Swift compiler oddity

URLSessionProtocol.swift

protocol URLSessionProtocol {
    func dataTask(with url: URL, completionHandler: @escaping (Data?, URLResponse?, Error?) -> Void) -> URLSessionDataTaskProtocol
}

protocol URLSessionDataTaskProtocol {
    func resume()
}

extension URLSessionDataTask: URLSessionDataTaskProtocol {}

//Compiler complains of non conformance here
//Need to implement method and do casts to URLSessionDataTaskProtocol
extension URLSession: URLSessionProtocol {}