Unexpected `"cannot assign to property: 'self' is immutable"` error in extension of `AnyObject`-constrained protocol

[calda]

Description

Mutating method on extension of AnyObject-constrained protocol results in unexpected "cannot assign to property: 'self' is immutable" error.

Steps to reproduce

Attempt to build the following code in Swift 5.7 (playground):

class MyClass: MyProtocol {
  var property: String = ""
}

protocol ProtocolSettable {
  var property: String { get set }
}

protocol MyProtocol: AnyObject, ProtocolSettable {

}

extension MyProtocol {
  func updateProperty() {
    // 🛑 "cannot assign to property: 'self' is immutable"
    property = "test"
  }
}

It fails to compile with this error:

/main.swift:15:5: error: cannot assign to property: 'self' is immutable
    property = "test"

Expected behavior

Since MyProtocol is guaranteed to be a class, I expect this to compile.

[LucianoPAlmeida]

This is indeed a bug, interesting that protocol ProtocolSettable: AnyObject (we can even remove AnyObject ) makes it works. My guess is that is assuming ProtocolSettable as base for property.

Simplified a bit

protocol P {
  var a: String { get set }
}
protocol P1: AnyObject, P {}

extension P1 {
  func mut() {
    a = "test" // error: cannot assign to property: 'self' is immutable
  }
}

[AnthonyLatsis]

@LucianoPAlmeida I think the compiler is correct to complain about your example. As in the case of a struct, the setter of a protocol or protocol extension member is considered mutating by default.

[AnthonyLatsis]

I agree this error makes no sense for a class-constrained Self though.

[LucianoPAlmeida]

@LucianoPAlmeida I think the compiler is correct to complain about your example. As in the case of a struct, the setter of a protocol or protocol extension member is considered mutating by default.

Ah yeah, I simplified incorrectly. Just fixed