[gcc r12-10146] libstdc++: Allow explicit conversion of string views with different traits

[Jonathan Wakely]

https://gcc.gnu.org/g:6e7b1527902e1e54c1a24af5bec0cc11a9af3ea4

commit r12-10146-g6e7b1527902e1e54c1a24af5bec0cc11a9af3ea4
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Feb 1 21:40:33 2024 +0000

    libstdc++: Allow explicit conversion of string views with different traits
    
    This was changed by LWG 3857.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/string_view (basic_string_view(R&&)): Remove
            constraint that traits_type must be the same, as per LWG 3857.
            * testsuite/21_strings/basic_string_view/cons/char/range_c++20.cc:
            Explicit conversion between different specializations should be
            allowed.
            * testsuite/21_strings/basic_string_view/cons/wchar_t/range_c++20.cc:
            Likewise.
    
    (cherry picked from commit f60d7e1c64518936797ec1009cb49f72f8fe45b9)

Diff:
---
 libstdc++-v3/include/std/string_view                |  2 --
 .../basic_string_view/cons/char/range_c++20.cc      | 21 ++++-----------------
 .../basic_string_view/cons/wchar_t/range_c++20.cc   | 21 ++++-----------------
 3 files changed, 8 insertions(+), 36 deletions(-)

diff --git a/libstdc++-v3/include/std/string_view b/libstdc++-v3/include/std/string_view
index e8c8bf7e4cd2..9ee888363813 100644
--- a/libstdc++-v3/include/std/string_view
+++ b/libstdc++-v3/include/std/string_view
@@ -160,8 +160,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  && (!requires (_DRange& __d) {
 		__d.operator ::std::basic_string_view<_CharT, _Traits>();
 	      })
-	  && (!requires { typename _DRange::traits_type; }
-	      || is_same_v<typename _DRange::traits_type, _Traits>)
 	constexpr explicit
 	basic_string_view(_Range&& __r)
 	noexcept(noexcept(ranges::size(__r)) && noexcept(ranges::data(__r)))
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/char/range_c++20.cc b/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/char/range_c++20.cc
index a5745fcb6033..1ba72a03f274 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/char/range_c++20.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/char/range_c++20.cc
@@ -116,23 +116,10 @@ test02()
   static_assert( ! std::is_constructible_v<std::string_view, V1> );
 
   using V2 = std::basic_string_view<char, __gnu_cxx::char_traits<char>>;
-  // V2::traits_type is not the right type
-  static_assert( ! std::is_constructible_v<std::string_view, V2> );
-
-  struct V3 : V2
-  {
-  private:
-    using V2::traits_type;
-  };
-  // V3::traits_type is not a valid (accessible) type
-  static_assert( std::is_constructible_v<std::string_view, V3> );
-
-  struct V4 : V2
-  {
-    using traits_type = std::string_view::traits_type;
-  };
-  // V4::traits_type is the right type
-  static_assert( std::is_constructible_v<std::string_view, V4> );
+  // LWG 3857
+  // basic_string_view should allow explicit conversion when only traits vary
+  static_assert( std::is_constructible_v<std::string_view, V2> );
+  static_assert( ! std::is_convertible_v<V2, std::string_view> );
 }
 
 void
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/wchar_t/range_c++20.cc b/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/wchar_t/range_c++20.cc
index af3c986e56f5..85e0531d2d7d 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/wchar_t/range_c++20.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/wchar_t/range_c++20.cc
@@ -116,23 +116,10 @@ test02()
   static_assert( ! std::is_constructible_v<std::wstring_view, V1> );
 
   using V2 = std::basic_string_view<wchar_t, __gnu_cxx::char_traits<wchar_t>>;
-  // V2::traits_type is not the right type
-  static_assert( ! std::is_constructible_v<std::wstring_view, V2> );
-
-  struct V3 : V2
-  {
-  private:
-    using V2::traits_type;
-  };
-  // V3::traits_type is not a valid (accessible) type
-  static_assert( std::is_constructible_v<std::wstring_view, V3> );
-
-  struct V4 : V2
-  {
-    using traits_type = std::wstring_view::traits_type;
-  };
-  // V4::traits_type is the right type
-  static_assert( std::is_constructible_v<std::wstring_view, V4> );
+  // LWG 3857
+  // basic_string_view should allow explicit conversion when only traits vary
+  static_assert( std::is_constructible_v<std::wstring_view, V2> );
+  static_assert( ! std::is_convertible_v<V2, std::wstring_view> );
 }
 
 void