That would unnecessarily constraint `a` to be known at compile time though? I imagine that would be incredibly limiting.
E.g., if you're implementing an iterator that takes a reference to the length of the container being iterated, and you require that that reference is constexpr, that means the container will have a fixed, compile-time-known, size.
Since you didn't reply to the other parts of the comment, I'll presume that C++ doesn't have the mutex I'm talking about (apart from this attempt[0], which I just learnt about and commented on in a different reply), and that it's not possible to have non-ref-counted shared references while statically avoiding dangling pointers.