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libstdc++: Simplify detection idiom using concepts

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Add a simpler definition of std::__detected_or using concepts.  This
also replaces the __detector::value_t member which should have been using
a reserved name.

Use __detected_or in pointer_traits.

libstdc++-v3/ChangeLog:

	* include/bits/alloc_traits.h (allocator_traits::is_always_equal):
	Only instantiate is_empty if needed.
	* include/bits/ptr_traits.h (__ptr_traits_impl::difference_type)
	(__ptr_traits_impl::rebind): Use __detected_or.
	* include/experimental/type_traits (is_same_v): Add a partial
	specialization instead of instantiating the std::is_same class
	template.
	(detected_t): Redefine in terms of detected_or_t.
	(is_detected, is_detected_v): Redefine in terms of detected_t.
	* include/std/type_traits [__cpp_concepts] (__detected_or): Add
	new definition using concepts.
	(__detector::value_t): Rename to __is_detected.
	* testsuite/17_intro/names.cc: Check value_t isn't used.
This commit is contained in:
Jonathan Wakely 2022-09-23 23:16:30 +01:00
parent ad2aab5c81
commit 2b667beba6
5 changed files with 44 additions and 39 deletions
libstdc++-v3
include
bits
alloc_traits.hptr_traits.h
experimental
type_traits
std
type_traits
testsuite/17_intro
names.cc

4
libstdc++-v3/include/bits/alloc_traits.h

@ -74,7 +74,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename _Tp>
using __pocs = typename _Tp::propagate_on_container_swap;
template<typename _Tp>
using __equal = typename _Tp::is_always_equal;
using __equal = __type_identity<typename _Tp::is_always_equal>;
};
template<typename _Alloc, typename _Up>
@ -209,7 +209,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
* otherwise @c is_empty<Alloc>::type
*/
using is_always_equal
= __detected_or_t<typename is_empty<_Alloc>::type, __equal, _Alloc>;
= typename __detected_or_t<is_empty<_Alloc>, __equal, _Alloc>::type;
template<typename _Tp>
using rebind_alloc = __alloc_rebind<_Alloc, _Tp>;

27
libstdc++-v3/include/bits/ptr_traits.h

@ -144,29 +144,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
struct __ptr_traits_impl : __ptr_traits_ptr_to<_Ptr, _Elt>
{
private:
template<typename _Tp, typename = void>
struct __difference { using type = ptrdiff_t; };
template<typename _Tp>
#if __cpp_concepts
requires requires { typename _Tp::difference_type; }
struct __difference<_Tp>
#else
struct __difference<_Tp, __void_t<typename _Tp::difference_type>>
#endif
{ using type = typename _Tp::difference_type; };
template<typename _Tp, typename _Up, typename = void>
struct __rebind : __replace_first_arg<_Tp, _Up> { };
using __diff_t = typename _Tp::difference_type;
template<typename _Tp, typename _Up>
#if __cpp_concepts
requires requires { typename _Tp::template rebind<_Up>; }
struct __rebind<_Tp, _Up>
#else
struct __rebind<_Tp, _Up, __void_t<typename _Tp::template rebind<_Up>>>
#endif
{ using type = typename _Tp::template rebind<_Up>; };
using __rebind = __type_identity<typename _Tp::template rebind<_Up>>;
public:
/// The pointer type.
@ -176,11 +158,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
using element_type = _Elt;
/// The type used to represent the difference between two pointers.
using difference_type = typename __difference<_Ptr>::type;
using difference_type = __detected_or_t<ptrdiff_t, __diff_t, _Ptr>;
/// A pointer to a different type.
template<typename _Up>
using rebind = typename __rebind<_Ptr, _Up>::type;
using rebind = typename __detected_or_t<__replace_first_arg<_Ptr, _Up>,
__rebind, _Ptr, _Up>::type;
};
// _GLIBCXX_RESOLVE_LIB_DEFECTS

24
libstdc++-v3/include/experimental/type_traits

@ -223,7 +223,9 @@ template <typename _Tp, unsigned _Idx = 0>
// See C++14 20.10.6, type relations
template <typename _Tp, typename _Up>
constexpr bool is_same_v = is_same<_Tp, _Up>::value;
constexpr bool is_same_v = false;
template <typename _Tp>
constexpr bool is_same_v<_Tp, _Tp> = true;
template <typename _Base, typename _Derived>
constexpr bool is_base_of_v = is_base_of<_Base, _Derived>::value;
template <typename _From, typename _To>
@ -266,23 +268,21 @@ struct nonesuch : private __nonesuchbase
};
#pragma GCC diagnostic pop
template<template<typename...> class _Op, typename... _Args>
using is_detected
= typename std::__detector<nonesuch, void, _Op, _Args...>::value_t;
template<template<typename...> class _Op, typename... _Args>
constexpr bool is_detected_v = is_detected<_Op, _Args...>::value;
template<template<typename...> class _Op, typename... _Args>
using detected_t
= typename std::__detector<nonesuch, void, _Op, _Args...>::type;
template<typename _Default, template<typename...> class _Op, typename... _Args>
using detected_or = std::__detected_or<_Default, _Op, _Args...>;
template<typename _Default, template<typename...> class _Op, typename... _Args>
using detected_or_t = typename detected_or<_Default, _Op, _Args...>::type;
template<template<typename...> class _Op, typename... _Args>
using detected_t = detected_or_t<nonesuch, _Op, _Args...>;
template<template<typename...> class _Op, typename... _Args>
using is_detected = typename detected_or<void, _Op, _Args...>::__is_detected;
template<template<typename...> class _Op, typename... _Args>
constexpr bool is_detected_v = is_detected<_Op, _Args...>::value;
template<typename _Expected, template<typename...> class _Op, typename... _Args>
using is_detected_exact = is_same<_Expected, detected_t<_Op, _Args...>>;

27
libstdc++-v3/include/std/type_traits

@ -2551,13 +2551,34 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
/// @cond undocumented
// Detection idiom.
// Detect whether _Op<_Args...> is a valid type, use default _Def if not.
#if __cpp_concepts
// Implementation of the detection idiom (negative case).
template<typename _Def, template<typename...> class _Op, typename... _Args>
struct __detected_or
{
using type = _Def;
using __is_detected = false_type;
};
// Implementation of the detection idiom (positive case).
template<typename _Def, template<typename...> class _Op, typename... _Args>
requires requires { typename _Op<_Args...>; }
struct __detected_or<_Def, _Op, _Args...>
{
using type = _Op<_Args...>;
using __is_detected = true_type;
};
#else
/// Implementation of the detection idiom (negative case).
template<typename _Default, typename _AlwaysVoid,
template<typename...> class _Op, typename... _Args>
struct __detector
{
using value_t = false_type;
using type = _Default;
using __is_detected = false_type;
};
/// Implementation of the detection idiom (positive case).
@ -2565,14 +2586,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
typename... _Args>
struct __detector<_Default, __void_t<_Op<_Args...>>, _Op, _Args...>
{
using value_t = true_type;
using type = _Op<_Args...>;
using __is_detected = true_type;
};
// Detect whether _Op<_Args...> is a valid type, use _Default if not.
template<typename _Default, template<typename...> class _Op,
typename... _Args>
using __detected_or = __detector<_Default, void, _Op, _Args...>;
#endif // __cpp_concepts
// _Op<_Args...> if that is a valid type, otherwise _Default.
template<typename _Default, template<typename...> class _Op,

1
libstdc++-v3/testsuite/17_intro/names.cc

@ -112,6 +112,7 @@
#define tmp (
#define sz (
#define token (
#define value_t (
#if __cplusplus < 201103L
#define uses_allocator (