Extending a Collection where the Element type is generic (Swift)

It's simple to extend a Collection type in swift to have a custom function for particular Element types:

struct MyStruct {
    let string: String
}

extension Set where Element == MyStruct {
    func getFirstWith(string: String) -> Element? {
        return filter({ $0.string == string }).first
    }
}

But suppose your Element type is generic?

protocol MyProtocol {}

struct MyGenericStruct<T: MyProtocol> {
    let string: String
}

// error on next line:
// error: expected '>' to complete generic argument list
// extension Set where Element == MyGenericStruct<T: MyProtocol> {
//                                                 ^
extension Set where Element == MyGenericStruct<T: MyProtocol> {
    func getFirstWith(string: String) -> Element? {
        return filter({ $0.string == string }).first
    }
}

// error on next line:
// error: use of undeclared type 'T' 
// extension Set where Element == MyGenericStruct<T> {
//                                                ^
extension Set where Element == MyGenericStruct<T> {
    func getFirstWith(string: String) -> Element? {
        return filter({ $0.string == string }).first
    }
}

It's unclear to me how to declare that my element is a generic.