[libcxx][format] Fix how we handle char traits in formatter<string> a…

…nd formatter<string_view>

Right now we drop the char_traits template argument, which presumes that
string<_CharT, _Traits> and string<_CharT> are interchangeable.

Reviewed By: Mordante, #libc, Quuxplusone

Differential Revision: https://reviews.llvm.org/D112017

libcxx/include/__format/formatter_string.h

@@ -119,7 +119,7 @@ struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT
 
   _LIBCPP_HIDE_FROM_ABI auto format(const _CharT __str[_Size], auto& __ctx)
       -> decltype(__ctx.out()) {
-    return _Base::format(_VSTD::basic_string_view<_CharT>(__str, _Size), __ctx);
+    return _Base::format(basic_string_view<_CharT>(__str, _Size), __ctx);
   }
 };
 
@@ -133,15 +133,24 @@ struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT
   _LIBCPP_HIDE_FROM_ABI auto
   format(const basic_string<_CharT, _Traits, _Allocator>& __str, auto& __ctx)
       -> decltype(__ctx.out()) {
-    return _Base::format(_VSTD::basic_string_view<_CharT>(__str), __ctx);
+    // drop _Traits and _Allocator
+    return _Base::format(basic_string_view<_CharT>(__str.data(), __str.size()), __ctx);
   }
 };
 
 // Formatter std::string_view.
 template <class _CharT, class _Traits>
-struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT
-    formatter<basic_string_view<_CharT, _Traits>, _CharT>
-    : public __format_spec::__formatter_string<_CharT> {};
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT formatter<basic_string_view<_CharT, _Traits>, _CharT>
+    : public __format_spec::__formatter_string<_CharT> {
+  using _Base = __format_spec::__formatter_string<_CharT>;
+
+  _LIBCPP_HIDE_FROM_ABI auto
+  format(basic_string_view<_CharT, _Traits> __str, auto& __ctx)
+      -> decltype(__ctx.out()) {
+    // drop _Traits
+    return _Base::format(basic_string_view<_CharT>(__str.data(), __str.size()), __ctx);
+  }
+};
 
 #endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)