Description

While reducing the code to demonstrate this other bug I discovered another behavior that surprised me. I don't know if it is a bug or just unhelpful diagnostic messaging for a genuine problem.

Reproduction

protocol A0 {
    associatedtype C: C0<Self>
}

protocol B0<K>: A0 where C: D0<K> {
    associatedtype K
}

protocol C0<A> {
    associatedtype A: A0
}

protocol D0<K>: C0 {
    associatedtype K
}

struct A1<K>: B0 {
    typealias C = C1<Self, K> // Error: Type alias 'C' references itself
//    typealias K = K // Uncommenting this fixes it
}

struct C1<A: B0<K>, K>: D0 {}

Expected behavior

I expect it to either compile or explain to me more clearly what the problem is

Environment

Swift 6.1
Xcode Version 16.4 (16F6)

Additional information

No response