[Bug libstdc++/99058] Consider adding a note about std::optional ABI break to the C++17 status table

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99058

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #1)
> C++17 support isn't stable until GCC 9 so there is no guarantee of
> compatibility between 7 and 8 or 8 and 9. That applies to the entire library
> (and language features) not just std::optional.

I should clarify that what "applies to the entire library" is the experimental
nature of C++17 features. But C++98/11/14 features like std::vector and
std::shared_ptr are compatible between GCC 7 and later releases.