[Bug libstdc++/80451] [6/7 Regression] return implicit type conversion to std::experimental::optional does not compile

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80451

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Денис Крыськов from comment #6)
> Seems like a serious bug in compiler.

No, it's just an incomplete implementation of a new feature added in C++14.