[commit: ghc] master: Make worker/wrapper robust to bogus unsafeCorece (a1b6932)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/a1b6932419a2d425b2a3b7672926a0f9c368f234/ghc

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

commit a1b6932419a2d425b2a3b7672926a0f9c368f234
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Fri Nov 8 10:51:20 2013 +0000

    Make worker/wrapper robust to bogus unsafeCorece
    
    Fixes Trac #8037


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

a1b6932419a2d425b2a3b7672926a0f9c368f234
 compiler/stranal/WwLib.lhs |    9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/compiler/stranal/WwLib.lhs b/compiler/stranal/WwLib.lhs
index 2548b0e..10e5ed3 100644
--- a/compiler/stranal/WwLib.lhs
+++ b/compiler/stranal/WwLib.lhs
@@ -444,6 +444,8 @@ mkWWstr_one dflags arg
       -- See Note [Unpacking arguments with product and polymorphic demands]
   , Just (data_con, inst_tys, inst_con_arg_tys, co)
              <- deepSplitProductType_maybe (idType arg)
+  , cs `equalLength` inst_con_arg_tys
+      -- See Note [mkWWstr and unsafeCore]
   =  do { (uniq1:uniqs) <- getUniquesM
         ; let   unpk_args      = zipWith mk_ww_local uniqs inst_con_arg_tys
                 unpk_args_w_ds = zipWithEqual "mkWWstr" set_worker_arg_info unpk_args cs
@@ -473,6 +475,13 @@ nop_fn :: CoreExpr -> CoreExpr
 nop_fn body = body
 \end{code}
 
+Note [mkWWstr and unsafeCoerce]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Byy using usafeCoerce, it is possible to make the number of demands fail to
+match the number of constructor arguments; this happened in Trac #8037.
+If so, the worker/wrapper split doesn't work right and we get a Core Lint
+bug.  The fix here is simply to decline to do w/w if that happens.
+
 \begin{code}
 deepSplitProductType_maybe :: Type -> Maybe (DataCon, [Type], [Type], Coercion)
 -- If    deepSplitProductType_maybe ty = Just (dc, tys, arg_tys, co)