duplicates:

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

Issue Description:

Protocols can't conform to themselves in the general case because of static requirements, but given class existentials comprise of a concrete type, the following should really compile:

protocol P {}
class Foo : P {}

func foo<T : P>(_ t: T) {}

let f: (Foo & P) = Foo()
foo(f) // Cannot invoke 'foo' with an argument list of type '((Foo & P))'

But currently doesn't.