[libcxx-commits] [libcxx] [libc++] Ensure that `std::expected` has no tail padding (PR #69673)

[Louis Dionne via libcxx-commits]

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In-Reply-To: <llvm/llvm-project/pull/69673/libcxx at github.com>


ldionne wrote:

> My personal preference would be to make it experimental again. If something's affected, I'd rather have it break verbosely on next build rather than silently start misbehaving.

Yeah, it seems like this might be the most reasonable thing to do given the timeline.

> As for the permanent solution, I suppose it wouldn't be possible to employ some namespace magic to expose two versions of the class, one preserving compatibility with the old ABI (and slow), and another exposing the new ABI?

The problem is that this patch is not a difference between "slow" and "fast". The current ABI is fast, but it's also broken in the sense that it's not correct. If this were "just" an optimization, it would be a lot simpler to handle indeed (we would probably not have made so much waves and just made the better ABI opt-in).

Pinging other @llvm/libcxx-vendors for awareness.

@jiixyj Can you update this on top of `main`? I merged your other patch.

https://github.com/llvm/llvm-project/pull/69673