 +17,7 @@
 #include <string.h>
 
 LUAU_FASTFLAG(LuauUniformTopHandling)
+LUAU_FASTFLAG(LuauGetImportDirect)
 
 // Disable c99-designator to avoid the warning in CGOTO dispatch table
 #ifdef __clang__
@@ -101,7 +102,7 @@ LUAU_FASTFLAG(LuauUniformTopHandling)
         VM_DISPATCH_OP(LOP_CONCAT), VM_DISPATCH_OP(LOP_NOT), VM_DISPATCH_OP(LOP_MINUS), VM_DISPATCH_OP(LOP_LENGTH), VM_DISPATCH_OP(LOP_NEWTABLE), \
         VM_DISPATCH_OP(LOP_DUPTABLE), VM_DISPATCH_OP(LOP_SETLIST), VM_DISPATCH_OP(LOP_FORNPREP), VM_DISPATCH_OP(LOP_FORNLOOP), \
         VM_DISPATCH_OP(LOP_FORGLOOP), VM_DISPATCH_OP(LOP_FORGPREP_INEXT), VM_DISPATCH_OP(LOP_DEP_FORGLOOP_INEXT), VM_DISPATCH_OP(LOP_FORGPREP_NEXT), \
-        VM_DISPATCH_OP(LOP_DEP_FORGLOOP_NEXT), VM_DISPATCH_OP(LOP_GETVARARGS), VM_DISPATCH_OP(LOP_DUPCLOSURE), VM_DISPATCH_OP(LOP_PREPVARARGS), \
+        VM_DISPATCH_OP(LOP_NATIVECALL), VM_DISPATCH_OP(LOP_GETVARARGS), VM_DISPATCH_OP(LOP_DUPCLOSURE), VM_DISPATCH_OP(LOP_PREPVARARGS), \
         VM_DISPATCH_OP(LOP_LOADKX), VM_DISPATCH_OP(LOP_JUMPX), VM_DISPATCH_OP(LOP_FASTCALL), VM_DISPATCH_OP(LOP_COVERAGE), \
         VM_DISPATCH_OP(LOP_CAPTURE), VM_DISPATCH_OP(LOP_DEP_JUMPIFEQK), VM_DISPATCH_OP(LOP_DEP_JUMPIFNOTEQK), VM_DISPATCH_OP(LOP_FASTCALL1), \
         VM_DISPATCH_OP(LOP_FASTCALL2), VM_DISPATCH_OP(LOP_FASTCALL2K), VM_DISPATCH_OP(LOP_FORGPREP), VM_DISPATCH_OP(LOP_JUMPXEQKNIL), \
@@ -210,7 +211,7 @@ static void luau_execute(lua_State* L)
     LUAU_ASSERT(!isblack(obj2gco(L))); // we don't use luaC_threadbarrier because active threads never turn black
 
 #if LUA_CUSTOM_EXECUTION
-    if ((L->ci->flags & LUA_CALLINFO_CUSTOM) && !SingleStep)
+    if ((L->ci->flags & LUA_CALLINFO_NATIVE) && !SingleStep)
     {
         Proto* p = clvalue(L->ci->func)->l.p;
         LUAU_ASSERT(p->execdata);
@@ -432,12 +433,20 @@ reentry:
                 {
                     uint32_t aux = *pc++;
 
-                    VM_PROTECT(luaV_getimport(L, cl->env, k, aux, /* propagatenil= */ false));
+                    if (FFlag::LuauGetImportDirect)
-                    ra = VM_REG(LUAU_INSN_A(insn)); // previous call may change the stack
+                    {
+                        VM_PROTECT(luaV_getimport(L, cl->env, k, ra, aux, /* propagatenil= */ false));
+                        VM_NEXT();
+                    }
+                    else
+                    {
+                        VM_PROTECT(luaV_getimport_dep(L, cl->env, k, aux, /* propagatenil= */ false));
+                        ra = VM_REG(LUAU_INSN_A(insn)); // previous call may change the stack
 
-                    setobj2s(L, ra, L->top - 1);
+                        setobj2s(L, ra, L->top - 1);
-                    L->top--;
+                        L->top--;
-                    VM_NEXT();
+                        VM_NEXT();
+                    }
                 }
             }
 
@@ -954,21 +963,11 @@ reentry:
                         setnilvalue(argi++); // complete missing arguments
                     L->top = p->is_vararg ? argi : ci->top;
 
-#if LUA_CUSTOM_EXECUTION
-                    if (LUAU_UNLIKELY(p->execdata && !SingleStep))
-                    {
-                        ci->flags = LUA_CALLINFO_CUSTOM;
-                        ci->savedpc = p->code;
-
-                        if (L->global->ecb.enter(L, p) == 1)
-                            goto reentry;
-                        else
-                            goto exit;
-                    }
-#endif
-
                     // reentry
-                    pc = p->code;
+                    // codeentry may point to NATIVECALL instruction when proto is compiled to native code
+                    // this will result in execution continuing in native code, and is equivalent to if (p->execdata) but has no additional overhead
+                    // note that p->codeentry may point *outside* of p->code..p->code+p->sizecode, but that pointer never gets saved to savedpc.
+                    pc = SingleStep ? p->code : p->codeentry;
                     cl = ccl;
                     base = L->base;
                     k = p->k;
@@ -1055,7 +1054,7 @@ reentry:
                 Proto* nextproto = nextcl->l.p;
 
 #if LUA_CUSTOM_EXECUTION
-                if (LUAU_UNLIKELY((cip->flags & LUA_CALLINFO_CUSTOM) && !SingleStep))
+                if (LUAU_UNLIKELY((cip->flags & LUA_CALLINFO_NATIVE) && !SingleStep))
                 {
                     if (L->global->ecb.enter(L, nextproto) == 1)
                         goto reentry;
@@ -2380,10 +2379,24 @@ reentry:
                 VM_NEXT();
             }
 
-            VM_CASE(LOP_DEP_FORGLOOP_NEXT)
+            VM_CASE(LOP_NATIVECALL)
             {
-                LUAU_ASSERT(!"Unsupported deprecated opcode");
+                Proto* p = cl->l.p;
+                LUAU_ASSERT(p->execdata);
+
+                CallInfo* ci = L->ci;
+                ci->flags = LUA_CALLINFO_NATIVE;
+                ci->savedpc = p->code;
+
+#if LUA_CUSTOM_EXECUTION
+                if (L->global->ecb.enter(L, p) == 1)
+                    goto reentry;
+                else
+                    goto exit;
+#else
+                LUAU_ASSERT(!"Opcode is only valid when LUA_CUSTOM_EXECUTION is defined");
                 LUAU_UNREACHABLE();
+#endif
             }
 
             VM_CASE(LOP_GETVARARGS)
@@ -2896,7 +2909,7 @@ int luau_precall(lua_State* L, StkId func, int nresults)
 
 #if LUA_CUSTOM_EXECUTION
         if (p->execdata)
-            ci->flags = LUA_CALLINFO_CUSTOM;
+            ci->flags = LUA_CALLINFO_NATIVE;
 #endif
 
         return PCRLUA;

VM/src/lvmload.cpp

@@ -13,6 +13,8 @@
 
 #include <string.h>
 
+LUAU_FASTFLAGVARIABLE(LuauGetImportDirect, false)
+
 // TODO: RAII deallocation doesn't work for longjmp builds if a memory error happens
 template<typename T>
 struct TempBuffer
@@ -40,8 +42,45 @@ struct TempBuffer
     }
 };
 
-void luaV_getimport(lua_State* L, Table* env, TValue* k, uint32_t id, bool propagatenil)
+void luaV_getimport(lua_State* L, Table* env, TValue* k, StkId res, uint32_t id, bool propagatenil)
 {
+    int count = id >> 30;
+    LUAU_ASSERT(count > 0);
+
+    int id0 = int(id >> 20) & 1023;
+    int id1 = int(id >> 10) & 1023;
+    int id2 = int(id) & 1023;
+
+    // after the first call to luaV_gettable, res may be invalid, and env may (sometimes) be garbage collected
+    // we take care to not use env again and to restore res before every consecutive use
+    ptrdiff_t resp = savestack(L, res);
+
+    // global lookup for id0
+    TValue g;
+    sethvalue(L, &g, env);
+    luaV_gettable(L, &g, &k[id0], res);
+
+    // table lookup for id1
+    if (count < 2)
+        return;
+
+    res = restorestack(L, resp);
+    if (!propagatenil || !ttisnil(res))
+        luaV_gettable(L, res, &k[id1], res);
+
+    // table lookup for id2
+    if (count < 3)
+        return;
+
+    res = restorestack(L, resp);
+    if (!propagatenil || !ttisnil(res))
+        luaV_gettable(L, res, &k[id2], res);
+}
+
+void luaV_getimport_dep(lua_State* L, Table* env, TValue* k, uint32_t id, bool propagatenil)
+{
+    LUAU_ASSERT(!FFlag::LuauGetImportDirect);
+
     int count = id >> 30;
     int id0 = count > 0 ? int(id >> 20) & 1023 : -1;
     int id1 = count > 1 ? int(id >> 10) & 1023 : -1;
@@ -114,7 +153,17 @@ static void resolveImportSafe(lua_State* L, Table* env, TValue* k, uint32_t id)
             // note: we call getimport with nil propagation which means that accesses to table chains like A.B.C will resolve in nil
             // this is technically not necessary but it reduces the number of exceptions when loading scripts that rely on getfenv/setfenv for global
             // injection
-            luaV_getimport(L, L->gt, self->k, self->id, /* propagatenil= */ true);
+            if (FFlag::LuauGetImportDirect)
+            {
+                // allocate a stack slot so that we can do table lookups
+                luaD_checkstack(L, 1);
+                setnilvalue(L->top);
+                L->top++;
+
+                luaV_getimport(L, L->gt, self->k, L->top - 1, self->id, /* propagatenil= */ true);
+            }
+            else
+                luaV_getimport_dep(L, L->gt, self->k, self->id, /* propagatenil= */ true);
         }
     };
 
@@ -204,6 +253,8 @@ int luau_load(lua_State* L, const char* chunkname, const char* data, size_t size
         for (int j = 0; j < p->sizecode; ++j)
             p->code[j] = read<uint32_t>(data, size, offset);
 
+        p->codeentry = p->code;
+
         p->sizek = readVarInt(data, size, offset);
         p->k = luaM_newarray(L, p->sizek, TValue, p->memcat);
 

tests/AssemblyBuilderA64.test.cpp

@@ -455,6 +455,11 @@ TEST_CASE_FIXTURE(AssemblyBuilderA64Fixture, "Conditionals")
     SINGLE_COMPARE(cset(x1, ConditionA64::Less), 0x9A9FA7E1);
 }
 
+TEST_CASE_FIXTURE(AssemblyBuilderA64Fixture, "Undefined")
+{
+    SINGLE_COMPARE(udf(), 0x00000000);
+}
+
 TEST_CASE("LogTest")
 {
     AssemblyBuilderA64 build(/* logText= */ true);

tests/AssemblyBuilderX64.test.cpp

@@ -537,6 +537,7 @@ TEST_CASE_FIXTURE(AssemblyBuilderX64Fixture, "AVXTernaryInstructionForms")
 TEST_CASE_FIXTURE(AssemblyBuilderX64Fixture, "MiscInstructions")
 {
     SINGLE_COMPARE(int3(), 0xcc);
+    SINGLE_COMPARE(ud2(), 0x0f, 0x0b);
     SINGLE_COMPARE(bsr(eax, edx), 0x0f, 0xbd, 0xc2);
     SINGLE_COMPARE(bsf(eax, edx), 0x0f, 0xbc, 0xc2);
 }

tests/Compiler.test.cpp

@@ -5796,7 +5796,9 @@ RETURN R3 1
 
 TEST_CASE("InlineRecurseArguments")
 {
-    // we can't inline a function if it's used to compute its own arguments
+    ScopedFastFlag sff("LuauCompileInlineDefer", true);
+
+    // the example looks silly but we preserve it verbatim as it was found by fuzzer for a previous version of the compiler
     CHECK_EQ("\n" + compileFunction(R"(
 local function foo(a, b)
 end
@@ -5805,15 +5807,82 @@ foo(foo(foo,foo(foo,foo))[foo])
                         1, 2),
         R"(
 DUPCLOSURE R0 K0 ['foo']
-MOVE R2 R0
+LOADNIL R3
-MOVE R3 R0
+LOADNIL R2
-MOVE R4 R0
-MOVE R5 R0
-MOVE R6 R0
-CALL R4 2 -1
-CALL R2 -1 1
 GETTABLE R1 R2 R0
 RETURN R0 0
+)");
+
+    // verify that invocations of the inlined function in any position for computing the arguments to itself compile
+    CHECK_EQ("\n" + compileFunction(R"(
+local function foo(a, b)
+    return a + b
+end
+
+local x, y, z = ...
+
+return foo(foo(x, y), foo(z, 1))
+)",
+                        1, 2),
+        R"(
+DUPCLOSURE R0 K0 ['foo']
+GETVARARGS R1 3
+ADD R5 R1 R2
+ADDK R6 R3 K1 [1]
+ADD R4 R5 R6
+RETURN R4 1
+)");
+
+    // verify that invocations of the inlined function in any position for computing the arguments to itself compile, including constants and locals
+    // note that foo(k1, k2) doesn't get constant folded, so there's still actual math emitted for some of the calls below
+    CHECK_EQ("\n" + compileFunction(R"(
+local function foo(a, b)
+    return a + b
+end
+
+local x, y, z = ...
+
+return
+    foo(foo(1, 2), 3),
+    foo(1, foo(2, 3)),
+    foo(x, foo(2, 3)),
+    foo(x, foo(y, 3)),
+    foo(x, foo(y, z)),
+    foo(x+0, foo(y, z)),
+    foo(x+0, foo(y+0, z)),
+    foo(x+0, foo(y, z+0)),
+    foo(1, foo(x, y))
+)",
+                        1, 2),
+        R"(
+DUPCLOSURE R0 K0 ['foo']
+GETVARARGS R1 3
+LOADN R5 3
+ADDK R4 R5 K1 [3]
+LOADN R6 5
+LOADN R7 1
+ADD R5 R7 R6
+LOADN R7 5
+ADD R6 R1 R7
+ADDK R8 R2 K1 [3]
+ADD R7 R1 R8
+ADD R9 R2 R3
+ADD R8 R1 R9
+ADDK R10 R1 K2 [0]
+ADD R11 R2 R3
+ADD R9 R10 R11
+ADDK R11 R1 K2 [0]
+ADDK R13 R2 K2 [0]
+ADD R12 R13 R3
+ADD R10 R11 R12
+ADDK R12 R1 K2 [0]
+ADDK R14 R3 K2 [0]
+ADD R13 R2 R14
+ADD R11 R12 R13
+ADD R13 R1 R2
+LOADN R14 1
+ADD R12 R14 R13
+RETURN R4 9
 )");
 }
 

tests/IrBuilder.test.cpp

@@ -80,6 +80,8 @@ public:
     static const int tnil = 0;
     static const int tboolean = 1;
     static const int tnumber = 3;
+    static const int tstring = 5;
+    static const int ttable = 6;
 };
 
 TEST_SUITE_BEGIN("Optimization");
@@ -1286,8 +1288,8 @@ TEST_CASE_FIXTURE(IrBuilderFixture, "IntEqRemoval")
     IrOp falseBlock = build.block(IrBlockKind::Internal);
 
     build.beginBlock(block);
-    IrOp value = build.inst(IrCmd::LOAD_INT, build.vmReg(1));
     build.inst(IrCmd::STORE_INT, build.vmReg(1), build.constInt(5));
+    IrOp value = build.inst(IrCmd::LOAD_INT, build.vmReg(1));
     build.inst(IrCmd::JUMP_EQ_INT, value, build.constInt(5), trueBlock, falseBlock);
 
     build.beginBlock(trueBlock);
@@ -1317,8 +1319,8 @@ TEST_CASE_FIXTURE(IrBuilderFixture, "NumCmpRemoval")
     IrOp falseBlock = build.block(IrBlockKind::Internal);
 
     build.beginBlock(block);
-    IrOp value = build.inst(IrCmd::LOAD_DOUBLE, build.vmReg(1));
     build.inst(IrCmd::STORE_DOUBLE, build.vmReg(1), build.constDouble(4.0));
+    IrOp value = build.inst(IrCmd::LOAD_DOUBLE, build.vmReg(1));
     build.inst(IrCmd::JUMP_CMP_NUM, value, build.constDouble(8.0), build.cond(IrCondition::Greater), trueBlock, falseBlock);
 
     build.beginBlock(trueBlock);
@@ -1551,6 +1553,50 @@ bb_0:
 )");
 }
 
+TEST_CASE_FIXTURE(IrBuilderFixture, "InvalidateReglinkVersion")
+{
+    IrOp block = build.block(IrBlockKind::Internal);
+    IrOp fallback = build.block(IrBlockKind::Fallback);
+
+    build.beginBlock(block);
+
+    build.inst(IrCmd::STORE_TAG, build.vmReg(2), build.constTag(tstring));
+    IrOp tv2 = build.inst(IrCmd::LOAD_TVALUE, build.vmReg(2));
+    build.inst(IrCmd::STORE_TVALUE, build.vmReg(1), tv2);
+    IrOp ft = build.inst(IrCmd::NEW_TABLE);
+    build.inst(IrCmd::STORE_POINTER, build.vmReg(2), ft);
+    build.inst(IrCmd::STORE_TAG, build.vmReg(2), build.constTag(ttable));
+    IrOp tv1 = build.inst(IrCmd::LOAD_TVALUE, build.vmReg(1));
+    build.inst(IrCmd::STORE_TVALUE, build.vmReg(0), tv1);
+    IrOp tag = build.inst(IrCmd::LOAD_TAG, build.vmReg(0));
+    build.inst(IrCmd::CHECK_TAG, tag, build.constTag(ttable), fallback);
+    build.inst(IrCmd::RETURN, build.constUint(0));
+
+    build.beginBlock(fallback);
+    build.inst(IrCmd::RETURN, build.constUint(1));
+
+    updateUseCounts(build.function);
+    constPropInBlockChains(build, true);
+
+    CHECK("\n" + toString(build.function, /* includeUseInfo */ false) == R"(
+bb_0:
+   STORE_TAG R2, tstring
+   %1 = LOAD_TVALUE R2
+   STORE_TVALUE R1, %1
+   %3 = NEW_TABLE
+   STORE_POINTER R2, %3
+   STORE_TAG R2, ttable
+   STORE_TVALUE R0, %1
+   %8 = LOAD_TAG R0
+   CHECK_TAG %8, ttable, bb_fallback_1
+   RETURN 0u
+
+bb_fallback_1:
+   RETURN 1u
+
+)");
+}
+
 TEST_SUITE_END();
 
 TEST_SUITE_BEGIN("LinearExecutionFlowExtraction");
@@ -2257,7 +2303,7 @@ TEST_CASE_FIXTURE(IrBuilderFixture, "NoPropagationOfCapturedRegs")
     IrOp entry = build.block(IrBlockKind::Internal);
 
     build.beginBlock(entry);
-    build.inst(IrCmd::CAPTURE, build.vmReg(0), build.constBool(true));
+    build.inst(IrCmd::CAPTURE, build.vmReg(0), build.constUint(1));
     IrOp op1 = build.inst(IrCmd::LOAD_DOUBLE, build.vmReg(0));
     IrOp op2 = build.inst(IrCmd::LOAD_DOUBLE, build.vmReg(0));
     IrOp sum = build.inst(IrCmd::ADD_NUM, op1, op2);
@@ -2273,7 +2319,7 @@ TEST_CASE_FIXTURE(IrBuilderFixture, "NoPropagationOfCapturedRegs")
 
 bb_0:
 ; in regs: R0
-   CAPTURE R0, true
+   CAPTURE R0, 1u
    %1 = LOAD_DOUBLE R0
    %2 = LOAD_DOUBLE R0
    %3 = ADD_NUM %1, %2

tests/TypeFamily.test.cpp

@@ -213,4 +213,36 @@ TEST_CASE_FIXTURE(Fixture, "add_family_at_work")
     CHECK(toString(result.errors[1]) == "Type family instance Add<string, number> is uninhabited");
 }
 
+TEST_CASE_FIXTURE(Fixture, "internal_families_raise_errors")
+{
+    if (!FFlag::DebugLuauDeferredConstraintResolution)
+        return;
+
+    CheckResult result = check(R"(
+        local function innerSum(a, b)
+            local _ = a + b
+        end
+    )");
+
+    LUAU_REQUIRE_ERROR_COUNT(1, result);
+    CHECK(toString(result.errors[0]) == "Type family instance Add<a, b> depends on generic function parameters but does not appear in the function signature; this construct cannot be type-checked at this time");
+}
+
+TEST_CASE_FIXTURE(BuiltinsFixture, "type_families_inhabited_with_normalization")
+{
+    ScopedFastFlag sff{"DebugLuauDeferredConstraintResolution", true};
+
+    CheckResult result = check(R"(
+        local useGridConfig : any
+        local columns = useGridConfig("columns", {}) or 1
+        local gutter = useGridConfig('gutter', {}) or 0
+        local margin = useGridConfig('margin', {}) or 0
+        return function(frameAbsoluteWidth: number)
+            local cellAbsoluteWidth = (frameAbsoluteWidth - 2 * margin + gutter) / columns - gutter
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
 TEST_SUITE_END();

tests/TypeInfer.functions.test.cpp

@@ -1922,6 +1922,7 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "dont_assert_when_the_tarjan_limit_is_exceede
         {"LuauClonePublicInterfaceLess2", true},
         {"LuauSubstitutionReentrant", true},
         {"LuauSubstitutionFixMissingFields", true},
+        {"LuauCloneSkipNonInternalVisit", true},
     };
 
     CheckResult result = check(R"(
@@ -1930,13 +1931,10 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "dont_assert_when_the_tarjan_limit_is_exceede
         end
     )");
 
-    LUAU_REQUIRE_ERROR_COUNT(2, result);
+    LUAU_REQUIRE_ERROR_COUNT(1, result);
 
     CHECK_MESSAGE(get<CodeTooComplex>(result.errors[0]), "Expected CodeTooComplex but got: " << toString(result.errors[0]));
     CHECK(Location({1, 17}, {1, 18}) == result.errors[0].location);
-
-    CHECK_MESSAGE(get<UnificationTooComplex>(result.errors[1]), "Expected UnificationTooComplex but got: " << toString(result.errors[1]));
-    CHECK(Location({0, 0}, {4, 4}) == result.errors[1].location);
 }
 
 /* We had a bug under DCR where instantiated type packs had a nullptr scope.

tests/TypeInfer.loops.test.cpp

@@ -60,23 +60,44 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "iteration_regression_issue_69967")
 {
     ScopedFastFlag sff{"DebugLuauDeferredConstraintResolution", true};
     CheckResult result = check(R"(
+        type Iterable = typeof(setmetatable(
+            {},
+            {}::{
+                __iter: (self: Iterable) -> () -> (number, string)
+            }
+        ))
 
-type Iterable = typeof(setmetatable(
+        local t: Iterable
-    {},
-    {}::{
-        __iter: (self: Iterable) -> () -> (number, string)
-    }
-))
 
-local t: Iterable
+        for a, b in t do end
+    )");
 
-for a, b in t do end
+    LUAU_REQUIRE_NO_ERRORS(result);
-)");
+}
 
-    LUAU_REQUIRE_ERROR_COUNT(1, result);
+TEST_CASE_FIXTURE(BuiltinsFixture, "iteration_regression_issue_69967_alt")
-    GenericError* ge = get<GenericError>(result.errors[0]);
+{
-    REQUIRE(ge);
+    ScopedFastFlag sff{"DebugLuauDeferredConstraintResolution", true};
-    CHECK_EQ("__iter metamethod must return (next[, table[, state]])", ge->message);
+    CheckResult result = check(R"(
+        type Iterable = typeof(setmetatable(
+            {},
+            {}::{
+                __iter: (self: Iterable) -> () -> (number, string)
+            }
+        ))
+
+        local t: Iterable
+        local x, y
+
+        for a, b in t do
+            x = a
+            y = b
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+    CHECK_EQ("number", toString(requireType("x")));
+    CHECK_EQ("string", toString(requireType("y")));
 }
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "for_in_loop")
@@ -777,4 +798,130 @@ TEST_CASE_FIXTURE(Fixture, "iterate_over_free_table")
     CHECK("Cannot iterate over a table without indexer" == ge->message);
 }
 
+TEST_CASE_FIXTURE(BuiltinsFixture, "dcr_iteration_explore_raycast_minimization")
+{
+    CheckResult result = check(R"(
+        local testResults = {}
+        for _, testData in pairs(testResults) do
+        end
+
+        table.insert(testResults, {})
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
+TEST_CASE_FIXTURE(BuiltinsFixture, "dcr_iteration_minimized_fragmented_keys_1")
+{
+    CheckResult result = check(R"(
+        local function rawpairs(t)
+            return next, t, nil
+        end
+
+        local function getFragmentedKeys(tbl)
+            local _ = rawget(tbl, 0)
+            for _ in rawpairs(tbl) do
+            end
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
+TEST_CASE_FIXTURE(BuiltinsFixture, "dcr_iteration_minimized_fragmented_keys_2")
+{
+    CheckResult result = check(R"(
+        local function getFragmentedKeys(tbl)
+            local _ = rawget(tbl, 0)
+            for _ in next, tbl, nil do
+            end
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
+TEST_CASE_FIXTURE(BuiltinsFixture, "dcr_iteration_minimized_fragmented_keys_3")
+{
+    CheckResult result = check(R"(
+        local function getFragmentedKeys(tbl)
+            local _ = rawget(tbl, 0)
+            for _ in pairs(tbl) do
+            end
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
+TEST_CASE_FIXTURE(BuiltinsFixture, "dcr_iteration_fragmented_keys")
+{
+    CheckResult result = check(R"(
+        local function isIndexKey(k, contiguousLength)
+            return true
+        end
+
+        local function getTableLength(tbl)
+            local length = 1
+            local value = rawget(tbl, length)
+            while value ~= nil do
+                length += 1
+                value = rawget(tbl, length)
+            end
+            return length - 1
+        end
+
+        local function rawpairs(t)
+            return next, t, nil
+        end
+
+        local function getFragmentedKeys(tbl)
+            local keys = {}
+            local keysLength = 0
+            local tableLength = getTableLength(tbl)
+            for key, _ in rawpairs(tbl) do
+                if not isIndexKey(key, tableLength) then
+                    keysLength = keysLength + 1
+                    keys[keysLength] = key
+                end
+            end
+            return keys, keysLength, tableLength
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
+TEST_CASE_FIXTURE(BuiltinsFixture, "dcr_xpath_candidates")
+{
+    CheckResult result = check(R"(
+        type Instance = {}
+        local function findCandidates(instances: { Instance },  path: { string })
+            for _, name in ipairs(path) do
+            end
+            return {}
+        end
+
+        local canditates = findCandidates({}, {})
+        for _, canditate in ipairs(canditates) do end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
+TEST_CASE_FIXTURE(BuiltinsFixture, "dcr_iteration_on_never_gives_never")
+{
+    ScopedFastFlag sff{"DebugLuauDeferredConstraintResolution", true};
+    CheckResult result = check(R"(
+        local iter: never
+        local ans
+        for xs in iter do
+            ans = xs
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+    CHECK(toString(requireType("ans")) == "never");
+}
+
 TEST_SUITE_END();

tests/TypeInfer.operators.test.cpp

@@ -679,10 +679,9 @@ TEST_CASE_FIXTURE(Fixture, "strict_binary_op_where_lhs_unknown")
 
     if (FFlag::DebugLuauDeferredConstraintResolution)
     {
-        // TODO: This will eventually entirely go away, but for now the Add
+        LUAU_REQUIRE_ERROR_COUNT(ops.size(), result);
-        // family will ensure there's one less error.
+        CHECK_EQ("Type family instance Add<a, b> depends on generic function parameters but does not appear in the function signature; this construct cannot be type-checked at this time", toString(result.errors[0]));
-        LUAU_REQUIRE_ERROR_COUNT(ops.size() - 1, result);
+        CHECK_EQ("Unknown type used in - operation; consider adding a type annotation to 'a'", toString(result.errors[1]));
-        CHECK_EQ("Unknown type used in - operation; consider adding a type annotation to 'a'", toString(result.errors[0]));
     }
     else
     {

tests/TypeInfer.rwprops.test.cpp

@@ -0,0 +1,70 @@
+// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+#include "Fixture.h"
+
+#include "doctest.h"
+
+LUAU_FASTFLAG(DebugLuauReadWriteProperties)
+
+using namespace Luau;
+
+namespace
+{
+
+struct ReadWriteFixture : Fixture
+{
+    ScopedFastFlag dcr{"DebugLuauDeferredConstraintResolution", true};
+
+    ReadWriteFixture()
+        : Fixture()
+    {
+        if (!FFlag::DebugLuauReadWriteProperties)
+            return;
+
+        TypeArena* arena = &frontend.globals.globalTypes;
+        NotNull<Scope> globalScope{frontend.globals.globalScope.get()};
+
+        unfreeze(*arena);
+
+        TypeId genericT = arena->addType(GenericType{"T"});
+
+        TypeId readonlyX = arena->addType(TableType{TableState::Sealed, TypeLevel{}, globalScope});
+        getMutable<TableType>(readonlyX)->props["x"] = Property::readonly(genericT);
+        globalScope->addBuiltinTypeBinding("ReadonlyX", TypeFun{{{genericT}}, readonlyX});
+
+        freeze(*arena);
+    }
+};
+
+} // namespace
+
+TEST_SUITE_BEGIN("ReadWriteProperties");
+
+TEST_CASE_FIXTURE(ReadWriteFixture, "read_from_a_readonly_prop")
+{
+    if (!FFlag::DebugLuauReadWriteProperties)
+        return;
+
+    CheckResult result = check(R"(
+        function f(rx: ReadonlyX<string>)
+            local x = rx.x
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
+TEST_CASE_FIXTURE(ReadWriteFixture, "write_to_a_readonly_prop")
+{
+    if (!FFlag::DebugLuauReadWriteProperties)
+        return;
+
+    CheckResult result = check(R"(
+        function f(rx: ReadonlyX<string>)
+            rx.x = "hello!" -- error
+        end
+    )");
+
+    LUAU_REQUIRE_ERROR_COUNT(1, result);
+}
+
+TEST_SUITE_END();

tests/TypeInfer.tryUnify.test.cpp

@@ -20,7 +20,7 @@ struct TryUnifyFixture : Fixture
     InternalErrorReporter iceHandler;
     UnifierSharedState unifierState{&iceHandler};
     Normalizer normalizer{&arena, builtinTypes, NotNull{&unifierState}};
-    Unifier state{NotNull{&normalizer}, Mode::Strict, NotNull{globalScope.get()}, Location{}, Variance::Covariant};
+    Unifier state{NotNull{&normalizer}, NotNull{globalScope.get()}, Location{}, Variance::Covariant};
 };
 
 TEST_SUITE_BEGIN("TryUnifyTests");

tests/TypeInfer.unionTypes.test.cpp

@@ -810,7 +810,7 @@ TEST_CASE_FIXTURE(Fixture, "free_options_can_be_unified_together")
     InternalErrorReporter iceHandler;
     UnifierSharedState sharedState{&iceHandler};
     Normalizer normalizer{&arena, builtinTypes, NotNull{&sharedState}};
-    Unifier u{NotNull{&normalizer}, Mode::Strict, NotNull{scope.get()}, Location{}, Variance::Covariant};
+    Unifier u{NotNull{&normalizer}, NotNull{scope.get()}, Location{}, Variance::Covariant};
 
     u.tryUnify(option1, option2);
 

tests/TypeInfer.unknownnever.test.cpp

@@ -324,4 +324,16 @@ TEST_CASE_FIXTURE(Fixture, "math_operators_and_never")
     CHECK_EQ("<a>(nil, a) -> boolean", toString(requireType("mul")));
 }
 
+TEST_CASE_FIXTURE(Fixture, "compare_never")
+{
+    CheckResult result = check(R"(
+        local function cmp(x: nil, y: number)
+            return x ~= nil and x > y and x < y -- infers boolean | never, which is normalized into boolean
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+    CHECK_EQ("(nil, number) -> boolean", toString(requireType("cmp")));
+}
+
 TEST_SUITE_END();

tests/conformance/native.lua

@@ -14,4 +14,27 @@ assert((function(x, y)
   return c, b, t, t1, t2
 end)(5, 10) == 50)
 
+local function fuzzfail1(...)
+  repeat
+    _ = nil
+  until not {}
+  for _ in ... do
+    for l0=_,_ do
+    end
+    return
+  end
+end
+
+local function fuzzFail2()
+  local _
+  do
+    repeat
+      _ = typeof(_),{_=_,}
+      _ = _(_._)
+    until _
+  end
+end
+
+assert(pcall(fuzzFail2) == false)
+
 return('OK')

tools/faillist.txt

@@ -151,8 +151,6 @@ TypeInferFunctions.too_many_arguments_error_location
 TypeInferFunctions.too_many_return_values_in_parentheses
 TypeInferFunctions.too_many_return_values_no_function
 TypeInferLoops.for_in_loop_error_on_factory_not_returning_the_right_amount_of_values
-TypeInferLoops.for_in_loop_with_next
-TypeInferLoops.for_in_with_generic_next
 TypeInferLoops.loop_iter_trailing_nil
 TypeInferLoops.unreachable_code_after_infinite_loop
 TypeInferModules.do_not_modify_imported_types_5
@@ -175,7 +173,6 @@ TypeInferOperators.unrelated_classes_cannot_be_compared
 TypeInferOperators.unrelated_primitives_cannot_be_compared
 TypeInferPrimitives.CheckMethodsOfNumber
 TypeInferPrimitives.string_index
-TypeInferUnknownNever.dont_unify_operands_if_one_of_the_operand_is_never_in_any_ordering_operators
 TypeInferUnknownNever.math_operators_and_never
 TypePackTests.detect_cyclic_typepacks2
 TypePackTests.pack_tail_unification_check