> But why does this matter?

Because I want the semantics of that equality constraint, without switching on any of these, which I don't otherwise use:

GADTs

TypeFamilies

TypeOperators

And if that means I can't use infix `~` in my constraints, I'll put up with that. (I'd user-define a conventional class, say `TypeCast`. Could be that has to be defined in terms of `~`, in which case I'd put that in a shim module as the only place with those extensions.)