[GHC] #7578: Instance selection regression from 7.4 to 7.6

[GHC]

#7578: Instance selection regression from 7.4 to 7.6
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Reporter:  nomeata           |          Owner:                         
    Type:  bug               |         Status:  new                    
Priority:  normal            |      Component:  Compiler (Type checker)
 Version:  7.6.1             |       Keywords:                         
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple       
 Failure:  None/Unknown      |      Blockedby:                         
Blocking:                    |        Related:                         
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 The following code (useless due to minification) works in GHC 7.4.1:

 {{{
 {-# LANGUAGE FlexibleInstances, UndecidableInstances, OverlappingInstances
 #-}

 class CollectArgs a where
     randomEvaluate :: a -> ()

 instance CollectArgs b => CollectArgs (a -> b) where
     randomEvaluate f = randomEvaluate (f undefined)
         where () = randomEvaluate (f undefined)

 instance Show a => CollectArgs a where
     randomEvaluate x = ()
 }}}

 In 7.6.2-rc1, I get this error message

 {{{
 test.hs:8:20:
     Could not deduce (Show b) arising from a use of `randomEvaluate'
     from the context (CollectArgs b)
       bound by the instance declaration at test.hs:6:10-46
     Possible fix:
       add (Show b) to the context of the instance declaration
     In the expression: randomEvaluate (f undefined)
     In a pattern binding: () = randomEvaluate (f undefined)
     In an equation for `randomEvaluate':
         randomEvaluate f
           = randomEvaluate (f undefined)
           where
               () = randomEvaluate (f undefined)
 }}}

 The error message goes away if I remove the {{{() = randomEvaluate (f
 undefined)}}} binding, i.e. the first recursive use of
 {{{randomEvaluate}}} works fine.

 (This came up at http://stackoverflow.com/questions/14294802/evaluating-
 function-at-random-arguments-using-quickcheck/14295179#14295179)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7578>
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