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[SR-2220] A protocol's mutating keyword is enforced on AnyObject when using generics #44827

swift-ci opened this issue Jul 30, 2016 · 1 comment


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swift-ci commented Jul 30, 2016

Previous ID SR-2220
Radar None
Original Reporter bnut (JIRA User)
Type Bug
Status Resolved
Resolution Done
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug
Assignee None
Priority Medium

md5: 68a8c8b12a6c73d972f7bfeeba12af89

relates to:

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

Issue Description:

The `mutating` keyword on protocols is enforced by the compiler on Object types when the type is given by a generic constraint. The `mutating` keyword is not otherwise enforced on objects.

This is inconsistent, I think it should work in this case as well.


Cannot use mutating member on immutable value: 'base' is a 'let' constant.
Fix-it     Change 'let' to 'var' to make it mutable


protocol SomeProtocol {
    mutating func update()
struct ObjectWrapper<T: AnyObject> {
    let base: T

This works:

struct Example: SomeProtocol { func update() { } }

let x = ObjectWrapper(base: Example())

This does not work:

extension ObjectWrapper where T: SomeProtocol {
    func update() {

This does not work:

func update
    <S: Sequence where S.Iterator.Element: SomeProtocol, S.Iterator.Element: AnyObject>
    (sequence: S)
    for element in sequence {
        let wrapper = ObjectWrapper(base: element)
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slavapestov commented Nov 2, 2017

Swift 4's implementation of subclass existentials fixed a lot of problems here.

Note that the behavior is correct though – a mutating member on a class-constrained type still has to be invoked on an lvalue, because it might be implemented in a protocol extension method that re-assigns self.

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