[SR-4541] Class-Only protocol inheriting from normal protocol require to be var to allow assign to their properties.

[swift-ci]

Previous ID	SR-4541
Radar	None
Original Reporter	erne (JIRA User)
Type	Bug
Status	Resolved
Resolution	Invalid

Environment

OS X 10.11.6 (15G1217)
Xcode Version 8.2.1 (8C1002)

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 160ee33eef73361ca49d17b79be58103

is duplicated by:

• SR-11298 Writable property declaration in a conditional-conforming protocol extension has incorrect mutability

relates to:

• SR-142 mutating function in protocol extension erroneously requires var declaration of class variables

Issue Description:

In this scenario:

protocol CoolProtocol {
    var coolness: Int { get set }
}

protocol CoolClassProtocol: class, CoolProtocol {
   // if you re-declare coolness here all works as expected
   // var coolness: Int { get set }
}

class Cool {
    var coolness = 1
}

func incrementCool(_ cool: CoolClassProtocol) {
   // here you get compiler error saying cool should be var.
   // if you do it as in: 
   // var cool = cool
   // you'll get a warning telling you that cool is never mutated and should be made a let constant.
    cool.coolness += 1
}

[slavapestov]

This is working as intended.

Imagine if the CoolProtocol had a default implementation of coolness in a protocol extension:

extension CoolProtocol {
  var coolness: Int { get { return 0 } set { self = ... } }
}

Here it's assigning to self (the conforming type) so it really does have to be a var, even if its a class.