I might be missing something, but as for as I can tell, if QOAuthHttpServerReplyHandler cannot bind to the specified port, no error is reported.

Example

Run an application that start an OAuth authorization. After the browser/webview is opened do not login, but start a second instance of your application. In this instance also start the OAuth authorization.

Expected behavior

The signal QAbstractOAuth2::error or some other error signal is emitted

Actual behavior

No signal is emitted, but the OAuth flow just continues with http://localhost:0/ (so with port 0) as the redirect URI being specified in the authorization URL.

QOAuthHttpServerReplyHandler::isListening() does correctly return false, and QOAuthHttpServerReplyHandler::port() returns 0.