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[SR-4229] Cannot use mutating member on immutable value: 'self' is immutable #46812

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ankitspd opened this issue Mar 13, 2017 · 2 comments
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[SR-4229] Cannot use mutating member on immutable value: 'self' is immutable #46812

ankitspd opened this issue Mar 13, 2017 · 2 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself

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@ankitspd
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Previous ID SR-4229
Radar None
Original Reporter @aciidb0mb3r
Type Bug
Status Resolved
Resolution Duplicate
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug
Assignee None
Priority Medium

md5: 89dcbdfbbd2ca57390d28d060b60ed39

duplicates:

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

Issue Description:

Is this error correct? If yes, how to work around it because mutating doesn't work on classes

protocol Foo {
    mutating func restoreState()
}

extension Foo {
    mutating func restoreState() {}
}

class Bar: Foo {
    func foo() {
        restoreState()
    }
}
@belkadan
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It's sort of correct. :-/ The problem is that mutating methods implemented in protocol extensions are allowed to re-assign self, which is something that doesn't exactly make sense for a class: it will reassign the reference, but not change any other references. There's a whole family of issues that show that protocols with mutating methods produce surprising and potentially-bug-prone results with classes.

You can work around this by using a temporary var:

var mutableSelfHack = self
mutableSelfHack.restoreState()

but you may want to reconsider whether it makes sense for a class to conform to a protocol with mutating methods at all.

@belkadan
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Consolidating.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
This issue was closed.
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself
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@belkadan @ankitspd