[Bug libstdc++/86188] Enhancement to std::merge, constexpr check of iterator types

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86188

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
See Bug 65861 which looks relevant, especially Bug 65861 comment 9