// same pointer (t1 == t2), so there's no chance of chasing
	// cycles ad infinitum, so no need for a depth counter.
	if assumedEqual == nil {
		assumedEqual = make(map[typePair]struct{})
	} else if _, ok := assumedEqual[typePair{t1, t2}]; ok {
		return true
	}
	assumedEqual[typePair{t1, t2}] = struct{}{}

	switch t1.kind {
	case TIDEAL:
		// Historically, cmd/compile used a single "untyped