[gcc(refs/users/wschmidt/heads/builtins3)] libstdc++: Fix std::indirectly_readable ambiguity [LWG 3446]
William Schmidt
wschmidt@gcc.gnu.org
Fri Aug 28 19:57:33 GMT 2020
https://gcc.gnu.org/g:186aa6304570e15065f31482e9c27326a3a6679f
commit 186aa6304570e15065f31482e9c27326a3a6679f
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Mon Aug 24 16:18:31 2020 +0100
libstdc++: Fix std::indirectly_readable ambiguity [LWG 3446]
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.
Diff:
---
libstdc++-v3/include/bits/iterator_concepts.h | 30 ++++++++++++++++++++--
.../associated_types/readable.traits.cc | 26 +++++++++++++++++++
2 files changed, 54 insertions(+), 2 deletions(-)
diff --git a/libstdc++-v3/include/bits/iterator_concepts.h b/libstdc++-v3/include/bits/iterator_concepts.h
index bd6660c5f22..a568f2ab825 100644
--- a/libstdc++-v3/include/bits/iterator_concepts.h
+++ b/libstdc++-v3/include/bits/iterator_concepts.h
@@ -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>
diff --git a/libstdc++-v3/testsuite/24_iterators/associated_types/readable.traits.cc b/libstdc++-v3/testsuite/24_iterators/associated_types/readable.traits.cc
index b503b0cdc1e..3c9c3927153 100644
--- a/libstdc++-v3/testsuite/24_iterators/associated_types/readable.traits.cc
+++ b/libstdc++-v3/testsuite/24_iterators/associated_types/readable.traits.cc
@@ -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> );
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