[GHC] #7453: unsafe coerce without extensions

[GHC]

#7453: unsafe coerce without extensions
------------------------------------------+---------------------------------
  Reporter:  guest                        |          Owner:                  
      Type:  bug                          |         Status:  new             
  Priority:  normal                       |      Milestone:                  
 Component:  Compiler (Type checker)      |        Version:  7.6.1           
Resolution:                               |       Keywords:                  
        Os:  Unknown/Multiple             |   Architecture:  Unknown/Multiple
   Failure:  GHC accepts invalid program  |     Difficulty:  Unknown         
  Testcase:  T7453                        |      Blockedby:                  
  Blocking:                               |        Related:                  
------------------------------------------+---------------------------------

Comment(by simonpj):

 Here is a fix for the 7.6 branch
 {{{
 commit b637a24dbb471d99887e5544037a2abe10af26ee
 Author: Simon Peyton Jones <simonpj at microsoft.com>
 Date:   Mon Dec 3 16:57:30 2012 +0000

     Fix Trac #7453 on the 7.6 branch

     This patch fixes a quite egregious bug. runTcS was intialising
     the 'untouchables' to NoUntouchables, which is quite wrong for
     the invocation in TcSimplify.simplifyInfer.  Result: missed
     skolem-escape check and seg-fault city.

     All this is done differently, and better, in HEAD, so this
     patch is needed only on the branch.

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

  compiler/typecheck/TcSMonad.lhs |   10 +++++++++-
  1 files changed, 9 insertions(+), 1 deletions(-)

 diff --git a/compiler/typecheck/TcSMonad.lhs
 b/compiler/typecheck/TcSMonad.lhs index eaaa8f6..8adb1d5 100644
 --- a/compiler/typecheck/TcSMonad.lhs
 +++ b/compiler/typecheck/TcSMonad.lhs
 @@ -935,6 +935,15 @@ runTcSWithEvBinds ev_binds_var tcs
         ; inert_var <- TcM.newTcRef is
         ; wl_var <- TcM.newTcRef wl

 +       -- The "low end" of the untouchable range should come from the
 +       -- ambient tcl_untch; the high end is the highest allocated to
 +       -- date. 'untouch' used (in 7.6.1, entirely wrongly) to be
 +       -- set to NoUntouchables, causing #7453.
 +       -- All this is done much better in 7.8.
 +       ; tc_lenv <- TcM.getLclEnv
 +       ; tcm_high <- TcM.readTcRef (tcl_meta tc_lenv)
 +       ; let untouch = TouchableRange tcm_low tcm_high
 +             tcm_low = tcl_untch tc_lenv
         ; let env = TcSEnv { tcs_ev_binds = ev_binds_var
                            , tcs_ty_binds = ty_binds_var
                            , tcs_untch    = (untouch, emptyVarSet) -- No
 Tcs untouchables yet
 @@ -960,7 +969,6 @@ runTcSWithEvBinds ev_binds_var tcs
         ; return res }
    where
      do_unification (tv,ty) = TcM.writeMetaTyVar tv ty
 -    untouch = NoUntouchables
      is = emptyInert
      wl = emptyWorkList
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7453#comment:14>
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