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

[swift-ci]

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.

Error:

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

Example

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

This works:

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

let x = ObjectWrapper(base: Example())
x.base.update()

This does not work:

extension ObjectWrapper where T: SomeProtocol {
    func update() {
        self.base.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)
        wrapper.base.update()
    }
}

[slavapestov]

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.