Fix constraint handling for lazy patterns
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.
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