When using authentication (Basic, Digest...) with SPDY enabled, Qt doesn't emit any QNetworkAccessManager::authenticationRequired, and simply emit a QNetworkReply::error on the corresponding reply with QNetworkReply::AuthenticationRequiredError as argument.
After browsing the code a bit, Qt doesn't seem to check for 401 or 407 and handle it as an error.

I joined a little client displaying the behavior, but you will need a SPDY-enabled webserver, with any type of authentication supported by Qt enabled, to test it. When using SPDY and the authentication together, authenticationRequired will not be called and the connection will end with an error.