duplicates:

• SR-522 Protocol funcs cannot have covariant returns

relates to:

• SR-2020 Protocol composition doesn't conform to associated type requirement

Issue Description:

The type protocol<A,B> should be treated as covariant with the type A since all types conforming to protocol<A,B> conform to A.

This works for function calling:

protocol A {}
protocol B {}
struct Foo: A, B {}
func bar(a: A) {}
let composite: protocol<A,B> = Foo()
bar(composite)

However, it does not work for protocol conformance. Given A and B above, I expect the following to compile, but it does not:

protocol HasA {
    var a: A { get }
}
struct Baz: HasA {
    var a: protocol<A,B>
}

Similarly, I expect function return values to work as well, but this does not compile:

protocol Baffer {
    func baf() -> A
}
struct Baz: Baffer {
    func baf() -> protocol<A,B> { return Foo() }
}