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

[Richard Smith via llvm-commits]

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zygoloid wrote:

> > ... then apply `std::construct_at` to the `repr` member from `emplace`.
> 
> Wouldn't `std::construct_at` still be invalid, technically, when called on `__repr_.__union_.__val_`? Isn't that still a partially overlapping subobject?

Yes, that would be invalid. You need to apply `std::construct_at` to the `repr` member itself, not to the value inside the union.

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