[Bug c++/69436] Method returning "auto&" fails to resolve "*this" (-std=c++17)

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69436

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to vmorgulys from comment #0)
>      auto& f2(auto v)

This is not standard C++, it's part of the Concepts TS, and so doesn't work
properly until GCC 6.