[GHC] #11728: Core lint errors

[GHC]

#11728: Core lint errors
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

 * status:  new => merge


Comment:

 The original report is fixed by this commit (I made a typo in the ticket
 number).
 {{{
 commit 356e5e03e63558019fd0571b6c462740aceb7810
 Author: Simon Peyton Jones <simonpj at microsoft.com>
 Date:   Fri Mar 25 09:23:17 2016 +0000

     Do not eta-reduce across Ticks in CorePrep

     The function tryEtaReducePrep was being over-ambitious.
     When Breakpoint ticks were involved (i.e. in GHCi), eta
     reduction left an out-of-scope variable in the Tick.

     Easily fixed. Fixes the original report in Trac #111728.


 >---------------------------------------------------------------

 356e5e03e63558019fd0571b6c462740aceb7810
  compiler/coreSyn/CorePrep.hs | 9 +++++++--
  1 file changed, 7 insertions(+), 2 deletions(-)

 diff --git a/compiler/coreSyn/CorePrep.hs b/compiler/coreSyn/CorePrep.hs
 index 58eda2f..fb00f2b 100644
 --- a/compiler/coreSyn/CorePrep.hs
 +++ b/compiler/coreSyn/CorePrep.hs
 @@ -967,8 +967,13 @@ tryEtaReducePrep bndrs (Let bind@(NonRec _ r) body)
    where
      fvs = exprFreeVars r

 -tryEtaReducePrep bndrs (Tick tickish e)
 -  = fmap (mkTick tickish) $ tryEtaReducePrep bndrs e
 +-- NB: do not attempt to eta-reduce across ticks
 +-- Otherwise we risk reducing
 +--       \x. (Tick (Breakpoint {x}) f x)
 +--   ==> Tick (breakpoint {x}) f
 +-- which is bogus (Trac #17228)
 +-- tryEtaReducePrep bndrs (Tick tickish e)
 +--   = fmap (mkTick tickish) $ tryEtaReducePrep bndrs e

  tryEtaReducePrep _ _ = Nothing
 }}}
 I'll add a test.  Please merge.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11728#comment:4>
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