[SR-8473] Broken protocol/class interaction

[dabrahams]

Previous ID	SR-8473
Radar	None
Original Reporter	@dabrahams
Type	Bug
Status	Resolved
Resolution	Duplicate

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 3cf7eef6af2f039969919debd16c6dd2

duplicates:

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

Issue Description:

protocol P {
    func f() -> Int
}

extension P {
    func f() -> Int { return 0 }
    func g() -> Int { return f() }
}

class X : P {
    init() {  }
}

class Y : X {
    // Compiler lets me write this, and claims that Y conforms to P (see
    // "redundant conformance" error below), however...
    func f() -> Int { return 1 }
}

print(Y().g()) // prints 0!

class Y1 : X {
    // error: method does not override any method from its superclass
    override func f() -> Int { return 1 }
}

// error: redundant conformance of 'Y2' to protocol 'P'
class Y2 : X, P {
    func f() -> Int { return 1 }
}