protocol P {} struct A: P {} struct B: P {} struct C: P {} struct D: P {} protocol FooProtocol { associatedtype X: P = A associatedtype Y: P = B associatedtype Z: P = C } struct S<Foo: FooProtocol> {} struct MyFoo: FooProtocol { typealias X = D } let s: S<MyFoo> = .init() print(s) (edited)