Consider the following:

{-# LANGUAGE StandaloneDeriving, GADTs, FlexibleInstances #-}

module StandaloneDerivingGADT where

data T a where
  MkT1 :: T Int
  MkT2 :: (Bool -> Bool) -> T Bool

deriving instance Show (T Int)

This gives the error:

StandaloneDerivingGADT.hs:9:1:
    Couldn't match type ‛Int’ with ‛Bool’
    Inaccessible code in
      a pattern with constructor
        MkT2 :: (Bool -> Bool) -> T Bool,
      in an equation for ‛showsPrec’
    In the pattern: MkT2 b1
    In an equation for ‛showsPrec’:
        showsPrec a (MkT2 b1)
          = showParen
              ((a >= 11)) ((.) (showString "MkT2 ") (showsPrec 11 b1))
    When typechecking the code for  ‛showsPrec’
      in a standalone derived instance for ‛Show (T Int)’:
      To see the code I am typechecking, use -ddump-deriv
    In the instance declaration for ‛Show (T Int)’

The derived instance declaration matches on all the constructors, even if they cannot possibly match. It should omit obviously inaccessible constructors so that this example is accepted. For reference, the derived code is:

  instance GHC.Show.Show
             (StandaloneDerivingGADT.T GHC.Types.Int) where
    GHC.Show.showsPrec _ StandaloneDerivingGADT.MkT1
      = GHC.Show.showString "MkT1"
    GHC.Show.showsPrec a_aij (StandaloneDerivingGADT.MkT2 b1_aik)
      = GHC.Show.showParen
          ((a_aij GHC.Classes.>= 11))
          ((GHC.Base..)
             (GHC.Show.showString "MkT2 ") (GHC.Show.showsPrec 11 b1_aik))
    GHC.Show.showList = GHC.Show.showList__ (GHC.Show.showsPrec 0)

The same problem applies to other derivable classes (e.g. Eq).