The solution reported by @GyratoryCircus (down below) worked for me too!

Still happening for me for a project with siri intent definition which causes autogenerated header file. Async methods (available in iOS 15) added in the generated intent definition header file have the same signature as the ones defined in iOS 13, but using completion blocks.  This is just an example, but I have many of these and I couldn't find any solution so far:  (void)handleCheckInChild:(CheckInChildIntent * _Nonnull)intent completion:(void (^ _Nonnull)(CheckInChildIntentResponse * _Nonnull))completion;(void)handleCheckInChild:(CheckInChildIntent * _Nonnull)intent completion:(void (^ _Nonnull)(CheckInChildIntentResponse * _Nonnull))completionHandler SWIFT_AVAILABILITY(watchos,introduced=8.0) SWIFT_AVAILABILITY(macos,introduced=12.0) SWIFT_AVAILABILITY(ios,introduced=15.0);  Error: Multiple declarations of method 'handleCheckInChild:completion:' found and ignored Opened: https://feedbackassistant.apple.com/feedback/9632938