As of now, compile and upload require the -b flag to supply the board FQBN, eg:

arduino-cli compile -u -p /dev/cu.usbmodem1D11 -b arduino:avr:mega

It would be nice to let users omit -b whenever -p is supplied:

arduino-cli compile -u -p /dev/cu.usbmodem1D11

In this case, FQBN can be tentatively detected using the same logic we already have for board list which provides the port-FQBN mapping.

The advantage of this would be a greatly simplified UX because we would remove the extra step of calling board list to read the FQBN (or the need for users to understand/find the correct FQBN manually from other source), thus reducing the needed awareness of the FQBN concept and the cognitive load related to it.