[Bug libstdc++/100226] [11/12 Regression] c++/11/bits/stl_tree.h:770:8: error: static assertion failed: comparison object must be invocable as const

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100226

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The template argument '_Compare = paircmp' shows the type user as the
comparison object.

So paircmp::operator() needs to be const.