protocol Fooable {
    func foo()
}
protocol Barable {
    func bar()
}
protocol Foobarable : Fooable, Barable {
}

// Conditionally conforming Array to Foobarable works as expected:
extension Array: Foobarable where Element: Foobarable {
    func foo() { forEach { $0.foo() } }
    func bar() { forEach { $0.bar() } }
}
// And it also works on eg Optional:
extension Optional: Foobarable where Wrapped: Foobarable {
    func foo() { self?.foo() }
    func bar() { self?.bar() }
}

// But, for some reason, it doesn't work on for example this:
struct MyArrayOfOne<Element> {
    let element: Element
}
extension MyArrayOfOne: Foobarable where Element: Foobarable { // <- Error
    func foo() { element.foo() }
    func bar() { element.bar() }
}
// Error: Type 'MyArrayOfOne<Element>' does not conform to protocol 'Barable'