[GHC] #13941: STG linter's type equality can loop

8 Jul 2017

      #13941: STG linter's type equality can loop
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.2
          Component:  Compiler       |           Version:  8.0.1
           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:
-------------------------------------+-------------------------------------
 `stgEqType` will currently loop when given the following,
 {{{#!hs
 stgEqType (a -> IO ()) (a -> IO ())
 }}}
 The problem is that the `(->)` tycon now takes runtime rep arguments.

 To see why, let's look at the (paraphrased) implementation,
 {{{#!hs
 stgEqType orig_ty1 orig_ty2
   = gos (typePrimRep orig_ty1) (typePrimRep orig_ty2)
   where
     gos :: [PrimRep] -> [PrimRep] -> Bool
     gos [_]   [_]   = go orig_ty1 orig_ty2
     gos reps1 reps2 = reps1 == reps2

     go :: UnaryType -> UnaryType -> Bool
     go ty1 ty2
       | Just (tc1, tc_args1) <- splitTyConApp_maybe ty1
       , Just (tc2, tc_args2) <- splitTyConApp_maybe ty2
       , let res = if tc1 == tc2
                   then equalLength tc_args1 tc_args2
                        && and (zipWith (gos `on` typePrimRep) tc_args1
 tc_args2)
                   else (isFamilyTyCon tc1 || isFamilyTyCon tc2)
       = res

       | otherwise = True  -- Conservatively say "fine".
 }}}

 Our example will begin by looking at the `gos (typePrimRep orig_ty1)
 (typePrimRep orig_ty2)` and, seeing that the `orig_ty`s are unary, enter
 `go`. We will then split the applications such that,
 {{{
 tc1, tc2 = (->)
 tc_args1, tc_args2 = ['LiftedPtrRep, 'LiftedPtrRep, a, IO ()]
 }}}
 Seeing that the tycons are equal, we will enter `gos` on each of the
 `tc_args`. Of course, one would think that `typePrimRep 'LiftedPtrRep`
 should throw an error, but because `gos` only forces the spine of the
 list, that error doesn't get thrown. Instead we incorrectly conclude that
 `'LiftedPtrRep` is a unary type and recurse into `go orig_ty1 orig_ty2`,
 thus looping.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13941
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