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

[Richard Smith via libcxx-commits]

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https://github.com/zygoloid commented:

I don't think that filling in the tail padding is sufficient here. Once we fix Clang to properly implement the "transparent replacement" rule for `std::construct_at`, constant evaluation of `expected::emplace` is going to stop working, and this change doesn't help there.

If you don't want to rely on a language extension, the rule you need to conform to is that you can't transparently replace a potentially overlapping subboject -- you  can't use `construct_at` on a base class or a `[[no_unique_address]]` member.

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