[committed] libstdc++: Allow explicit conversion of string views with different traits

[Jonathan Wakely]

Tested aarch64-linux. Pushed to trunk.

-- >8 --

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.
---
 libstdc++-v3/include/std/string_view          |  2 --
 .../cons/char/range_c++20.cc                  | 21 ++++---------------
 .../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 b444b5b9a39..e30a9c1768e 100644
--- a/libstdc++-v3/include/std/string_view
+++ b/libstdc++-v3/include/std/string_view
@@ -167,8 +167,6 @@ _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 5d0396563fd..bae8da1d264 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
@@ -115,23 +115,10 @@ 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 d6996a1dd6f..3cf154d8602 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
@@ -115,23 +115,10 @@ 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
-- 
2.43.0