[GHC] #12787: Weird type constraint with undecidable instances

[GHC]

#12787: Weird type constraint with undecidable instances
-------------------------------------+-------------------------------------
           Reporter:  nome           |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Linux
  UndecidableInstances               |
       Architecture:  x86_64         |   Type of failure:  GHC rejects
  (amd64)                            |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Minimal example:

 {{{#!hs
 {-# LANGUAGE FlexibleInstances    #-}
 {-# LANGUAGE UndecidableInstances #-}
 module Test where

 class PreOrd a where
     (<~) :: a -> a -> Bool

 class PreOrd a => PartialOrd a where
     tryCompare :: a -> a -> Maybe Ordering
 instance PartialOrd a => PreOrd a where
     x <~ y = let cmp = tryCompare x y in cmp == Just LT || cmp == Just EQ

 class PartialOrd a => TotalOrd a where
     tcompare :: a -> a -> Ordering
 instance TotalOrd a => PartialOrd a where
     tryCompare x y = Just $ tcompare x y

 instance TotalOrd Int where
     tcompare x y = compare x y
 }}}

 GHC (tested with 7.10.3 and 8.0.1) rejects this with the following error:
 {{{
 test.hs:11:24: error:
     • Could not deduce (TotalOrd a) arising from a use of ‘tryCompare’
       from the context: PartialOrd a
         bound by the instance declaration at test.hs:10:10-33
       Possible fix:
         add (TotalOrd a) to the context of the instance declaration
     • In the expression: tryCompare x y
       In an equation for ‘cmp’: cmp = tryCompare x y
       In the expression:
         let cmp = tryCompare x y in cmp == Just LT || cmp == Just EQ
 }}}

 After banging my head against this for a while, and having successfully
 run this in Hugs, I tend to see it as a bug in GHC. Which is to say, I
 have no idea why tryCompare could possibly require TotalOrd when it's
 actually defined in its superclass.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12787>
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