Here's a patch.

I'm still not 100% convinced this is a good idea.  Part of my objection is 
that it seems likely that these comparisons are fairly useless, in that a 
mixed-type comparison is probably going to be followed by a mixed-type 
arithmetic operation at some point (unless people are misspelling "x < 0" 
as "x < 0.0").  So all that's really gained is a noisy failure instead of 
a silent one.  Still, I suppose that's something.