Associatedtype not inferred?

[andyhill]

On Xcode13beta3, BigSur 11.5:

protocol MyProtocol {
    associatedtype T: SignedNumeric & Comparable
    
    var prop: T { get }
    func myMax(x: T, y: T) -> T
    func myAbs(x: T) -> T
}

struct MyStruct: MyProtocol {    
    var prop: Int
    func myMax(x: T, y: T) -> T { x < y ? y : x }
    func myAbs(x: T) -> T { x < 0 ? -x : x }
}

for myAbs(x:) Xcode first complains of the return T, "Reference to invalid associated type 'T' of type 'MyStruct'". If I change the return T to Int and cast the return value to Int, then it makes the same complain about the parameter type T.

Two workarounds:

1. Change both Ts in myAbs(x:) to Int:

struct MyStruct: MyProtocol {    
    var prop: Int
    func myMax(x: T, y: T) -> T { x < y ? y : x }
    func myAbs(x: Int) -> Int { x < 0 ? -x : x }
}

or

2. add typealias T = Int in MyStruct:

struct MyStruct: MyProtocol { 
    typealias T = Int
    var prop: T
    func myMax(x: T, y: T) -> T { x < y ? y : x }
    func myAbs(x: T) -> T { x < 0 ? -x : x }
}

My question is whether it is expected behavior that the two Ts in myAbs(x:) cannot be inferred or am I doing something wrong?

Also, if I swap the ordering of myAbs(x:) and myMax(x:y:) in MyStruct, Xcode happily accepts myAbs(x:) and complains about the Ts in myMax(x:y:) instead.

Of the five threads this topic is similar to, three asked about why an associatedtype inference succeeded, one was a pitch to remove associatedtype inference, only one asked about associatedtype not inferred. My case seems to be much simpler than the one reported in the last thread.

Thanks.

[Jens]

This looks like a bug, and it's in Xcode 12.5.1 (12E507) too, ie:

protocol P {
  associatedtype T
  var p: T { get }
  func foo(x: T)
  func bar(x: T)
}

struct S: P {
  var p: Int
  func foo(x: T) {}
  func bar(x: T) {} // ERROR: Reference to invalid associated type 'T' of type 'S'
}

Here's a variation without a property, which compiles with two methods, but not with three:

protocol P {
  associatedtype T
  func foo(x: T)
  func bar(x: T)
  func baz(x: T)
}

struct S: P {
  func foo(x: Int) {}
  func bar(x: T) {}
  func baz(x: T) {} // ERROR: Reference to invalid associated type 'T' of type 'S'
}

[andyhill]

Thanks for looking into it.

Apparently even with one method it won't compile if the method returns T:

protocol P {
  associatedtype T
  func foo(x: T) -> T
}

struct S: P {
  func foo(x: Int) -> T { return 0 } // ERROR: Reference to invalid associated type 'T' of type 'S'
}

[Jens]

Filed SR-15027