[GHC] #13750: GHC produces incorrect coercions in hairy code

24 May 2017


      #13750: GHC produces incorrect coercions in hairy code
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime crash
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Andres Löh has been working with an optimized version of generics-sop, and
 ran into segfaults. He was able to reduce the test case to the attached
 (still not tiny) one, and could reduce it no further. He discovered the
 following utterly bogus definition in `-ddump-simpl` (with -O):

 {{{
 Main.gshowP_$dAll :: All MyShow GHC.Exts.Any
 [GblId,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=False, ConLike=False,
          WorkFree=False, Expandable=True, Guidance=IF_ARGS [] 20 0}]
 Main.gshowP_$dAll
   = ghc-prim-0.5.0.0:GHC.Classes.$p1(%,%)
       @ (All MyShow GHC.Exts.Any)
       @ (All (All MyShow) GHC.Exts.Any)
       (ghc-prim-0.5.0.0:GHC.Classes.C:(%%)
        `cast` (Sub (Sym (Main.D:R:AllFk_c[][0] <[*]>_N <All MyShow>_N))
                ; Sym (Main.N:All[0] <[*]>_N <All MyShow>_N <'[]>_N)
                ; Sub
                    (Nth:1
                       (Sub
                          (Sym
                             (Main.D:R:AllFkc:[0] <[*]>_N <All MyShow>_N
 <'[Char]>_N <'[]>_N))
                        ; (AllF
                             <[*]>_N
                             <All MyShow>_N
                             ((':)
                                <[*]>_N
                                (UnsafeCo nominal '[Char] GHC.Exts.Any)
                                (UnsafeCo nominal '[] GHC.Exts.Any))_N)_R
                        ; Sub
                            (Main.D:R:AllFkc:[0]
                               <[*]>_N <All MyShow>_N <GHC.Exts.Any>_N
 <GHC.Exts.Any>_N)))
                ; Main.N:All[0]
                    <[*]>_N
                    <All MyShow>_N
                    (UnsafeCo nominal GHC.Exts.Any (GHC.Exts.Any :
 GHC.Exts.Any))
                ; Sub
                    (Main.D:R:AllFkc:[0]
                       <[*]>_N <All MyShow>_N <GHC.Exts.Any>_N
 <GHC.Exts.Any>_N)
                :: (() :: Constraint :: Constraint)
                   ~R#
                   ((All MyShow GHC.Exts.Any,
                     All (All MyShow) GHC.Exts.Any) :: Constraint)))
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13750>
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