Reduced case where Swift produces a warning with a fix-it that removes its ability to infer types in otherwise working code.

I-can-infer-that-but-I'd-rather-not.swift

// Yes, this is reduced from code that works and does something arguably useful.

protocol P1 {
    associatedtype A1
    func f(_: A1)
}

protocol P2: P1 {
    associatedtype A2: P2
    var a2: A2 { get }

    // Warning: Typealias overriding associated type 'A1' from protocol 'P1' is better
    // expressed as same-type constraint on the protocol [with fix-it]
    typealias A1 = A2.A1
}

extension P2 {
    func f(_: A1) {}
}

struct T1: P2 {
    struct A1 {}
    var a2: Self { self }
}

struct T2: P2 {
    var a2: some P2 { T1() }

    // If I follow the warning’s advice, *or* remove f() from P1, this is required:
    // typealias A1 = A2.A1
}


// Things that don’t work:
// • Flattening P2 into P1. It still requires the extra typealias in T2.
// • Using the latest toolchain build with all the -requirment-machine-... flags set to on.
// • Making a marker protocol for terminal cases like T1 and constraining P2 where A2: TerminalP2:
//
//       protocol TerminalP2: P2 {
//           var a2: Self { get }
//       }
//
//       extension TerminalP2 {
//           var a2: Self { self }
//       }
//
//       extension P2 where A2: TerminalP2 {
//           typealias A1 = A2.A1
//       }
//
//   This actually causes a new hilarious error,
//   Type 'Self.A2' does not conform to protocol 'TerminalP2' on line 18.