Not all boards that provide a Arduino pin layout break out all GPIOs. A good example are Adafruit `feather-m0` boards. GPIOs that are not broken out have to be defined as `GPIO_UNDEF` to preserve the Arduino pin layout. However, GPIO functions lead to a complete system lock on `feather-m0` boards if a pin is used that is defined as `GPIO_UNDEF`. Therefore, at least an assert should blow up in this case.
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