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

[Richard Smith via libcxx-commits]

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


zygoloid wrote:

> 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.

Here's one approach you could take:
```
template<typename T, typename E> struct expected {
  struct repr_type {
    [[no_unique_address]] union {
      [[no_unique_address]] T value;
      [[no_unique_address]] E error;
    };
    bool is_error;
  } repr;
};
```
... then apply `std::construct_at` to the `repr` member from `emplace`.

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