[SR-11398] Self type does not behave correctly in subclasses of a protcol implementation

[swift-ci]

Previous ID	SR-11398
Radar	None
Original Reporter	ezamagni (JIRA User)
Type	Bug
Status	Resolved
Resolution	Duplicate

Attachment: Download

Environment

Xcode 11.0 beta 6 (11M392r)

Apple Swift version 5.1 (swiftlang-1100.0.270.6 clang-1100.0.32.1)

macOS 10.14.6 (18G95)

Additional Detail from JIRA

Votes	0
Component/s
Labels	Bug
Assignee	None
Priority	Medium

md5: 7bbc923a93acf7925ca4d3fc49375d04

duplicates:

• SR-103 Protocol Extension: function's implementation cannot be overridden by a subclass

Issue Description:

Note: please open the attached code sample and have a look at the README

---

The following code defines a simple SomeProtocol with a static var definition svar.
A default implementation of svar is provided via extension, along with an instance method that prints its value via Self.

protocol SomeProtocol {
    static var svar: String { get }
    func printStaticVar()
}

extension SomeProtocol {
    static var svar: String {
        return "Default svar"
    }
    func printStaticVar() {
        print(Self.svar)
    }
}

Considering the following class hierarchy of protocol adoption class:

class BaseImplementation: SomeProtocol {
    
}

class ChildImplementation: BaseImplementation {
    class var svar: String {
        return "ChildImplementation's svar"
    }
}

we have an inconsistency if we try to execute the following code:

let childImpl = ChildImplementation()
childImpl.printStaticVar()      // output: Default svar
print(type(of: childImpl).svar) // output: ChildImplementation's svar

According to the official Self type documentation the two outputs should be the same:

Writing Self.someStaticMember to access a member of the current type is the same as writing type(of: self).someStaticMember

A more thorough explanation of the wrong behavior is provided in the attached sample project.