[Bug libstdc++/77288] Std::experimental::optional::operator= implementation is broken in gcc 6.1

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77288

--- Comment #6 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
There's a superior fix that retains conversions but doesn't cause this
regression. Stay tuned.