[Bug c++/115960] gcc throws an error when I use Optional in c++17.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115960

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
> The problem seems to be that, despite passing "-std=c++17", it doesn't use c++17
> header files for the Optional identifier.

Why should it? The name "Optional" is not part of any C++ standard.