With the following module: module Demo where class Class13 a where f :: a -> a infixl 9 `f` I get Fail: Need.hs:246: Non-exhaustive patterns in function needClassInst I noticed lots of warning while building nhc (one of which I'd have thought would have highlighted the existence of this bug), as well as some dangerous mixing of tabs and spaces. Would you apply patches to fix these (not that I am at all sure if/when I will have time to make them)? I think that if the builds were generally warning free then the existence of a warning may have helped detect the bug earlier. For the latter I would be changing tabs to spaces as tabs in Haskell are harder to do correctly than other languages, and looking at the source code out there people have enough trouble with languages like C. Thanks Ian