> I played around with this, I don't think we could make it work for instance doubles, since we only have the class handy.

You're right -- I didn't think that through.

> It could be made to work for dynamically defined class methods, but I think it would be less confusing to just not support them at all.

Hmm.  I'm on the fence about this.  I get your point, but given that we can make it work with class doubles, I would be frustrated as a user if it didn't.  Do you think it would add much complexity?

Actually, thinking a bit more about `method_defined?...that's for an `instance_double` and `respond_to?` is the natural analog to it.  Seems like it would simplify the implementation by using `method_defined?` and `instance_method` (for the arity check) for `instance_double` and `respond_to?` and `method` (for the arity check) for `class_double`.