[SR-4680] The compiler complains about the wrong error when trying to mutate oldValue in didSet

[swift-ci]

Previous ID	SR-4680
Radar	None
Original Reporter	alinradut (JIRA User)
Type	Bug

Attachment: Download

Environment

Sierra, Xcode 8.3.1

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 0 | |Component/s | Compiler | |Labels | Bug, DiagnosticsQoI, TypeChecker | |Assignee | None | |Priority | Medium | md5: ee5549c90681d2280969444c1e2f80a2

Issue Description:

I discovered a confusing bug in the compiler when attempting to modify a member in the didSet observer. This bug requires calling a mutating function on oldValue and it needs to be defined more than once.

In my code, I created an Observable protocol which contains a mutating removeObserver() function. I then have a class which inherits from NSObject (which defines its own removeObserver as part of KVO) and implements my Observable protocol. An instance of this SomeClass is attached to an AnotherClass instance as a member variable.

When attempting to call oldValue.removeObserver() in the didSet of the SomeClass variable, the compiler will throw the following errors:

error: cannot invoke 'removeObserver' with an argument list of type '(AnotherClass)'
oldValue.removeObserver(self)
note: overloads for 'removeObserver' exist with these partially matching parameter lists: (NSObject, forKeyPath: String, context: UnsafeMutableRawPointer?), (NSObject, forKeyPath: String)

If the function is called something else, for example helloWorld, the correct error is shown:

error: cannot use mutating member on immutable value: 'oldValue' is immutable

I believe this is an issue which requires attention, because it leads developers to a seriously wrong path when trying to debug the code.

Code is below and in the attached file.

import Foundation

protocol Observable {
    associatedtype T
    var observers: [T] { get set }

    mutating func removeObserver(_ observer: T)
}

extension Observable {
    mutating func removeObserver(_ observer: T) {
        print("hello world")
    }
}

protocol SomeProtocol { }

class SomeClass: Observable {
    var observers: [SomeProtocol] = []
}

class AnotherClass: SomeProtocol {
    var someObject = SomeClass() {

        didSet {
            oldValue.removeObserver(self)
            someObject.removeObserver(self)
        }
    }
}

[swift-ci]

Comment by Brian (JIRA)

I believe this is a dupe of SR-142