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

[krisk0.2017.02.27 at protonmail dot com]

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

--- Comment #2 from Денис Крыськов <krisk0.2017.02.27 at protonmail dot com> ---
None of official documents I read says that optional<T> invokes
copy-constructor of T.

Jonathan, did you describe a change in specification, or a temporary
walkaround?