protocol P1 {
    var foo: Foo! { get }
}

protocol P2: AnyObject {
    var foo: Foo! { get set }
}

struct Foo {
    func doSomething() {}
}

protocol P3 {}

extension P3 where Self: P1 & P2 {
    func bar() {
        self.foo?.doSomething()
    }
}

bar.swift:17:9: error: ambiguous use of 'foo'
        self.foo?.doSomething()
        ^
bar.swift:6:9: note: found this candidate
    var foo: Foo! { get set }
        ^
bar.swift:2:9: note: found this candidate
    var foo: Foo! { get set }
        ^

let foo: Foo? = self.foo
foo?.doSomething()