[Bug libstdc++/98725] New: Review what is disabled in libstdc++ by --disable-wchar_t

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98725

            Bug ID: 98725
           Summary: Review what is disabled in libstdc++ by
                    --disable-wchar_t
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Currently libstdc++ guards *any* use of wchar_t with a check for
_GLIBCXX_USE_WCHAR_T. This has unfortunate consequences such as
std::wstring_convert being unusable to convert between char and char16_t (which
doesn't use wchar_t at all). That's because we do this in <bits/locale_conv.h>:

 #ifdef _GLIBCXX_USE_WCHAR_T

 _GLIBCXX_BEGIN_NAMESPACE_CXX11

   /// String conversions
   template<typename _Codecvt, typename _Elem = wchar_t,
           typename _Wide_alloc = allocator<_Elem>,
           typename _Byte_alloc = allocator<char>>
     class wstring_convert


The wchar_t type exists unconditionally for C++, so traits like
std::is_integral_v<wchar_t> are already always defined as true.
std::char_traits<wchar_t> also works, by using the primary template (which
works for any character-like type). That means that even std::wstring works,
albeit slower than if we had optimized support for <wchar.h> in libc.

The consequences of --disable-wchar_t (whether implicit or explicit) should to
disable explicit instantiations for std::wstring, std::wifstream etc. and to
disable wchar_t support in locales and std::filesystem. It doesn't need to
disable everything that refers to wchar_t.