> It would've been much more useful to make the move constructor of indirect be deleted
No, that would be less useful.
As-is, you can use a class with a `std::indirect<T>` member in a `std::vector` just fine, thanks to the move constructor.
With `std::indirect` in `std::vector`, the vector can use move the `indirect` internally when reallocating, and as a user of the vector you will never encounter the moved-from state.
It's annoying that C++ doesn't have destructive move and thus needs to relax class invariants after a move happened, but in practice it's usually not a big deal because use-after-move is usually a bug, nothing anyone would do intentionally.
No, that would be less useful. As-is, you can use a class with a `std::indirect<T>` member in a `std::vector` just fine, thanks to the move constructor.
With `std::indirect` in `std::vector`, the vector can use move the `indirect` internally when reallocating, and as a user of the vector you will never encounter the moved-from state.
It's annoying that C++ doesn't have destructive move and thus needs to relax class invariants after a move happened, but in practice it's usually not a big deal because use-after-move is usually a bug, nothing anyone would do intentionally.