Open Radar

 
Summary:
If you have a protocol with an associated type that isn’t used in a method or something, you have to explicitly specify it with typealias in your conforming type. You cannot for example specify it with a type parameter of the same name, i.e.:

protocol P { typealias T }
struct S<T> : P {}

does not compile because S does not provide the associated type T.

One obvious thing to try is just `typealias T`, which also doesn’t compile because it doesn’t have `= …` following it.

The next thing to try is `typealias T = T`, which crashes the compiler.


Steps to Reproduce:
1. Compile this code:
struct S<T> {
	typealias T = T
}


Expected Results:
I didn’t really expect it to work but I certainly didn’t expect the compiler to crash.


Actual Results:
Among Swift’s weaponry are such diverse elements as fear, surprise, ruthless efficiency, and compiler crashes.


Regression:
If you do typealias T = T at the top level it correctly informs you that it’s a circular reference.


Notes:
This crash occurred while I was trying to reduce another crash that was in the way of what I’m trying to do. The yaks have gone recursive here, people!