[Bug libstdc++/124396] Optimize compile times for internal uses of std::string

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124396

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yeah we can remove about 10% of <string> very easily, by moving non-member
functions out of bits/basic_string.h

Doing more than that would take a bit of effort, but I think operator<=> and
operator== could be moved to a separate header, and all the operator+
overloads.