luau/tests/TypeInfer.unionTypes.test.cpp

// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
#include "Luau/TypeInfer.h"
#include "Luau/Type.h"

#include "Fixture.h"

#include "doctest.h"

using namespace Luau;

TEST_SUITE_BEGIN("UnionTypes");

TEST_CASE_FIXTURE(Fixture, "return_types_can_be_disjoint")
{
    CheckResult result = check(R"(
        local count = 0
        function most_of_the_natural_numbers(): number?
            if count < 10 then
                count = count + 1
                return count
            else
                return nil
            end
        end
    )");

    LUAU_REQUIRE_NO_ERRORS(result);

    const FunctionType* utv = get<FunctionType>(requireType("most_of_the_natural_numbers"));
    REQUIRE(utv != nullptr);
}

TEST_CASE_FIXTURE(Fixture, "allow_specific_assign")
{
    CheckResult result = check(R"(
        local a:number|string = 22
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "allow_more_specific_assign")
{
    CheckResult result = check(R"(
        local a:number|string = 22
        local b:number|string|nil = a
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "disallow_less_specific_assign")
{
    CheckResult result = check(R"(
        local a:number = 10
        local b:number|string = 20
        a = b
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
}

TEST_CASE_FIXTURE(Fixture, "disallow_less_specific_assign2")
{
    CheckResult result = check(R"(
        local a:number? = 10
        local b:number|string? = 20
        a = b
    )");

    REQUIRE_EQ(1, result.errors.size());
}

TEST_CASE_FIXTURE(Fixture, "optional_arguments")
{
    CheckResult result = check(R"(
        function f(a:string, b:string?)
        end
        f("s")
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "optional_arguments_table")
{
    CheckResult result = check(R"(
        local a:{a:string, b:string?}
        a = {a="ok"}
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "optional_arguments_table2")
{
    CheckResult result = check(R"(
        local a:{a:string, b:string}
        a = {a=""}
    )");
    REQUIRE(!result.errors.empty());
}

TEST_CASE_FIXTURE(BuiltinsFixture, "error_takes_optional_arguments")
{
    CheckResult result = check(R"(
        error("message")
        error("message", 2)
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "error_optional_argument_enforces_type")
{
    CheckResult result = check(R"(
        error("message", "2")
    )");

    REQUIRE(result.errors.size() == 1);
}

TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_with_property_guaranteed_to_exist")
{
    CheckResult result = check(R"(
        type A = {x: number}
        type B = {x: number}
        local t: A | B

        local r = t.x
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
    CHECK_EQ(*builtinTypes->numberType, *requireType("r"));
}

TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_with_mixed_types")
{
    CheckResult result = check(R"(
        type A = {x: number}
        type B = {x: string}
        local t: A | B

        local r = t.x
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
    CHECK_EQ("number | string", toString(requireType("r")));
}

TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_works_at_arbitrary_depth")
{
    CheckResult result = check(R"(
        type A = {x: {y: {z: {thing: number}}}}
        type B = {x: {y: {z: {thing: string}}}}
        local t: A | B

        local r = t.x.y.z.thing
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
    CHECK_EQ("number | string", toString(requireType("r")));
}

TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_with_one_optional_property")
{
    CheckResult result = check(R"(
        type A = {x: number}
        type B = {x: number?}
        local t: A | B

        local r = t.x
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
    CHECK_EQ("number?", toString(requireType("r")));
}

TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_with_missing_property")
{
    CheckResult result = check(R"(
        type A = {x: number}
        type B = {}
        local t: A | B

        local r = t.x
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);

    MissingUnionProperty* mup = get<MissingUnionProperty>(result.errors[0]);
    REQUIRE(mup);
    CHECK_EQ(mup->type, requireType("t"));
    REQUIRE(mup->missing.size() == 1);
    std::optional<TypeId> bTy = lookupType("B");
    REQUIRE(bTy);
    CHECK_EQ(mup->missing[0], *bTy);
    CHECK_EQ(mup->key, "x");
    CHECK_EQ("*error-type*", toString(requireType("r")));
}

TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_with_one_property_of_type_any")
{
    CheckResult result = check(R"(
        type A = {x: number}
        type B = {x: any}
        local t: A | B

        local r = t.x
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
    CHECK_EQ(*builtinTypes->anyType, *requireType("r"));
}

TEST_CASE_FIXTURE(Fixture, "union_equality_comparisons")
{
    CheckResult result = check(R"(
        type A = number | string | nil
        type B = number | nil
        type C = number | boolean

        local a: A = 1
        local b: B = nil
        local c: C = true
        local n = 1

        local x = a == b
        local y = a == n
        local z = a == c
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "optional_union_members")
{
    CheckResult result = check(R"(
local a = { a = { x = 1, y = 2 }, b = 3 }
type A = typeof(a)
local b: A? = a
local bf = b
local c = bf.a.y
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ(*builtinTypes->numberType, *requireType("c"));
    CHECK_EQ("Value of type 'A?' could be nil", toString(result.errors[0]));
}

TEST_CASE_FIXTURE(Fixture, "optional_union_functions")
{
    CheckResult result = check(R"(
        local a = {}
        function a.foo(x:number, y:number) return x + y end
        type A = typeof(a)
        local b: A? = a
        local c = b.foo(1, 2)
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ(*builtinTypes->numberType, *requireType("c"));
    CHECK_EQ("Value of type 'A?' could be nil", toString(result.errors[0]));
}

TEST_CASE_FIXTURE(Fixture, "optional_union_methods")
{
    CheckResult result = check(R"(
local a = {}
function a:foo(x:number, y:number) return x + y end
type A = typeof(a)
local b: A? = a
local c = b:foo(1, 2)
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ(*builtinTypes->numberType, *requireType("c"));
    CHECK_EQ("Value of type 'A?' could be nil", toString(result.errors[0]));
}

TEST_CASE_FIXTURE(Fixture, "optional_union_follow")
{
    CheckResult result = check(R"(
local y: number? = 2
local x = y
local function f(a: number, b: typeof(x), c: typeof(x)) return -a end
return f()
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);

    auto acm = get<CountMismatch>(result.errors[0]);
    REQUIRE(acm);
    CHECK_EQ(1, acm->expected);
    CHECK_EQ(0, acm->actual);
    CHECK_FALSE(acm->isVariadic);
}

TEST_CASE_FIXTURE(Fixture, "optional_field_access_error")
{
    CheckResult result = check(R"(
type A = { x: number }
local b: A? = { x = 2 }
local c = b.x
local d = b.y
    )");

    LUAU_REQUIRE_ERROR_COUNT(3, result);
    CHECK_EQ("Value of type 'A?' could be nil", toString(result.errors[0]));
    CHECK_EQ("Value of type 'A?' could be nil", toString(result.errors[1]));
    CHECK_EQ("Key 'y' not found in table 'A'", toString(result.errors[2]));
}

TEST_CASE_FIXTURE(Fixture, "optional_index_error")
{
    CheckResult result = check(R"(
type A = {number}
local a: A? = {1, 2, 3}
local b = a[1]
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ("Value of type 'A?' could be nil", toString(result.errors[0]));
}

TEST_CASE_FIXTURE(Fixture, "optional_call_error")
{
    CheckResult result = check(R"(
type A = (number) -> number
local a: A? = function(a) return -a end
local b = a(4)
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ("Value of type '((number) -> number)?' could be nil", toString(result.errors[0]));
}

TEST_CASE_FIXTURE(Fixture, "optional_assignment_errors")
{
    CheckResult result = check(R"(
type A = { x: number }
local a: A? = { x = 2 }
a.x = 2
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ("Value of type 'A?' could be nil", toString(result.errors[0]));

    result = check(R"(
type A = { x: number } & { y: number }
local a: A? = { x = 2, y = 3 }
a.x = 2
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    auto s = toString(result.errors[0]);
    if (FFlag::DebugLuauDeferredConstraintResolution)
        CHECK_EQ("Value of type '{| x: number, y: number |}?' could be nil", s);
    else
        CHECK_EQ("Value of type '({| x: number |} & {| y: number |})?' could be nil", s);
}

TEST_CASE_FIXTURE(Fixture, "optional_length_error")
{
    CheckResult result = check(R"(
type A = {number}
local a: A? = {1, 2, 3}
local b = #a
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ("Value of type 'A?' could be nil", toString(result.errors[0]));
}

TEST_CASE_FIXTURE(Fixture, "optional_missing_key_error_details")
{
    CheckResult result = check(R"(
type A = { x: number, y: number }
type B = { x: number, y: number }
type C = { x: number }
type D = { x: number }

local a: A|B|C|D
local b = a.y

local c: A|(B|C)?|D
local d = c.y

local e = a.z
    )");

    LUAU_REQUIRE_ERROR_COUNT(4, result);
    CHECK_EQ("Key 'y' is missing from 'C', 'D' in the type 'A | B | C | D'", toString(result.errors[0]));

    CHECK_EQ("Value of type '(A | B | C | D)?' could be nil", toString(result.errors[1]));
    CHECK_EQ("Key 'y' is missing from 'C', 'D' in the type 'A | B | C | D'", toString(result.errors[2]));

    CHECK_EQ("Type 'A | B | C | D' does not have key 'z'", toString(result.errors[3]));
}

TEST_CASE_FIXTURE(Fixture, "optional_iteration")
{
    CheckResult result = check(R"(
function foo(values: {number}?)
    local s = 0
    for _, value in values do
        s += value
    end
end
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ("Value of type '{number}?' could be nil", toString(result.errors[0]));
}

TEST_CASE_FIXTURE(Fixture, "unify_unsealed_table_union_check")
{
    CheckResult result = check(R"(
local x = { x = 3 }
type A = number?
type B = string?
local y: { x: number, y: A | B }
y = x
    )");

    LUAU_REQUIRE_NO_ERRORS(result);

    result = check(R"(
local x = { x = 3 }

local a: number? = 2
local y = {}
y.x = 2
y.y = a

y = x
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "unify_sealed_table_union_check")
{
    CheckResult result = check(R"(
 -- the difference between this and unify_unsealed_table_union_check is the type annotation on x
local t = { x = 3, y = true }
local x: { x: number } = t
type A = number?
type B = string?
local y: { x: number, y: A | B }
-- Shouldn't typecheck!
y = x
-- If it does, we can convert any type to any other type
y.y = 5
local oh : boolean = t.y
    )");

    LUAU_REQUIRE_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "error_detailed_union_part")
{
    CheckResult result = check(R"(
type X = { x: number }
type Y = { y: number }
type Z = { z: number }

type XYZ = X | Y | Z

local a: XYZ
local b: { w: number } = a
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ(toString(result.errors[0]), R"(Type 'X | Y | Z' could not be converted into '{| w: number |}'
caused by:
  Not all union options are compatible. Table type 'X' not compatible with type '{| w: number |}' because the former is missing field 'w')");
}

TEST_CASE_FIXTURE(Fixture, "error_detailed_union_all")
{
    CheckResult result = check(R"(
type X = { x: number }
type Y = { y: number }
type Z = { z: number }

type XYZ = X | Y | Z

local a: XYZ = { w = 4 }
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ(toString(result.errors[0]), R"(Type 'a' could not be converted into 'X | Y | Z'; none of the union options are compatible)");
}

TEST_CASE_FIXTURE(Fixture, "error_detailed_optional")
{
    CheckResult result = check(R"(
type X = { x: number }

local a: X? = { w = 4 }
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ(toString(result.errors[0]), R"(Type 'a' could not be converted into 'X?'
caused by:
  None of the union options are compatible. For example: Table type 'a' not compatible with type 'X' because the former is missing field 'x')");
}

// We had a bug where a cyclic union caused a stack overflow.
// ex type U = number | U
TEST_CASE_FIXTURE(Fixture, "dont_allow_cyclic_unions_to_be_inferred")
{
    CheckResult result = check(R"(
        --!strict

        function f(a, b)
            a:g(b or {})
            a:g(b)
        end
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(BuiltinsFixture, "table_union_write_indirect")
{
    CheckResult result = check(R"(
        type A = { x: number, y: (number) -> string } | { z: number, y: (number) -> string }

        local a:A = nil

        function a.y(x)
            return tostring(x * 2)
        end

        function a.y(x: string): number
            return tonumber(x) or 0
        end
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    // NOTE: union normalization will improve this message
    CHECK_EQ(toString(result.errors[0]),
        R"(Type '(string) -> number' could not be converted into '((number) -> string) | ((number) -> string)'; none of the union options are compatible)");
}

TEST_CASE_FIXTURE(Fixture, "union_true_and_false")
{
    CheckResult result = check(R"(
        local x : boolean
        local y1 : (true | false) = x -- OK
        local y2 : (true | false | (string & number)) = x -- OK
        local y3 : (true | (string & number) | false) = x -- OK
        local y4 : (true | (boolean & true) | false) = x -- OK
     )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "union_of_functions")
{
    CheckResult result = check(R"(
        local x : (number) -> number?
        local y : ((number?) -> number?) | ((number) -> number) = x -- OK
     )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "union_of_generic_functions")
{
    CheckResult result = check(R"(
        local x : <a>(a) -> a?
        local y : (<a>(a?) -> a?) | (<b>(b) -> b) = x -- Not OK
     )");

    // TODO: should this example typecheck?
    LUAU_REQUIRE_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "union_of_generic_typepack_functions")
{
    CheckResult result = check(R"(
        local x : <a...>(number, a...) -> (number?, a...)
        local y : (<a...>(number?, a...) -> (number?, a...)) | (<b...>(number, b...) -> (number, b...)) = x -- Not OK
     )");

    // TODO: should this example typecheck?
    LUAU_REQUIRE_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "union_of_functions_mentioning_generics")
{
    CheckResult result = check(R"(
      function f<a,b>()
        local x : (a) -> a?
        local y : ((a?) -> nil) | ((a) -> a) = x -- OK
        local z : ((b?) -> nil) | ((b) -> b) = x -- Not OK
      end
     )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ(toString(result.errors[0]),
        "Type '(a) -> a?' could not be converted into '((b) -> b) | ((b?) -> nil)'; none of the union options are compatible");
}

TEST_CASE_FIXTURE(Fixture, "union_of_functions_mentioning_generic_typepacks")
{
    CheckResult result = check(R"(
      function f<a...>()
        local x : (number, a...) -> (number?, a...)
        local y : ((number | string, a...) -> (number, a...)) | ((number?, a...) -> (nil, a...)) = x -- OK
        local z : ((number) -> number) | ((number?, a...) -> (number?, a...)) = x -- Not OK
      end
     )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ(toString(result.errors[0]), "Type '(number, a...) -> (number?, a...)' could not be converted into '((number) -> number) | ((number?, "
                                         "a...) -> (number?, a...))'; none of the union options are compatible");
}

TEST_CASE_FIXTURE(Fixture, "union_of_functions_with_mismatching_arg_arities")
{
    CheckResult result = check(R"(
        local x : (number) -> number?
        local y : ((number?) -> number) | ((number | string) -> nil) = x -- OK
        local z : ((number, string?) -> number) | ((number) -> nil) = x -- Not OK
     )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ(toString(result.errors[0]), "Type '(number) -> number?' could not be converted into '((number) -> nil) | ((number, string?) -> "
                                         "number)'; none of the union options are compatible");
}

TEST_CASE_FIXTURE(Fixture, "union_of_functions_with_mismatching_result_arities")
{
    CheckResult result = check(R"(
        local x : () -> (number | string)
        local y : (() -> number) | (() -> string) = x -- OK
        local z : (() -> number) | (() -> (string, string)) = x -- Not OK
     )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ(toString(result.errors[0]), "Type '() -> number | string' could not be converted into '(() -> (string, string)) | (() -> number)'; none "
                                         "of the union options are compatible");
}

TEST_CASE_FIXTURE(Fixture, "union_of_functions_with_variadics")
{
    CheckResult result = check(R"(
        local x : (...nil) -> (...number?)
        local y : ((...string?) -> (...number)) | ((...number?) -> nil) = x -- OK
        local z : ((...string?) -> (...number)) | ((...string?) -> nil) = x -- OK
     )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ(toString(result.errors[0]), "Type '(...nil) -> (...number?)' could not be converted into '((...string?) -> (...number)) | ((...string?) "
                                         "-> nil)'; none of the union options are compatible");
}

TEST_CASE_FIXTURE(Fixture, "union_of_functions_with_mismatching_arg_variadics")
{
    CheckResult result = check(R"(
        local x : (number) -> ()
        local y : ((number?) -> ()) | ((...number) -> ()) = x -- OK
        local z : ((number?) -> ()) | ((...number?) -> ()) = x -- Not OK
     )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ(toString(result.errors[0]),
        "Type '(number) -> ()' could not be converted into '((...number?) -> ()) | ((number?) -> ())'; none of the union options are compatible");
}

TEST_CASE_FIXTURE(Fixture, "union_of_functions_with_mismatching_result_variadics")
{
    CheckResult result = check(R"(
        local x : () -> (number?, ...number)
        local y : (() -> (...number)) | (() -> nil) = x -- OK
        local z : (() -> (...number)) | (() -> number) = x -- OK
     )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);
    CHECK_EQ(toString(result.errors[0]), "Type '() -> (number?, ...number)' could not be converted into '(() -> (...number)) | (() -> number)'; none "
                                         "of the union options are compatible");
}

TEST_CASE_FIXTURE(Fixture, "less_greedy_unification_with_union_types")
{
    if (!FFlag::DebugLuauDeferredConstraintResolution)
        return;

    CheckResult result = check(R"(
        local function f(t): { x: number } | { x: string }
            local x = t.x
            return t
        end
    )");

    LUAU_REQUIRE_NO_ERRORS(result);

    CHECK_EQ("({| x: number |} | {| x: string |}) -> {| x: number |} | {| x: string |}", toString(requireType("f")));
}

TEST_CASE_FIXTURE(Fixture, "less_greedy_unification_with_union_types_2")
{
    if (!FFlag::DebugLuauDeferredConstraintResolution)
        return;

    CheckResult result = check(R"(
        local function f(t: { x: number } | { x: string })
            return t.x
        end
    )");

    LUAU_REQUIRE_NO_ERRORS(result);

    CHECK_EQ("({| x: number |} | {| x: string |}) -> number | string", toString(requireType("f")));
}

TEST_CASE_FIXTURE(Fixture, "union_table_any_property")
{
    CheckResult result = check(R"(
        function f(x)
            -- x : X
            -- sup : { p : { q : X } }?
            local sup = if true then { p = { q = x } } else nil
            local sub : { p : any }
            sup = nil
            sup = sub
        end
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "union_function_any_args")
{
    CheckResult result = check(R"(
        local sup : ((...any) -> (...any))?
        local sub : ((number) -> (...any))
        sup = sub
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "optional_any")
{
    CheckResult result = check(R"(
        local sup : any?
        local sub : number
        sup = sub
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "generic_function_with_optional_arg")
{
    ScopedFastFlag sff[] = {
        {"LuauTransitiveSubtyping", true},
    };

    CheckResult result = check(R"(
        function f<T>(x : T?) : {T}
            local result = {}
            if x then
                result[1] = x
            end
            return result
        end
        local t : {string} = f(nil)
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "lookup_prop_of_intersection_containing_unions")
{
    CheckResult result = check(R"(
        local function mergeOptions<T>(options: T & ({} | {}))
            return options.variables
        end
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);

    const UnknownProperty* unknownProp = get<UnknownProperty>(result.errors[0]);
    REQUIRE(unknownProp);

    CHECK("variables" == unknownProp->key);
}

TEST_CASE_FIXTURE(Fixture, "free_options_can_be_unified_together")
{
    ScopedFastFlag sff[] = {
        {"LuauTransitiveSubtyping", true},
        {"LuauUnifyTwoOptions", true}
    };

    TypeArena arena;
    TypeId nilType = builtinTypes->nilType;

    std::unique_ptr scope = std::make_unique<Scope>(builtinTypes->anyTypePack);

    TypeId free1 = arena.addType(FreeType{scope.get()});
    TypeId option1 = arena.addType(UnionType{{nilType, free1}});

    TypeId free2 = arena.addType(FreeType{scope.get()});
    TypeId option2 = arena.addType(UnionType{{nilType, free2}});

    InternalErrorReporter iceHandler;
    UnifierSharedState sharedState{&iceHandler};
    Normalizer normalizer{&arena, builtinTypes, NotNull{&sharedState}};
    Unifier u{NotNull{&normalizer}, Mode::Strict, NotNull{scope.get()}, Location{}, Variance::Covariant};

    u.tryUnify(option1, option2);

    CHECK(!u.failure);

    u.log.commit();

    ToStringOptions opts;
    CHECK("a?" == toString(option1, opts));
    CHECK("a?" == toString(option2, opts));
}

TEST_CASE_FIXTURE(Fixture, "unify_more_complex_unions_that_include_nil")
{
    CheckResult result = check(R"(
        type Record = {prop: (string | boolean)?}

        function concatPagination(prop: (string | boolean | nil)?): Record
            return {prop = prop}
        end
    )");

    LUAU_REQUIRE_NO_ERRORS(result);
}

TEST_CASE_FIXTURE(Fixture, "optional_class_instances_are_invariant")
{
    ScopedFastFlag sff[] = {
        {"LuauUnifyTwoOptions", true},
        {"LuauTypeMismatchInvarianceInError", true}
    };

    createSomeClasses(&frontend);

    CheckResult result = check(R"(
        function foo(ref: {current: Parent?})
        end

        function bar(ref: {current: Child?})
            foo(ref)
        end
    )");

    LUAU_REQUIRE_ERROR_COUNT(1, result);

    // The last line of this error is the most important part.  We need to
    // communicate that this is an invariant context.
    std::string expectedError =
        "Type '{| current: Child? |}' could not be converted into '{| current: Parent? |}'\n"
        "caused by:\n"
        "  Property 'current' is not compatible. Type 'Child' could not be converted into 'Parent' in an invariant context"
    ;

    CHECK(expectedError == toString(result.errors[0]));
}

TEST_SUITE_END();