[Bug libstdc++/86015] Better handling of iterator distances

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86015

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Every version of the standard says the same thing under iterator requirements:

"For every iterator type X for which equality is defined, there is a
corresponding signed integer type called the difference type of the iterator."

Your distance objects are class types, which are not signed integer types.