https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98978
--- Comment #6 from andysem at mail dot ru ---
(In reply to Jonathan Wakely from comment #5)
> (In reply to andysem from comment #3)
> > Is there no way to improve standard components implementation? I'd imagine
> > you could provide the new implementation in the new version inline namespace
> > and still support the old ABI for backward compatibility.
>
> To give a more complete answer: Inline namespaces don't help, that's a myth.
>
> struct X { std::optional<bool> b; };
>
> Now one translation unit has X using the old ABI and one has X using the new
> ABI, but they're the same X. The fact that the two versions of optional are
> in different namespaces is no help at all. You still have an ABI break.
>
> We did it for std::string and it was a multi-year effort that caused
> disruption across the industry. It's not worth doing that again to save a
> few bytes in std::optional.
You could include a hash of all members' namespaces in the X's mangled name. Or
maybe add an attribute that would propagate a tag to the outer class' mangled
name. Yes, that's also an ABI change in itself, but a useful one that would
allow inline namespaces to become useful.
I think a way to improve standard library components is essential. If not
inline namespaces then something else. Otherwise there is no evolution path of
the standard library components.
