fixes #70506 The detailed problem description is in #70506 The original proposed fix was to remove `[[no_unique_address]]` except when `_Tp` is empty. Edit: After the discussion in the comments below, the new fix here is to remove the `[[no_unique_address]]` from `movable_box` in the cases where we need to add our own assignment operator, which has contains the problematic `construct_at`