[GHC] #11562: WARNING: file compiler/stgSyn/CoreToStg.hs, line 250: $fCategoryConstraint:- True False

[GHC]

#11562: WARNING: file compiler/stgSyn/CoreToStg.hs, line 250:
$fCategoryConstraint:- True False
-------------------------------------+-------------------------------------
        Reporter:  slyfox            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by slyfox):

 Even less extensions, less external haskell module dependencies:
 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE Rank2Types #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE NoImplicitPrelude #-}

 module C () where
 import qualified GHC.Types as C (Constraint)

 class Category (cat :: k -> k -> *) where
   id :: cat a a
   (.) :: cat b c -> cat a b -> cat a c

 data Dict :: C.Constraint -> * where
   Dict :: a => Dict a

 newtype C2D a b = Sub (a => Dict b)

 instance Category C2D where
   id  = Sub Dict
   f . g = Sub (sub (sub Dict f) g)

 sub :: a => (b => r) -> (C2D a b) -> r
 sub r (Sub Dict) = r

 {-
 $ inplace/bin/ghc-stage2 -fforce-recomp -c C.hs -O0

 WARNING: file compiler/stgSyn/CoreToStg.hs, line 250
   $fCategoryConstraint:- True False
 -}
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11562#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler