RecordWildCards

[igloo]

With this module:

{-# LANGUAGE RecordWildCards #-}

module TT where

data T = T {t1, t2 :: Int}

f :: T -> Int
f d = x
    where T {t1 = x, ..} = d

g :: T -> Int
g (T {t1 = x, ..}) = x

f gives warnings about t2 being unused:

$ ghc -Wall -c n.hs

n.hs:9:11: Warning: Defined but not used: `t2'

which is probably not what we want for variables bound by a wildcard. Reported by Serge here:  http://www.haskell.org/pipermail/glasgow-haskell-bugs/2010-October/025858.html

[NeilMitchell]

A simpler example from bug 4411 (which I'm going to close as a dupe of this):

{-# LANGUAGE RecordWildCards #-}

module Test where

data T = MkT { f,g :: Int }

p1 x = let MkT{..} = x in f

p2 MkT{..} = f

With GHC 6.12.3 this gives the warning:

Test.hs:7:11: Warning: Defined but not used: `g'

i.e. p1 gives a warning, but p2 doesn't. Neither is meant to give a warning.

[batterseapower]

I have fixed this bug on branch patch-4404 in the GHC and testsuite repos. The patch validates. Please review.

[igloo]

Merged, thanks