[GHC] #9750: Core lint failure with TypeLits Symbol

[GHC]

#9750: Core lint failure with TypeLits Symbol
-------------------------------------+-------------------------------------
       Reporter:  dreixel            |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.9
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 The following module:

 {{{
 {-# LANGUAGE FlexibleInstances      #-}
 {-# LANGUAGE TypeFamilies           #-}
 {-# LANGUAGE DataKinds              #-}
 {-# LANGUAGE ScopedTypeVariables    #-}

 module Bug where

 import GHC.TypeLits ( Symbol, KnownSymbol )

 --------------------------------------------------------------------------------

 data Meta = MetaCons Symbol
 data M1 (c :: Meta) = M1

 class Generic a where
   type Rep a :: *
   from  :: a -> Rep a

 --------------------------------------------------------------------------------

 data A = A1

 instance Generic A where
   type Rep A = M1 ('MetaCons "test")
   from A1 = M1

 class GShow' f where
   gshowsPrec' :: f -> ShowS

 instance (KnownSymbol c) => GShow' (M1 ('MetaCons c))
 instance GShow' A where gshowsPrec' = gshowsPrec' . from
 }}}

 fails `-dcore-lint` in HEAD with:
 {{{
 *** Core Lint errors : in result of Desugar (after optimization) ***
 <no location info>: Warning:
     [RHS of $dKnownSymbol_azn :: GHC.TypeLits.KnownSymbol "test"]
     From-type of Cast differs from type of enclosed expression
     From-type: GHC.TypeLits.KnownSymbol "test"
     Type of enclosed expr: [GHC.Types.Char]
     Actual enclosed expr: GHC.CString.unpackCString# "test"#
     Coercion used in cast: GHC.TypeLits.NTCo:KnownSymbol[0] <"test">_N
                            ; GHC.TypeLits.NTCo:SSymbol[0] <"test">_P
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9750>
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