[GHC] #16141: StrictData and TypeFamilies regression

[GHC]

#16141: StrictData and TypeFamilies regression
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.6.3
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I'm starting to think that this is actually an old bug with `StrictData`,
 since the following program (which uses a plain old newtype, not a data
 family) also breaks Core Lint in a similar fashion with GHC 8.4.4 or
 later:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE StrictData #-}
 module Bug where

 newtype T a b where
   MkT :: forall b a. Int -> T a b
 }}}
 {{{
 $ /opt/ghc/8.4.4/bin/ghc -O -dcore-lint Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
 *** Core Lint errors : in result of Tidy Core ***
 <no location info>: warning:
     In a case alternative: (I# dt_aXx :: Int#)
     Type of case alternatives not the same as the annotation on case:
         Actual type: T a_atk b_atj
         Annotation on case: T b_atj a_atk
         Alt Rhs: dt_aXx
                  `cast` (Sym (N:T[0] <a_atk>_P <b_atj>_P)
                          :: (Int# :: TYPE 'IntRep) ~R# (T a_atk b_atj ::
 *))
 *** Offending Program ***
 $WMkT [InlPrag=INLINE[2]] :: forall b a. Int -> T a b
 [GblId[DataConWrapper],
  Arity=1,
  Caf=NoCafRefs,
  Str=<S,U>,
  Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True,
          Guidance=ALWAYS_IF(arity=1,unsat_ok=True,boring_ok=False)
          Tmpl= \ (@ b_atj) (@ a_atk) (dt_aXw [Occ=Once!] :: Int) ->
                  case dt_aXw of { I# dt_aXx [Occ=Once] ->
                  dt_aXx
                  `cast` (Sym (N:T[0] <a_atk>_P <b_atj>_P)
                          :: (Int# :: TYPE 'IntRep) ~R# (T a_atk b_atj ::
 *))
                  }}]
 $WMkT
   = \ (@ b_atj) (@ a_atk) (dt_aXw [Occ=Once!] :: Int) ->
       case dt_aXw of { I# dt_aXx [Occ=Once] ->
       dt_aXx
       `cast` (Sym (N:T[0] <a_atk>_P <b_atj>_P)
               :: (Int# :: TYPE 'IntRep) ~R# (T a_atk b_atj :: *))
       }

 <elided>
 }}}

 The issue appears to involve newtypes with wrappers in general. (The
 reason why the original program only started breaking with GHC 8.6 is
 because commit eb680f2c0a365b12f9b867da6bb10e9ab4b328e0 changed GHC's
 treatment of newtype instances so that they would have wrappers where they
 didn't before.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16141#comment:3>
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