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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/98978] Consider packing _M_Engaged in the tail
 padding of T in optional<>
Date: Wed, 07 Sep 2022 12:30:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98978

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(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 namesp=
ace
> 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 disrup=
tion
across the industry. It's not worth doing that again to save a few bytes in
std::optional.=