[Bug libstdc++/96088] Range insertion into unordered_map is less effective than a loop with insertion

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96088

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to François Dumont from comment #1)
> I'll check if we can be smarter here. A nice improvement would be to change
> std::hash<std::string> operator signature to:
> 
>       size_t
>       operator()(const string_view& __str) const noexcept
> 
> but that's a Standard modification.

Or use unordered_map<string, int, hash<string_view>, equal_to<>> which should
perform better.

We haven't implemented http://wg21.link/p0919r3 and http://wg21.link/p1690r1
yet, I wonder if those would help, especially if we make the internal helpers
available pre-C++20. That could allow the range insertion to use the
heteregenous lookup, to avoid creating temporaries. I'm not sure if that would
be conforming though. Heterogeneous lookup is observably different, and not
conforming in C++17.

Adding hash<string>::operator()(string_view) is an interesting idea for the
standard though.