An Atmega328 sketch using an added .cpp file with attachInterrupt(....) and detachInterrupt(...) statements compiles successfully in the Arduino IDE 1.8.6 with no errors. However the same sketch gives ...

Version: Arduino IDE 1.8.19, Arduino Mbed OS Nano Boards 2.8.0 Attach an input to pin 2 (e.g. button) which is high when active and high-z (disconnected) when inactive. Run the following sketch: ...