Incorrect "defined but not used" warning for data types using deriving

Consider:

module Main() where

import System

data Foo = Bar | Baz
           deriving (Show,Read)

main = do
    [x] <- getArgs
    print (read x :: Foo)

When we compile:

$ ghc -c Test.hs -fwarn-unused-binds
Test.hs:6:11: Warning: Defined but not used: data constructor `Bar'
Test.hs:6:17: Warning: Defined but not used: data constructor `Baz'

This is incorrect. If a data type derives Read, Enum or Bounded (and for Bounded, is either the first or last element), then the values are used - even if not by name.

Trac metadata

Trac field	Value
Version	6.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ndmitchell@gmail.com
Operating system
Architecture