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[SR-4541] Class-Only protocol inheriting from normal protocol require to be var to allow assign to their properties. #47118

swift-ci opened this issue Apr 8, 2017 · 1 comment


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@swift-ci swift-ci commented Apr 8, 2017

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

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
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@slavapestov slavapestov commented Apr 23, 2017

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.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
This issue was closed.
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