[SR-6184] Not expected behavior in Generic Type's Protocol Extension

[swift-ci]

Previous ID	SR-6184
Radar	None
Original Reporter	100mango (JIRA User)
Type	Bug

Environment

Xcode 9.0

Swift4

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 0bf8e790977c4215c3710b9e3a95f618

Issue Description:

extension MyProtocol {
    func log()
}
extension MyProtocol {
    func log() {
        print("MyProtocol default implementation")
    }
}

class MyGenericClass<T> : MyProtocol {
    let t: T
    init(_ t: T) {self.t = t}
}

extension MyProtocol where Self==MyGenericClass<Int> {
    func log() {
        print("MyProtocol where MyGenericClass<Int>")
    }
}

func logByProtocol(_ p: MyProtocol) {
    p.log()
}

let myGenericClassNumber = MyGenericClass(1)
let myGenericClassString = MyGenericClass("1")

myGenericClassNumber.log()//expect "MyGenericClass<Int>"   true
myGenericClassString.log()//expect "MyProtocol default implementation"   true

logByProtocol(myGenericClassNumber)//expect "MyGenericClass<Int>", But print "MyProtocol default implementation"   wrong

[belkadan]

A class can only conform to a protocol in one way, no matter what its generic parameters, so that means it has to pick a version of log that will work for any possible T. However, when you statically have more type information, the call to log can pick the "best" version that's visible.