__builtin_is_implicit_lifetime returns false for function pointers

[TymianekPL]

Clang 20 (MSVC STL version) returns false for is implicit lifetime builtin for function pointers

[TymianekPL]

#if _HAS_CXX23 && defined(__clang__)

_EXPORT_STD template <class _Ty>
struct is_implicit_lifetime : bool_constant<__builtin_is_implicit_lifetime(_Ty)> {};

_EXPORT_STD template <class _Ty>
constexpr bool is_implicit_lifetime_v = __builtin_is_implicit_lifetime(_Ty);

#endif // ^^^ _HAXCXX23 && __clang__

From the STL (MSVC)

Tests:

template <typename T>
constexpr bool test_implicit_lifetime = is_implicit_lifetime_v<T> && is_implicit_lifetime<T>::value;

// Basics (arrays thereof included in Arrays section)
static_assert(test_implicit_lifetime<int>);
static_assert(test_implicit_lifetime<int*>);
static_assert(test_implicit_lifetime<nullptr_t>);
static_assert(test_implicit_lifetime<UnscopedEnum>);
static_assert(test_implicit_lifetime<ScopedEnum>);
static_assert(test_implicit_lifetime<void (*)()>); // fails!!!
static_assert(test_implicit_lifetime<TrivialClass>);
static_assert(!test_implicit_lifetime<UserProvidedDestructorClass>);

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Tymianek (TymianekPL)

Clang 20 (MSVC STL version) returns false for is implicit lifetime builtin for function pointers

[shafik]

Can you provide a godbolt that shows this behavior, I am not having any success getting this to reproduce.

Nothing stands out here to me at least:

llvm-project/clang/lib/Sema/SemaTypeTraits.cpp

Lines 1141 to 1174 in d2fbca8

	case UTT_IsImplicitLifetime: {
	DiagnoseVLAInCXXTypeTrait(Self, TInfo,
	tok::kw___builtin_is_implicit_lifetime);
	DiagnoseAtomicInCXXTypeTrait(Self, TInfo,
	tok::kw___builtin_is_implicit_lifetime);
	// [basic.types.general] p9
	// Scalar types, implicit-lifetime class types ([class.prop]),
	// array types, and cv-qualified versions of these types
	// are collectively called implicit-lifetime types.
	QualType UnqualT = T->getCanonicalTypeUnqualified();
	if (UnqualT->isScalarType())
	return true;
	if (UnqualT->isArrayType() || UnqualT->isVectorType())
	return true;
	const CXXRecordDecl *RD = UnqualT->getAsCXXRecordDecl();
	if (!RD)
	return false;
	// [class.prop] p9
	// A class S is an implicit-lifetime class if
	// - it is an aggregate whose destructor is not user-provided or
	// - it has at least one trivial eligible constructor and a trivial,
	// non-deleted destructor.
	const CXXDestructorDecl *Dtor = RD->getDestructor();
	if (UnqualT->isAggregateType())
	if (Dtor && !Dtor->isUserProvided())
	return true;
	if (RD->hasTrivialDestructor() && (!Dtor || !Dtor->isDeleted()))
	if (RD->hasTrivialDefaultConstructor() ||
	RD->hasTrivialCopyConstructor() || RD->hasTrivialMoveConstructor())
	return true;
	return false;
	}

[cor3ntin]

I cannot reproduce either https://gcc.godbolt.org/z/a1bb5j8Y1

[frederick-vs-ja]

Possible background:

• https://github.com/microsoft/STL/pull/5445/checks?check_run_id=50466660599

Error messages

Build setup steps:
Build steps:
Build step failed unexpectedly.
Command: "C:\Program Files\Microsoft Visual Studio\18\Insiders\VC\Tools\Llvm\x64\bin\clang-cl.EXE" "-c" "C:\a\_work\1\s\tests\std\tests\P2674R1_is_implicit_lifetime\test.compile.pass.cpp" "-IC:\stlBuild\out\inc" "-IC:\a\_work\1\s\llvm-project\libcxx\test\support" "-IC:\a\_work\1\s\tests\std\include" "/nologo" "/Od" "/W4" "/w14061" "/w14242" "/w14265" "/w14582" "/w14583" "/w14587" "/w14588" "/w14749" "/w14841" "/w14842" "/w15038" "/w15214" "/w15215" "/w15216" "/w15217" "/w15262" "/sdl" "/WX" "/D_ENABLE_STL_INTERNAL_CHECK" "/bigobj" "/FIforce_include.hpp" "/w14365" "/w14668" "/w15267" "/D_ENFORCE_FACET_SPECIALIZATIONS=1" "/w14640" "/Zc:threadSafeInit-" "-fno-ms-compatibility" "-fno-delayed-template-parsing" "-Wno-unqualified-std-cast-call" "/EHsc" "/std:c++latest" "/permissive-" "/MTd" "/fp:strict" "--start-no-unused-arguments" "-m64"
Exit Code: 1 (0x1)
Standard Error:
--
C:\a\_work\1\s\tests\std\tests\P2674R1_is_implicit_lifetime\test.compile.pass.cpp(30,15): error: static assertion failed due to requirement '!test_implicit_lifetime<void (*)()>'
   30 | static_assert(!test_implicit_lifetime<void (*)()>);
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:\a\_work\1\s\tests\std\tests\P2674R1_is_implicit_lifetime\test.compile.pass.cpp(32,39): error: use of undeclared identifier 'UserProvidedDestructorClass'
   32 | static_assert(!test_implicit_lifetime<UserProvidedDestructorClass>);
      |                                       ^
C:\a\_work\1\s\tests\std\tests\P2674R1_is_implicit_lifetime\test.compile.pass.cpp(38,38): error: use of undeclared identifier 'UserProvidedDestructorClass'
   38 | static_assert(test_implicit_lifetime<UserProvidedDestructorClass[12]>);
      |                                      ^
3 errors generated.
--

It seems that you misread the error messages. The static assertion failure indicated that !test_implicit_lifetime<void (*)()> is false, i.e. test_implicit_lifetime<void (*)()> is true, which is perfectly fine.

Closing as invalid Resolved as invalid, i.e. not a bug .

[TymianekPL]

It fails when the exclamation mark is removed (at least for me)

[frederick-vs-ja]

It fails when the exclamation mark is removed (at least for me)

Clang at least did right things in CI. Could you post some proper error message?