Lazy patterns are quite tricky!  Consider
	f ~(C x) = 3
Can the Num constraint from the 3 be discharged by a Num dictionary
bound by the pattern?  Definitely not!  

See Note [Hopping the LIE in lazy patterns] in TcPat

The type checker wasn't ensuring this, and that was causing all
manner of strange things to happen.  It actually manifested as a
strictness bug reported by Sven Panne.

I've added his test case as tcrun040.