SAMD21G18: analogRead and digitalRead doesn’t work on AREF

I have made a SAMD21G Arduino compatible development board see here:

The SAM 15x15 has much more I/O pins than the Arduino Zero because I have changed the variant.cpp file.
Everything works fine, but just the AREF pin has a problem:
analogRead and digitalRead doesn't work on AREF.
There is no reason why this functionality is restricted in the Arduino IDE.
Can this be solved?