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[libc++][RFC] Use type traits builtins directly in <type_traits> and <concepts>
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‎libcxx/docs/CodingGuidelines.rst‎

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@@ -184,3 +184,13 @@ headers (which is sometimes required for ``constexpr`` support).
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When defining a function at the ABI boundary, it can also be useful to consider which attributes (like ``[[gnu::pure]]``
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and ``[[clang::noescape]]``) can be added to the function to improve the compiler's ability to optimize.
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Don't use type traits builtins directly
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=======================================
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Except for ``<__type_traits/*>`` and ``<__concepts/*>``, headers should avoid using the builtins for any type traits
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directly, since that makes it a lot harder to work around compiler bugs or add support for new compilers. Since the type
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traits and concepts themselves are instantiated a lot it is worth the speedup to use the builtins directly instead of
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instantiating additional templates. Note that the builtins cannot be used directly in cases where they may become part
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of the mangling of a function, since GCC doesn't mangle them. If you are unsure whether that is the case somewhere, use
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the library trait instead of the builtin.

‎libcxx/include/__concepts/class_or_enum.h‎

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@@ -10,10 +10,6 @@
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#define _LIBCPP___CONCEPTS_CLASS_OR_ENUM_H
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#include <__config>
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#include <__type_traits/is_class.h>
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#include <__type_traits/is_enum.h>
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#include <__type_traits/is_union.h>
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#include <__type_traits/remove_cvref.h>
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
@@ -26,7 +22,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
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// Whether a type is a class type or enumeration type according to the Core wording.
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template <class _Tp>
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concept __class_or_enum = is_class_v<_Tp> || is_union_v<_Tp> || is_enum_v<_Tp>;
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concept __class_or_enum = __is_class(_Tp) || __is_union(_Tp) || __is_enum(_Tp);
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#endif // _LIBCPP_STD_VER >= 20
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‎libcxx/include/__concepts/constructible.h‎

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@@ -12,7 +12,6 @@
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#include <__concepts/convertible_to.h>
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#include <__concepts/destructible.h>
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#include <__config>
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#include <__type_traits/is_constructible.h>
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
@@ -24,7 +23,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
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// [concept.constructible]
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template <class _Tp, class... _Args>
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concept constructible_from = destructible<_Tp> && is_constructible_v<_Tp, _Args...>;
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concept constructible_from = destructible<_Tp> && __is_constructible(_Tp, _Args...);
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// [concept.default.init]
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‎libcxx/include/__concepts/convertible_to.h‎

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@@ -24,7 +24,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
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// [concept.convertible]
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template <class _From, class _To>
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concept convertible_to = is_convertible_v<_From, _To> && requires { static_cast<_To>(std::declval<_From>()); };
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concept convertible_to = __is_convertible(_From, _To) && requires { static_cast<_To>(std::declval<_From>()); };
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#endif // _LIBCPP_STD_VER >= 20
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‎libcxx/include/__concepts/derived_from.h‎

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@@ -10,8 +10,6 @@
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#define _LIBCPP___CONCEPTS_DERIVED_FROM_H
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#include <__config>
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#include <__type_traits/is_base_of.h>
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#include <__type_traits/is_convertible.h>
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
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// [concept.derived]
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template <class _Dp, class _Bp>
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concept derived_from = is_base_of_v<_Bp, _Dp> && is_convertible_v<const volatile _Dp*, const volatile _Bp*>;
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concept derived_from = __is_base_of(_Bp, _Dp) && __is_convertible(const volatile _Dp*, const volatile _Bp*);
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#endif // _LIBCPP_STD_VER >= 20
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‎libcxx/include/__concepts/different_from.h‎

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@@ -11,7 +11,6 @@
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#include <__concepts/same_as.h>
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#include <__config>
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#include <__type_traits/remove_cvref.h>
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
@@ -22,7 +21,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
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#if _LIBCPP_STD_VER >= 20
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template <class _Tp, class _Up>
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concept __different_from = !same_as<remove_cvref_t<_Tp>, remove_cvref_t<_Up>>;
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concept __different_from = !same_as<__remove_cvref(_Tp), __remove_cvref(_Up)>;
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#endif // _LIBCPP_STD_VER >= 20
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‎libcxx/include/__concepts/movable.h‎

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@@ -13,7 +13,6 @@
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#include <__concepts/constructible.h>
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#include <__concepts/swappable.h>
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#include <__config>
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#include <__type_traits/is_object.h>
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
@@ -26,7 +25,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
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// [concepts.object]
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template <class _Tp>
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concept movable = is_object_v<_Tp> && move_constructible<_Tp> && assignable_from<_Tp&, _Tp> && swappable<_Tp>;
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concept movable = __is_object(_Tp) && move_constructible<_Tp> && assignable_from<_Tp&, _Tp> && swappable<_Tp>;
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#endif // _LIBCPP_STD_VER >= 20
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‎libcxx/include/__concepts/same_as.h‎

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#define _LIBCPP___CONCEPTS_SAME_AS_H
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#include <__config>
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#include <__type_traits/is_same.h>
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
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// [concept.same]
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template <class _Tp, class _Up>
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concept __same_as_impl = _IsSame<_Tp, _Up>::value;
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concept __same_as_impl = __is_same(_Tp, _Up);
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template <class _Tp, class _Up>
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concept same_as = __same_as_impl<_Tp, _Up> && __same_as_impl<_Up, _Tp>;

‎libcxx/include/__concepts/swappable.h‎

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@@ -18,7 +18,6 @@
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#include <__type_traits/extent.h>
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#include <__type_traits/is_nothrow_assignable.h>
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#include <__type_traits/is_nothrow_constructible.h>
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#include <__type_traits/remove_cvref.h>
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#include <__utility/exchange.h>
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#include <__utility/forward.h>
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#include <__utility/move.h>
@@ -46,7 +45,7 @@ void swap(_Tp&, _Tp&) = delete;
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// clang-format off
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template <class _Tp, class _Up>
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concept __unqualified_swappable_with =
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(__class_or_enum<remove_cvref_t<_Tp>> || __class_or_enum<remove_cvref_t<_Up>>) &&
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(__class_or_enum<__remove_cvref(_Tp)> || __class_or_enum<__remove_cvref(_Up)>) &&
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requires(_Tp&& __t, _Up&& __u) {
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swap(std::forward<_Tp>(__t), std::forward<_Up>(__u));
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};

‎libcxx/include/__type_traits/can_extract_key.h‎

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@@ -13,7 +13,6 @@
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#include <__fwd/pair.h>
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#include <__type_traits/conditional.h>
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#include <__type_traits/integral_constant.h>
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#include <__type_traits/is_same.h>
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#include <__type_traits/remove_const.h>
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#include <__type_traits/remove_const_ref.h>
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@@ -29,19 +28,18 @@ struct __extract_key_self_tag {};
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struct __extract_key_first_tag {};
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template <class _ValTy, class _Key, class _RawValTy = __remove_const_ref_t<_ValTy> >
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struct __can_extract_key
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: __conditional_t<_IsSame<_RawValTy, _Key>::value, __extract_key_self_tag, __extract_key_fail_tag> {};
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struct __can_extract_key : __conditional_t<__is_same(_RawValTy, _Key), __extract_key_self_tag, __extract_key_fail_tag> {
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};
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template <class _Pair, class _Key, class _First, class _Second>
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struct __can_extract_key<_Pair, _Key, pair<_First, _Second> >
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: __conditional_t<_IsSame<__remove_const_t<_First>, _Key>::value, __extract_key_first_tag, __extract_key_fail_tag> {
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};
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: __conditional_t<__is_same(__remove_const_t<_First>, _Key), __extract_key_first_tag, __extract_key_fail_tag> {};
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// __can_extract_map_key uses true_type/false_type instead of the tags.
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// It returns true if _Key != _ContainerValueTy (the container is a map not a set)
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// and _ValTy == _Key.
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template <class _ValTy, class _Key, class _ContainerValueTy, class _RawValTy = __remove_const_ref_t<_ValTy> >
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struct __can_extract_map_key : integral_constant<bool, _IsSame<_RawValTy, _Key>::value> {};
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struct __can_extract_map_key : integral_constant<bool, __is_same(_RawValTy, _Key)> {};
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// This specialization returns __extract_key_fail_tag for non-map containers
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// because _Key == _ContainerValueTy

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