[Bug libstdc++/80331] unused const std::string not optimized away

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80331

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #7)
> With -std=c++20 -O2 I get better code than just -std=c++17 -O2:
>   _34 = operator new (24);
>   __builtin_memcpy (_34, "a                      ", 23);
>   MEM[(char_type &)_34 + 23] = 0;
>   operator delete (_34, 24);
> 
> That is all I think due to what is referenced in PR 86590.

Yes, it's because the explicit instantiations are disabled for C++20:

# if __cplusplus <= 201703L && _GLIBCXX_EXTERN_TEMPLATE > 0
  extern template class basic_string<char>;