Class that conforms to protocol with generic function returning associatedtype does not compile

Using Swift
, , , ,
4

I looks like generics are required for the issue to show up:

// A protocol with an associated type
protocol PAT {
    associatedtype U
    func f() -> U
}

// ✅ Compiles
struct S1: PAT {
    func f() -> String  { "Hello" }
}

// ✅ Compiles
struct S2: PAT {
    func f() -> some StringProtocol { "Hello" }
}

// ===========

// A protocol with a constrained associated type
protocol ConstrainedPAT {
    associatedtype U: StringProtocol
    func f() -> U
}

// ✅ Compiles
struct S3: ConstrainedPAT {
    func f() -> String  { "Hello" }
}

// ✅ Compiles
struct S4: ConstrainedPAT {
    func f() -> some StringProtocol { "Hello" }
}

// ===========

// A protocol with an associated type and a generic requirement
protocol GenericPAT {
    associatedtype U
    func f<T>(t: T) -> U
}

// ✅ Compiles
struct S5: GenericPAT {
    func f<T>(t: T) -> String  { "Hello" }
}

// ❌ error: type 'S6' does not conform to protocol 'GenericPAT'
// struct S6: GenericPAT {
//       ^
// note: protocol requires nested type 'U'; do you want to add it?
//     associatedtype U
//                    ^
struct S6: GenericPAT {
    func f<T>(t: T) -> some StringProtocol { "Hello" }
}