Suppress errors from well-formed-testing type traits in SFINAE contexts

There are several type traits that produce a boolean value or type based
on the well-formedness of some expression (more precisely, the immediate
context, i.e. for example excluding nested template instantiation):
* `__is_constructible` and variants,
* `__is_convertible` and variants,
* `__is_assignable` and variants,
* `__reference_{binds_to,{constructs,converts}_from}_temporary`,
* `__is_trivially_equality_comparable`,
* `__builtin_common_type`.

(It should be noted that the standard doesn't always base this on the
immediate context being well-formed: for `std::common_type` it's based
on whether some expression "denotes a valid type." But I assume that's
an editorial issue and means the same thing.)

Errors in the immediate context are suppressed, instead the type traits
return another value or produce a different type if the expression is
not well-formed. This is achieved using an `SFINAETrap` with
`AccessCheckingSFINAE` set to true. If the type trait is used outside of
an SFINAE context, errors are discarded because in that case the
`SFINAETrap` sets `InNonInstantiationSFINAEContext`, which makes
`isSFINAEContext` return an `optional(nullptr)`, which causes the errors
to be discarded in `EmitDiagnostic`. However, in an SFINAE context this
doesn't happen, and errors are added to `SuppressedDiagnostics` in the
`TemplateDeductionInfo` returned by `isSFINAEContext`. Once we're done
with deducing template arguments and have decided which template is
going to be instantiated, the errors corresponding to the chosen
template are then emitted. At this point we get errors from those type
traits that we wouldn't have seen if used with the same arguments
outside of an SFINAE context. That doesn't seem right.

So what we want to do is always set `InNonInstantiationSFINAEContext`
when evaluating these well-formed-testing type traits, regardless of
whether we're in an SFINAE context or not. This should only affect the
immediate context, as nested contexts add a new `CodeSynthesisContext`
that resets `InNonInstantiationSFINAEContext` for the time it's active.

Going through uses of `SFINAETrap` with `AccessCheckingSFINAE` = `true`,
it occurred to me that all of them want this behavior and we can just
use this parameter to decide whether to use a non-instantiation context.
The uses are precisely the type traits mentioned above plus the
`TentativeAnalysisScope`, where I think it is also fine. (Though I think
we don't do tentative analysis in SFINAE contexts anyway.)

Because the parameter no longer just sets `AccessCheckingSFINAE` in Sema
but also `InNonInstantiationSFINAEContext`, I think it should be renamed
(along with uses, which also point the reviewer to the affected places).
Since we're testing for well-formedness of some expression, I think
`WellFormedSFINAE` is a good new name.

The added tests should more or less correspond to the users of
`SFINAETrap` with `AccessCheckingSFINAE` = `true`. I added a test for
errors outside of the immediate context for only one type trait, because
it requires some setup and is relatively noisy.

We put the `WellFormedSFINAE` condition first because it's constant in
all uses and would allow the compiler to prune the call to
`isSFINAEContext` when true.

Fixes #132044.

Author: aaronpuchert

clang/include/clang/Sema/Sema.h

@@ -12240,16 +12240,16 @@ class Sema final : public SemaBase {
     bool PrevLastDiagnosticIgnored;
 
   public:
-    explicit SFINAETrap(Sema &SemaRef, bool AccessCheckingSFINAE = false)
+    explicit SFINAETrap(Sema &SemaRef, bool TestWellformedSFINAE = false)
         : SemaRef(SemaRef), PrevSFINAEErrors(SemaRef.NumSFINAEErrors),
           PrevInNonInstantiationSFINAEContext(
               SemaRef.InNonInstantiationSFINAEContext),
           PrevAccessCheckingSFINAE(SemaRef.AccessCheckingSFINAE),
           PrevLastDiagnosticIgnored(
               SemaRef.getDiagnostics().isLastDiagnosticIgnored()) {
-      if (!SemaRef.isSFINAEContext())
+      if (TestWellformedSFINAE || !SemaRef.isSFINAEContext())
         SemaRef.InNonInstantiationSFINAEContext = true;
-      SemaRef.AccessCheckingSFINAE = AccessCheckingSFINAE;
+      SemaRef.AccessCheckingSFINAE = TestWellformedSFINAE;
     }
 
     ~SFINAETrap() {
@@ -12279,7 +12279,7 @@ class Sema final : public SemaBase {
 
   public:
     explicit TentativeAnalysisScope(Sema &SemaRef)
-        : SemaRef(SemaRef), Trap(SemaRef, true),
+        : SemaRef(SemaRef), Trap(SemaRef, /*TestWellformedSFINAE=*/true),
           PrevDisableTypoCorrection(SemaRef.DisableTypoCorrection) {
       SemaRef.DisableTypoCorrection = true;
     }

clang/lib/Sema/SemaConcept.cpp

@@ -908,7 +908,7 @@ static const Expr *SubstituteConstraintExpressionWithoutSatisfaction(
   if (MLTAL.getNumSubstitutedLevels() == 0)
     return ConstrExpr;
 
-  Sema::SFINAETrap SFINAE(S, /*AccessCheckingSFINAE=*/false);
+  Sema::SFINAETrap SFINAE(S);
 
   Sema::InstantiatingTemplate Inst(
       S, DeclInfo.getLocation(),

clang/lib/Sema/SemaExprCXX.cpp

@@ -5198,7 +5198,7 @@ static bool HasNonDeletedDefaultedEqualityComparison(Sema &S,
   {
     EnterExpressionEvaluationContext UnevaluatedContext(
         S, Sema::ExpressionEvaluationContext::Unevaluated);
-    Sema::SFINAETrap SFINAE(S, /*AccessCheckingSFINAE=*/true);
+    Sema::SFINAETrap SFINAE(S, /*TestWellformedSFINAE=*/true);
     Sema::ContextRAII TUContext(S, S.Context.getTranslationUnitDecl());
 
     // const ClassT& obj;
@@ -5809,7 +5809,7 @@ static ExprResult CheckConvertibilityForTypeTraits(
   // trap at translation unit scope.
   EnterExpressionEvaluationContext Unevaluated(
       Self, Sema::ExpressionEvaluationContext::Unevaluated);
-  Sema::SFINAETrap SFINAE(Self, /*AccessCheckingSFINAE=*/true);
+  Sema::SFINAETrap SFINAE(Self, /*TestWellformedSFINAE=*/true);
   Sema::ContextRAII TUContext(Self, Self.Context.getTranslationUnitDecl());
   InitializationSequence Init(Self, To, Kind, From);
   if (Init.Failed())
@@ -5932,7 +5932,7 @@ static bool EvaluateBooleanTypeTrait(Sema &S, TypeTrait Kind,
     // trap at translation unit scope.
     EnterExpressionEvaluationContext Unevaluated(
         S, Sema::ExpressionEvaluationContext::Unevaluated);
-    Sema::SFINAETrap SFINAE(S, /*AccessCheckingSFINAE=*/true);
+    Sema::SFINAETrap SFINAE(S, /*TestWellformedSFINAE=*/true);
     Sema::ContextRAII TUContext(S, S.Context.getTranslationUnitDecl());
     InitializedEntity To(
         InitializedEntity::InitializeTemporary(S.Context, Args[0]));
@@ -6272,7 +6272,7 @@ static bool EvaluateBinaryTypeTrait(Sema &Self, TypeTrait BTT, const TypeSourceI
     // trap at translation unit scope.
     EnterExpressionEvaluationContext Unevaluated(
         Self, Sema::ExpressionEvaluationContext::Unevaluated);
-    Sema::SFINAETrap SFINAE(Self, /*AccessCheckingSFINAE=*/true);
+    Sema::SFINAETrap SFINAE(Self, /*TestWellformedSFINAE=*/true);
     Sema::ContextRAII TUContext(Self, Self.Context.getTranslationUnitDecl());
     ExprResult Result = Self.BuildBinOp(/*S=*/nullptr, KeyLoc, BO_Assign, &Lhs,
                                         &Rhs);

clang/lib/Sema/SemaTemplate.cpp

@@ -3112,7 +3112,7 @@ static QualType builtinCommonTypeImpl(Sema &S, TemplateName BaseTemplate,
 
     EnterExpressionEvaluationContext UnevaluatedContext(
         S, Sema::ExpressionEvaluationContext::Unevaluated);
-    Sema::SFINAETrap SFINAE(S, /*AccessCheckingSFINAE=*/true);
+    Sema::SFINAETrap SFINAE(S, /*TestWellformedSFINAE=*/true);
     Sema::ContextRAII TUContext(S, S.Context.getTranslationUnitDecl());
 
     QualType BaseTemplateInst =
@@ -3158,7 +3158,7 @@ static QualType builtinCommonTypeImpl(Sema &S, TemplateName BaseTemplate,
       auto CheckConditionalOperands = [&](bool ConstRefQual) -> QualType {
         EnterExpressionEvaluationContext UnevaluatedContext(
             S, Sema::ExpressionEvaluationContext::Unevaluated);
-        Sema::SFINAETrap SFINAE(S, /*AccessCheckingSFINAE=*/true);
+        Sema::SFINAETrap SFINAE(S, /*TestWellformedSFINAE=*/true);
         Sema::ContextRAII TUContext(S, S.Context.getTranslationUnitDecl());
 
         // false

clang/test/SemaCXX/type-trait-common-type.cpp

@@ -34,6 +34,7 @@ void test_vla() {
 
 template <class... Args>
 using common_type_base = __builtin_common_type<common_type_t, type_identity, empty_type, Args...>;
+// expected-note@-1 {{in instantiation of default function argument expression for 'InvalidConversion<void>' required here}}
 
 template <class... Args>
 struct common_type : common_type_base<Args...> {};
@@ -208,3 +209,36 @@ struct common_type<PrivateTypeMember, PrivateTypeMember>
 };
 
 static_assert(__is_same(common_type_base<PrivateTypeMember, PrivateTypeMember, PrivateTypeMember>, empty_type));
+
+class PrivateConstructor {
+private:
+  PrivateConstructor(int);
+};
+
+static_assert(__is_same(common_type_base<int, PrivateConstructor>, empty_type));
+
+// expected-note@+1 {{in instantiation of template type alias 'common_type_base' requested here}}
+template<typename A, typename B, typename Res = common_type_base<A, B>>
+static Res common_type_sfinae();
+// expected-note@-1 {{in instantiation of default argument for 'common_type_sfinae<int, InvalidConversion>' required here}}
+
+// Make sure we swallow "calling a private constructor" even in SFINAE context ...
+static_assert(__is_same(decltype(common_type_sfinae<int, PrivateConstructor>()), empty_type));
+
+// ... but not when there is an error outside of the immediate context.
+template<typename T>
+struct Member {
+  T t; // expected-error {{field has incomplete type 'void'}}
+};
+
+// The conversion from int has a non-SFINAE error.
+class InvalidConversion {
+private:
+  template<typename T = void>
+  InvalidConversion(int, Member<T> = {});
+    // expected-note@-1 {{in instantiation of template class 'Member<void>' requested here}}
+    // expected-note@-2 {{passing argument to parameter here}}
+};
+
+// expected-note@+1 {{while substituting deduced template arguments into function template 'common_type_sfinae'}}
+static_assert(__is_same(decltype(common_type_sfinae<int, InvalidConversion>()), empty_type));

clang/test/SemaCXX/type-traits.cpp

@@ -2673,6 +2673,9 @@ struct FloatWrapper
   }
 };
 
+template<typename A, typename B, bool = __is_convertible(A, B)>
+static void is_convertible_sfinae();
+
 void is_convertible()
 {
   static_assert(__is_convertible(IntWrapper, IntWrapper));
@@ -2697,6 +2700,10 @@ void is_convertible()
   static_assert(__is_convertible(FloatWrapper, const float&));
   static_assert(__is_convertible(float, FloatWrapper&&));
   static_assert(__is_convertible(float, const FloatWrapper&));
+
+  static_assert(!__is_convertible(AllPrivate, AllPrivate));
+  // Make sure we swallow "calling a private constructor" even in SFINAE context.
+  using Result = decltype(is_convertible_sfinae<AllPrivate, AllPrivate>());
 }
 
 void is_nothrow_convertible()
@@ -2822,6 +2829,9 @@ void is_trivial()
 
 template<typename T> struct TriviallyConstructibleTemplate {};
 
+template<typename A, typename B, bool = __is_assignable(A, B)>
+static void is_assignable_sfinae();
+
 void trivial_checks()
 {
   static_assert(__is_trivially_copyable(int));
@@ -2995,6 +3005,10 @@ void trivial_checks()
   static_assert(!__is_assignable(AnIncompleteType[1], AnIncompleteType[1])); // expected-error {{incomplete type}}
   static_assert(!__is_assignable(void, void));
   static_assert(!__is_assignable(const volatile void, const volatile void));
+
+  static_assert(!__is_assignable(AllPrivate, AllPrivate));
+  // Make sure we swallow "'operator=' is a private member" even in SFINAE context.
+  using Result = decltype(is_assignable_sfinae<AllPrivate, AllPrivate>());
 }
 
 void constructible_checks() {
@@ -3191,6 +3205,9 @@ void reference_constructs_from_temporary_checks() {
 
 }
 
+template<typename A, typename B, bool = __reference_converts_from_temporary(A, B)>
+static void reference_converts_from_temporary_sfinae();
+
 void reference_converts_from_temporary_checks() {
   static_assert(!__reference_converts_from_temporary(int &, int &));
   static_assert(!__reference_converts_from_temporary(int &, int &&));
@@ -3241,6 +3258,9 @@ void reference_converts_from_temporary_checks() {
   static_assert(__reference_converts_from_temporary(const int&, ExplicitConversionRef));
   static_assert(__reference_converts_from_temporary(int&&, ExplicitConversionRvalueRef));
 
+  static_assert(!__reference_converts_from_temporary(AllPrivate, AllPrivate));
+  // Make sure we swallow "calling a private constructor" even in SFINAE context.
+  using Result = decltype(reference_converts_from_temporary_sfinae<AllPrivate, AllPrivate>());
 }
 
 void array_rank() {
@@ -4075,6 +4095,20 @@ struct NotTriviallyEqualityComparableNonTriviallyEqualityComparableArrs2 {
 
 static_assert(!__is_trivially_equality_comparable(NotTriviallyEqualityComparableNonTriviallyEqualityComparableArrs2));
 
+struct NotTriviallyEqualityComparablePrivateComparison {
+  int i;
+
+private:
+  bool operator==(const NotTriviallyEqualityComparablePrivateComparison&) const = default;
+};
+static_assert(!__is_trivially_equality_comparable(NotTriviallyEqualityComparablePrivateComparison));
+
+template<typename T, bool = __is_trivially_equality_comparable(T)>
+static void is_trivially_equality_comparable_sfinae();
+
+// Make sure we swallow "'operator==' is a private member" even in SFINAE context.
+using SFINAEResult = decltype(is_trivially_equality_comparable_sfinae<NotTriviallyEqualityComparablePrivateComparison>());
+
 template<bool B>
 struct MaybeTriviallyEqualityComparable {
     int i;