[Bug libstdc++/113200] New: std::char_traits<char>::move is not constexpr when the argument is a string literal

["pdimov at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113200

            Bug ID: 113200
           Summary: std::char_traits<char>::move is not constexpr when the
                    argument is a string literal
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pdimov at gmail dot com
  Target Milestone: ---

```
#include <string>
#include <cassert>

template<std::size_t N> struct S
{
    char data_[ N ];

    using traits_type = std::char_traits<char>;

    constexpr S( char const* p ): data_{}
    {
        std::size_t n = traits_type::length( p );

        assert( n < N );

        traits_type::move( data_, p, n + 1 );
    }
};

template<std::size_t N> S( char const(&)[N] ) -> S<N>;

constexpr S s( "test" );
```
(https://godbolt.org/z/PofY8MP6G)

fails with

```
In file included from
/opt/compiler-explorer/gcc-trunk-20240102/include/c++/14.0.0/string:42,
                 from <source>:1:
<source>:22:23:   in 'constexpr' expansion of 'S<5>(((const char*)"test"))'
<source>:16:26:   in 'constexpr' expansion of
'std::char_traits<char>::move(((char*)(&((S<5>*)this)->S<5>::data_)), p, (n +
1))'
/opt/compiler-explorer/gcc-trunk-20240102/include/c++/14.0.0/bits/char_traits.h:423:50:
  in 'constexpr' expansion of '__gnu_cxx::char_traits<char>::move(__s1, __s2,
__n)'
/opt/compiler-explorer/gcc-trunk-20240102/include/c++/14.0.0/bits/char_traits.h:230:20:
error: '(((const __gnu_cxx::char_traits<char>::char_type*)(& s.S<5>::data_)) ==
((const char*)"test"))' is not a constant expression
  230 |           if (__s1 == __s2) // unlikely, but saves a lot of work
      |               ~~~~~^~~~~~~
```

(Reduced from a similar failure in Boost.StaticString.)