[SR-2704] T: Protocol generic constraint doesn't accept protocol existentials

[swift-ci]

Previous ID	SR-2704
Radar	None
Original Reporter	chadaustin (JIRA User)
Type	Bug
Status	Resolved
Resolution	Duplicate

Environment

swift 3.0, both in xcode 8 and in the swiftc 3 toolchain.

Additional Detail from JIRA

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

md5: 2e6e3deb4b4efc9e73cefa9c355867b0

duplicates:

• SR-55 non-@objc protocol existentials do not conform to their own protocol type

Issue Description:

I would expect the following to compile. The protocol used as an existential type should also conform to the constraint given in the generic function concrete.

However, it gives the error: error: cannot invoke 'concrete' with an argument list of type '(P)'

protocol P {
    func go()
}

extension String: P {
    func go() {
        print("String")
    }
}
extension Int: P {
    func go() {
        print("Int")
    }
}

func concrete<T: P>(_ a: T) {
    a.go()
}

func abstract(a: P) {
    concrete(a)
}