Very magical—but I'm not convinced all types have meaningful equality or hashes. It's convenient to have, but thorny whenever you, say, have anonymous functions lying around.

My read of this is that tel (mis)interpreted andralex as saying that all types must support equality and hashing, not that types must support equality and hashing to be hashable. It would be tragic because there can be types with no meaningful way to produce a hash or test for equality.

If that's not the case, then I don't understand either.