[Bug libstdc++/87855] std::optional only copy-constructible if T is trivially copy-constructible

[fiesh at zefix dot tv]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87855

--- Comment #12 from fiesh at zefix dot tv ---
X(double) : X(X(42)) {} // clang doesn't like this

is also enough to show the difference, no need for an operator.