[Bug libstdc++/115939] Cannot unambiguously compare iterators in std::unordered_map with mapped type std::expected>

[de34 at live dot cn via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115939

--- Comment #7 from Jiang An <de34 at live dot cn> ---
(In reply to Jiang An from comment #6)
> I found that we may also need to add some operator- overloads
> (https://godbolt.org/z/jTTcYhxMc).
> 
> All standard library implementations are currently broken.

Oops, it's extremely difficult (if possible) to defend against forwarding
references. libstdc++ seems to be fine for const T& now.