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Merged
Vighnesh-V merged 2 commits into from
Sep 28, 2026
Merged

Skip subtyping reasoning that is never read #2992

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5848a91
Skip subtyping reasoning that is never read
Pyseph Sep 26, 2026
1edda12
Reword the union reasoning comment and drop an unneeded test flag
Pyseph Sep 28, 2026
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30 changes: 29 additions & 1 deletion Analysis/src/Subtyping.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,7 @@ LUAU_FASTFLAG(LuauRefactorStringSemanticSubtyping)
LUAU_FASTFLAG(DebugLuauExactTableTypes)
LUAU_FASTFLAGVARIABLE(LuauFixSuperNegationTypePaths)
LUAU_FASTFLAGVARIABLE(LuauDoNotIceForBindingGeneric)
LUAU_FASTFLAGVARIABLE(LuauSubtypingSkipUnreadReasoning)


namespace Luau
Expand Down Expand Up @@ -304,6 +305,11 @@ SubtypingResult& SubtypingResult::withBothComponent(TypePath::Component componen

SubtypingResult& SubtypingResult::withSubComponent(TypePath::Component component)
{
// Nothing reads the reasoning of a successful result: andAlso and orElse drop it, and negate
// discards it.
if (FFlag::LuauSubtypingSkipUnreadReasoning && isSubtype)
return *this;

if (reasoning.empty())
reasoning.insert(SubtypingReasoning{Path(std::move(component)), TypePath::kEmpty});
else
Expand All @@ -317,6 +323,9 @@ SubtypingResult& SubtypingResult::withSubComponent(TypePath::Component component

SubtypingResult& SubtypingResult::withSuperComponent(TypePath::Component component)
{
if (FFlag::LuauSubtypingSkipUnreadReasoning && isSubtype)
return *this;

if (reasoning.empty())
reasoning.insert(SubtypingReasoning{TypePath::kEmpty, Path(std::move(component))});
else
Expand All @@ -335,6 +344,9 @@ SubtypingResult& SubtypingResult::withBothPath(TypePath::Path path)

SubtypingResult& SubtypingResult::withSubPath(TypePath::Path path)
{
if (FFlag::LuauSubtypingSkipUnreadReasoning && isSubtype)
return *this;

if (reasoning.empty())
reasoning.insert(SubtypingReasoning{std::move(path), TypePath::kEmpty});
else
Expand All @@ -348,6 +360,9 @@ SubtypingResult& SubtypingResult::withSubPath(TypePath::Path path)

SubtypingResult& SubtypingResult::withSuperPath(TypePath::Path path)
{
if (FFlag::LuauSubtypingSkipUnreadReasoning && isSubtype)
return *this;

if (reasoning.empty())
reasoning.insert(SubtypingReasoning{TypePath::kEmpty, std::move(path)});
else
Expand Down Expand Up @@ -1532,6 +1547,9 @@ template<typename SubTy, typename SuperTy>
SubtypingResult Subtyping::isContravariantWith(SubtypingEnvironment& env, SubTy subTy, SuperTy superTy, NotNull<Scope> scope)
{
SubtypingResult result = isCovariantWith(env, superTy, subTy, scope);
if (FFlag::LuauSubtypingSkipUnreadReasoning && result.isSubtype)
return result;

if (result.reasoning.empty())
result.reasoning.insert(SubtypingReasoning{TypePath::kEmpty, TypePath::kEmpty, SubtypingVariance::Contravariant});
else
Expand Down Expand Up @@ -1564,6 +1582,9 @@ SubtypingResult Subtyping::isInvariantWith(SubtypingEnvironment& env, SubTy subT
SubtypingResult result = isCovariantWith(env, subTy, superTy, scope);
result.andAlso(isContravariantWith(env, subTy, superTy, scope));

if (FFlag::LuauSubtypingSkipUnreadReasoning && result.isSubtype)
return result;

if (result.reasoning.empty())
result.reasoning.insert(SubtypingReasoning{TypePath::kEmpty, TypePath::kEmpty, SubtypingVariance::Invariant});
else
Expand Down Expand Up @@ -1651,7 +1672,14 @@ SubtypingResult Subtyping::isCovariantWith(SubtypingEnvironment& env, TypeId sub
if (next.isSubtype)
return next;

result.andAlso(next.withSuperComponent(TypePath::Index{index, TypePath::Index::Variant::Union}));
if (FFlag::LuauSubtypingSkipUnreadReasoning)
{
// The failed reasoning emitted when subtyping `T <: A | B | C` is always cleared before returning, so we can simply skip its inclusion.
next.reasoning.clear();
result.andAlso(std::move(next));
}
else
result.andAlso(next.withSuperComponent(TypePath::Index{index, TypePath::Index::Variant::Union}));
++index;
}

Expand Down
10 changes: 10 additions & 0 deletions tests/Subtyping.test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,7 @@ LUAU_FASTFLAG(LuauNewTypePathErrorMessages)
LUAU_FASTFLAG(LuauRefactorStringSemanticSubtyping)
LUAU_FASTFLAG(DebugLuauParseExactTables)
LUAU_FASTFLAG(DebugLuauExactTableTypes)
LUAU_FASTFLAG(LuauSubtypingSkipUnreadReasoning)

using namespace Luau;

Expand Down Expand Up @@ -2251,6 +2252,15 @@ TEST_CASE_FIXTURE(SubtypeFixture, "multiple_reasonings")
);
}

TEST_CASE_FIXTURE(SubtypeFixture, "successful_subtyping_has_no_reasoning")
{
ScopedFastFlag sff{FFlag::LuauSubtypingSkipUnreadReasoning, true};

SubtypingResult result = isSubtype(getBuiltins()->numberType, negate(getBuiltins()->stringType));
CHECK(result.isSubtype);
CHECK(result.reasoning.empty());
}

TEST_CASE_FIXTURE(SubtypeFixture, "substitute_a_generic_for_a_negation")
{
// <A, B>(x: A, y: B) -> (A & ~(false?)) | B
Expand Down
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