[Bug libstdc++/70238] [5/6 Regression] std::future_category ABI change

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70238

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Not a regression, it is unsupported to mix C++11 objects from gcc4 and gcc5.