Isn't coupling both A and B to the global func(A, B) just as bad as coupling B to A in A::func(B) or A to B B::func(A)?

The thing you're trying to avoid with coupling is that when the dependency changes you might have to change the thing that depends on it, right? Isn't global func going to change with the same likelihood as A::func or B::func?

I'm not sure if this is a relevant or not, (but since it relates to your post,) I do often find myself wishing I had multiple dispatch and/or some way to extend interfaces to existing types when programming in a language without those features.