Open Radar

 
Summary:
Provide a descriptive summary of the issue.

Steps to Reproduce:
This code:
protocol A {
	typealias O : B
}
protocol B {
	typealias O : A
}


Expected Results:
should result in an unambiguous, easy-to-understand, and resolvable error message (or actually it should compile but that’s fodder for a different radar)


Actual Results:
but instead, you get this:
boom.swift:5:12: error: type may not reference itself as a requirement
        typealias O : A
                  ^


Regression:
It used to crash (rdar://problem/22064783), so this is much better.


Notes:
This error message is misleading (to say the least), because the type does _not_ reference itself as a requirement; it references another protocol as a requirement which references it (the first one) as a requirement. It’s still cyclic either way, but the direct/indirect distinction is significant to the programmer.