C++20 added support for NNTPs (Non-Type Template Parameters) of class type, provided the type is

1. literal
2. only public members (incl. bases)
3. has strong structural equality
   • public op==() = default, and within the class body, and all non-static data members have strong structural equality, too, recursively
4. no mutable or volatile members

It would be nice if e.g. QFlags was such a type (it currently isn't).

The C++20 semantics require such types to have public members. https://wg21.link/P2484 aims to remove this restriction, but it's not seen a revision since last year and won't be in C++23. We might need to wait until a version of that paper is accepted.