I agree that mail from address literals should be refused.
But the decision whether to accept mail from address literals or not is the
receiver's policy and therefore has nothing to do with the Sender Policy
Framework (SPF).
If the receiver's MTA passes an address literal to the check_host() routine,
then the receiver has already decided not to refuse the mail because it is
from an address literal. So check_host() should not return "fail". Rather it
should find out the sender's policy, or return "none" if it cannot find it
out.
Roger