GHC gives incorrect warnings with simple applications of the view patterns extension

[jmg]

GHC is not able to infer exhaustivity of f1 in the following example

{-# LANGUAGE ViewPatterns #-}
{-# OPTIONS_GHC -fwarn-incomplete-patterns #-}
module VP where


f1, f2 :: Either Int Int -> Either Int Int

f1 (id -> Left _ ) = undefined
f1 (id -> Right _ ) = undefined

f2 (Left _ ) = undefined
f2 (Right _ ) = undefined


f (id -> _) = undefined

GHC outputs:

[1 of 1] Compiling VP               ( VP.hs, interpreted )

VP.hs:8:1:
    Warning: Pattern match(es) are non-exhaustive
             In an equation for `f1': Patterns not matched: _
Ok, modules loaded: VP.

I've tested this with GHC 7.0.4, 7.2.2, and 7.4.1-rc1 on Mac OS X.

BTW: There is neither a version entry for 7.2.2 nor for 7.4.1-rc1 in Trac.

[simonpj]

Not easy to solve this; cf #595, #5728, #3927, #5724. Volunteers welcome.