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libstdc++: Fix std::indirectly_readable ambiguity [LWG 3446]

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This implements the proposed resolution of LWG 3446. I'm also adding
another new constrained specialization which isn't proposed by 3446, to
resolve the ambiguity when a type has both value_type and element_type
but denoting different types.

libstdc++-v3/ChangeLog:

	* include/bits/iterator_concepts.h (indirectly_readable): Add
	partial specializations to resolve ambiguities (LWG 3446).
	* testsuite/24_iterators/associated_types/readable.traits.cc:
	Check types with both value_type and element_type.
This commit is contained in:
Jonathan Wakely 2020-08-24 16:18:31 +01:00
parent a0e6f05d26
commit 186aa63045
2 changed files with 54 additions and 2 deletions
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30
libstdc++-v3/include/bits/iterator_concepts.h
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@ -220,6 +220,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename _Tp> requires is_object_v<_Tp>
struct __cond_value_type<_Tp>
{ using value_type = remove_cv_t<_Tp>; };
template<typename _Tp>
concept __has_member_value_type
= requires { typename _Tp::value_type; };
template<typename _Tp>
concept __has_member_element_type
= requires { typename _Tp::element_type; };
} // namespace __detail
template<typename> struct indirectly_readable_traits { };
@ -238,16 +247,33 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
: indirectly_readable_traits<_Iter>
{ };
template<typename _Tp> requires requires { typename _Tp::value_type; }
template<__detail::__has_member_value_type _Tp>
struct indirectly_readable_traits<_Tp>
: __detail::__cond_value_type<typename _Tp::value_type>
{ };
template<typename _Tp> requires requires { typename _Tp::element_type; }
template<__detail::__has_member_element_type _Tp>
struct indirectly_readable_traits<_Tp>
: __detail::__cond_value_type<typename _Tp::element_type>
{ };
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 3446. indirectly_readable_traits ambiguity for types with both [...]
template<__detail::__has_member_value_type _Tp>
requires __detail::__has_member_element_type<_Tp>
&& same_as<remove_cv_t<typename _Tp::element_type>,
remove_cv_t<typename _Tp::value_type>>
struct indirectly_readable_traits<_Tp>
: __detail::__cond_value_type<typename _Tp::value_type>
{ };
// LWG 3446 doesn't add this, but it's needed for the case where
// value_type and element_type are both present, but not the same type.
template<__detail::__has_member_value_type _Tp>
requires __detail::__has_member_element_type<_Tp>
struct indirectly_readable_traits<_Tp>
{ };
namespace __detail
{
template<typename _Tp>

26
libstdc++-v3/testsuite/24_iterators/associated_types/readable.traits.cc
View File

@ -141,3 +141,29 @@ struct J
// iterator_traits<J> matches constrained specialization in the library,
// so use its value_type.
static_assert( check_alias<J, int> );
struct I2
{
using element_type = int;
};
// iterator_traits<I2> is not specialized, and no standard specialization
// matches, so use indirectly_readable_traits.
static_assert( check_alias<I2, std::indirectly_readable_traits<I2>::value_type> );
// LWG 3446
struct I3
{
using value_type = long;
using element_type = const long;
};
// iterator_traits<I3> is not specialized, and no standard specialization
// matches, so use indirectly_readable_traits.
static_assert( check_alias<I3, std::indirectly_readable_traits<I3>::value_type> );
// Correction to LWG 3446
struct I4
{
using value_type = int;
using element_type = long;
};
static_assert( ! has_alias<I4> );