Late and confusing error on uncallable class method

This has been discussed on Haskell Café (http://www.haskell.org/pipermail/haskell-cafe/2008-December/051856.html), I'm reporting it here in case something can be done to help other confused souls in future.

In short, the error message reported by GHC (6.10.1):

    Could not deduce (Container x y) from the context (Container x y1)
      arising from a use of `wrapper' at Test.hs:11:22-30
    Possible fix:
      add (Container x y) to the context of
        the type signature for `liftWrap'
    In the expression: wrapper x
    In the expression:
        (if wrapper x then rewrap . f . unwrap else id) x
    In the definition of `liftWrap':
        liftWrap f x = (if wrapper x then rewrap . f . unwrap else id) x

does not begin to indicate that the actual problem with the following program is in the type class declaration:

   {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}

   import Data.Maybe

   class Container x y where
      wrapper :: x -> Bool
      unwrap :: x -> y
      rewrap :: y -> x

   liftWrap :: Container x y => (y -> y) -> (x -> x)
   liftWrap f x = (if wrapper x then rewrap . f . unwrap else id) x

   instance Container (Maybe x) x where
      wrapper = isJust
      unwrap = fromJust
      rewrap = Just

   main = print (liftWrap (succ :: Int -> Int) (Just 1 :: Maybe Int))

If the 'wrapper' method can never be called in any possible context, there should be an error report at the point where the method is declared. I don't think that's a controversial statement. Even if the compiler allows the declaration under the assumption that the useless method is never called, it should at least emit a strong warning.

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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