[GHC] #15957: Provide warning for when RecordWildCards LHS {..} doesn't bind anything.

27 Nov 2018

      #15957: Provide warning for when RecordWildCards LHS {..} doesn't bind anything.
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           Reporter:  Fuuzetsu       |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.3
          Component:  Compiler       |           Version:  8.6.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Given simple code like

 {{{#!haskell
 {-# LANGUAGE RecordWildCards #-}
 module M (foo) where

 data D = D { d1 :: Int, d2 :: Int }

 foo :: D -> Int -> Int
 foo D{..} x = x + 5
 }}}

 There is no way that I know of to get GHC to warn us that the {{{ D{..}
 }}} in {{{foo}}} is not binding anything at all.

 This is very common after refactoring (perhaps {{{foo}}} looked like
 {{{foo D{..} x = x + d1 + 5}}} before) and would be great to be able to
 get a warning about.

 Of course it's not _dead_ code as it still forces {{{D}}} to WHNF but if
 we're using {{{RecordWildCards}}} syntax then it's probably not what we
 were going for.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15957
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