[Bug libstdc++/109299] [12/13 Regression] wrong warning on std::wstring with -O2 -std=c++20 -D_FORTIFY_SOURCE=2

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109299

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is not a library bug. The library code is:

      basic_string(basic_string&& __str) noexcept
      : _M_dataplus(_M_local_data(), std::move(__str._M_get_allocator()))
      {
        if (__str._M_is_local())
          {
            traits_type::copy(_M_local_buf, __str._M_local_buf,
                              __str.length() + 1);
          }
        else
          {
            _M_data(__str._M_data());
            _M_capacity(__str._M_allocated_capacity);
          }


The warning is coming from the call to traits_type::copy which only happens if
the string fits in the local buffer. Warning that it overflows the buffer is
not helpful when we don't take that branch BECAUSE IT WOULD OVERFLOW THE
BUFFER.

These warnings need to be removed from the compiler.