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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

Trac metadata

Trac field	Value
Version	7.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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