[libc++] Simplify is_convertible

[philnik]

Summary

GCC 13 has added __is_convertible(), so we don't need to keep the fallback implementation around.

[Mordante]

LGTM! Thanks for the cleanup.

libcxx/include/__type_traits/is_convertible.h

 #include <cstddef>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if __has_builtin(__is_convertible) && !defined(_LIBCPP_USE_IS_CONVERTIBLE_FALLBACK)
-
 template <class _T1, class _T2>
 struct _LIBCPP_TEMPLATE_VIS is_convertible : public integral_constant<bool, __is_convertible(_T1, _T2)> {};
 
-#elif __has_builtin(__is_convertible_to) && !defined(_LIBCPP_USE_IS_CONVERTIBLE_FALLBACK)
-
-template <class _T1, class _T2>
-struct _LIBCPP_TEMPLATE_VIS is_convertible : public integral_constant<bool, __is_convertible_to(_T1, _T2)> {};
-
-// TODO: Remove this fallback when GCC < 13 support is no longer required.
-// GCC 13 has the __is_convertible built-in.
-#else // __has_builtin(__is_convertible_to) && !defined(_LIBCPP_USE_IS_CONVERTIBLE_FALLBACK)
-
-namespace __is_convertible_imp {
-template <class _Tp>
-void __test_convert(_Tp);
-
-template <class _From, class _To, class = void>
-struct __is_convertible_test : public false_type {};
-
-template <class _From, class _To>
-struct __is_convertible_test<_From, _To, decltype(__is_convertible_imp::__test_convert<_To>(std::declval<_From>()))>
-    : public true_type {};
-
-// clang-format off
-template <class _Tp,
-          bool _IsArray    = is_array<_Tp>::value,
-          bool _IsFunction = is_function<_Tp>::value,
-          bool _IsVoid     = is_void<_Tp>::value>
-                     struct __is_array_function_or_void                          { enum { value = 0 }; };
-template <class _Tp> struct __is_array_function_or_void<_Tp, true, false, false> { enum { value = 1 }; };
-template <class _Tp> struct __is_array_function_or_void<_Tp, false, true, false> { enum { value = 2 }; };
-template <class _Tp> struct __is_array_function_or_void<_Tp, false, false, true> { enum { value = 3 }; };
-// clang-format on
-} // namespace __is_convertible_imp
-
-template <class _Tp,
-          unsigned = __is_convertible_imp::__is_array_function_or_void<__libcpp_remove_reference_t<_Tp> >::value>
-struct __is_convertible_check {
-  static const size_t __v = 0;
-};
-
-template <class _Tp>
-struct __is_convertible_check<_Tp, 0> {
-  static const size_t __v = sizeof(_Tp);
-};
-
-template <class _T1,
-          class _T2,
-          unsigned _T1_is_array_function_or_void = __is_convertible_imp::__is_array_function_or_void<_T1>::value,
-          unsigned _T2_is_array_function_or_void = __is_convertible_imp::__is_array_function_or_void<_T2>::value>
-struct __is_convertible
-    : public integral_constant<bool, __is_convertible_imp::__is_convertible_test<_T1, _T2>::value >{};
-
-// clang-format off
-template <class _T1, class _T2> struct __is_convertible<_T1, _T2, 0, 1> : public false_type{};
-template <class _T1, class _T2> struct __is_convertible<_T1, _T2, 1, 1> : public false_type{};
-template <class _T1, class _T2> struct __is_convertible<_T1, _T2, 2, 1> : public false_type{};
-template <class _T1, class _T2> struct __is_convertible<_T1, _T2, 3, 1> : public false_type{};
-
-template <class _T1, class _T2> struct __is_convertible<_T1, _T2, 0, 2> : public false_type{};
-template <class _T1, class _T2> struct __is_convertible<_T1, _T2, 1, 2> : public false_type{};
-template <class _T1, class _T2> struct __is_convertible<_T1, _T2, 2, 2> : public false_type{};
-template <class _T1, class _T2> struct __is_convertible<_T1, _T2, 3, 2> : public false_type{};
-
-template <class _T1, class _T2> struct __is_convertible<_T1, _T2, 0, 3> : public false_type{};
-template <class _T1, class _T2> struct __is_convertible<_T1, _T2, 1, 3> : public false_type{};
-template <class _T1, class _T2> struct __is_convertible<_T1, _T2, 2, 3> : public false_type{};
-template <class _T1, class _T2> struct __is_convertible<_T1, _T2, 3, 3> : public true_type{};
-// clang-format on
-
-template <class _T1, class _T2>
-struct _LIBCPP_TEMPLATE_VIS is_convertible : public __is_convertible<_T1, _T2> {
-  static const size_t __complete_check1 = __is_convertible_check<_T1>::__v;
-  static const size_t __complete_check2 = __is_convertible_check<_T2>::__v;
-};
-
-#endif // __has_builtin(__is_convertible_to) && !defined(_LIBCPP_USE_IS_CONVERTIBLE_FALLBACK)
-
 #if _LIBCPP_STD_VER >= 17
 template <class _From, class _To>
-inline constexpr bool is_convertible_v = is_convertible<_From, _To>::value;
+inline constexpr bool is_convertible_v = __is_convertible(_From, _To);
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___TYPE_TRAITS_IS_CONVERTIBLE_H

libcxx/test/std/utilities/meta/meta.rel/is_convertible_fallback.pass.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// UNSUPPORTED: c++03
-
-// ADDITIONAL_COMPILE_FLAGS: -D _LIBCPP_USE_IS_CONVERTIBLE_FALLBACK
-
-// UNSUPPORTED: gcc-13
-
-// type_traits
-
-// is_convertible
-
-// Test the fallback implementation.
-
-// libc++ provides a fallback implementation of the compiler trait
-// `__is_convertible` with the same name when clang doesn't.
-// Because this test forces the use of the fallback even when clang provides
-// it causing a keyword incompatibility.
-
-#include "test_macros.h"
-TEST_CLANG_DIAGNOSTIC_IGNORED("-Wkeyword-compat")
-
-#include "is_convertible.pass.cpp"