Okay, it looks like I misunderstood your original point:

> "Are these two things equivalent?" has an unambiguous answer regardless of whether the things are of different types.

as

"Isomorphic things should compare equal"

So, to respond to (what I think is) your actual point: in my view, equality is defined on values of a single type, so I prefer to distinguish

"a isn't equal to b" (= False)

and

"there's no defined way to compare a and b, because they have different types" (= some kind of error or a null-like value)

To me, conflating the two seems counterproductive, but perhaps this is personal preference (probably correlated with how much one likes static typing).