[GHC] #6001: Wrong diagnostics for instance signature mismatch

[GHC]

#6001: Wrong diagnostics for instance signature mismatch
-----------------------------------------------+----------------------------
 Reporter:  heisenbug                          |          Owner:                  
     Type:  bug                                |         Status:  new             
 Priority:  normal                             |      Component:  Compiler        
  Version:  7.5                                |       Keywords:                  
       Os:  Unknown/Multiple                   |   Architecture:  Unknown/Multiple
  Failure:  Incorrect warning at compile-time  |       Testcase:                  
Blockedby:                                     |       Blocking:                  
  Related:                                     |  
-----------------------------------------------+----------------------------
 Consider:

 -------------------------------------------
 {-# LANGUAGE InstanceSigs, DataKinds, KindSignatures, GADTs #-}

 data DayKind = Work | Rest

 data Day :: DayKind -> * where
   Mon :: Day Work
   Sat :: Day Rest

 instance Num (Day c) where
   fromInteger :: Int -> Day p
   fromInteger 1 = Mon
   fromInteger 6 = Sat
 -------------------------------------------

 I get

 $ ~/bin/ghci NumGADT.hs
 GHCi, version 7.5.20120410: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 [1 of 1] Compiling Main             ( NumGADT.hs, interpreted )

 NumGADT.hs:10:18:
     Method signature does not match class; it should be
       fromInteger :: forall (p :: DayKind). Int -> Day p
     In the instance declaration for `Num (Day c)'
 Failed, modules loaded: none.
 Prelude>

 It appears to me that instead of telling me the expected signature
 GHCi simply echoes what I have provided. This is not really useful ;-)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6001>
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