Defined but not used is not detected for data types with instances

[maeder]

module Unused () where

data U = U deriving Show

does _not_ report

Unused.hs:3:6: Warning:
    Defined but not used: type constructor or class `U'

Unused.hs:3:10: Warning: Defined but not used: data constructor `U'

This message is only displayed without "deriving". I know, instances are always exported, but these instances cannot be used.

Considering that all types may get Typeable instances, no unused warning would ever be issued for data types.

[simonpj]

Good point. Here's a fix:

commit 910a642294eb3547d0cbb3d5735ad81b964f137b
Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Mon Oct 29 23:25:25 2012 +0000

    Do not treat a constructor in a *pattern* as a *use* of that constructor
    
    Occurrences in terms are uses, in patterns they are not.
    In this way we get unused-constructor warnings from modules like this
    
       module M( f, g, T ) where
         data T = T1 | T2 Bool
    
         f x = T2 x
    
         g T1 = True
         g (T2 x) = x
    
    Here a T1 value cannot be constructed, so we can warn. The use
    in a pattern doesn't count.  See Note [Patterns are not uses]
    in RnPat.
    
    Interestingly this change exposed three module in GHC itself
    that had unused constructors, which I duly removed:
      * ghc/Main.hs
      * compiler/ghci/ByteCodeAsm
      * compiler/nativeGen/PPC/RegInfo
    Their changes are in this patch.

The fix does not treat "deriving" specially; it simply changes the way that constructors appearing in pattern matches are treated. Nice and simple, and it showed up some unused constructors in GHC itself (see the commit message).

Thanks for the suggestion.

Simon