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 f60d7e1)

Author: jwakely

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)))

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

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