[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):

 It turns out you don't need `deriving` to notice something afoot with this
 program. Even if you just have this:

 {{{#!hs
 {-# LANGUAGE StrictData #-}
 {-# LANGUAGE TypeFamilies #-}
 module Bug where

 data family T
 newtype instance T = MkT Int
 }}}

 And compile this with `-O -dcore-lint`, it blows up:

 {{{
 $ /opt/ghc/8.6.3/bin/ghc Bug.hs -O -dcore-lint
 [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_aXp :: Int#)
     Type of case alternatives not the same as the annotation on case:
         Actual type: R:T
         Annotation on case: T
         Alt Rhs: dt_aXp `cast` (Sym (N:R:T[0]) :: Int# ~R# R:T)
 *** Offending Program ***
 $WMkT [InlPrag=INLINE[2]] :: Int -> T
 [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= \ (dt_aXo [Occ=Once!] :: Int) ->
                  (case dt_aXo of { I# dt_aXp [Occ=Once] ->
                   dt_aXp `cast` (Sym (N:R:T[0]) :: Int# ~R# R:T)
                   })
                  `cast` (Sym (D:R:T0[0]) :: R:T ~R# T)}]
 $WMkT
   = \ (dt_aXo [Occ=Once!] :: Int) ->
       (case dt_aXo of { I# dt_aXp [Occ=Once] ->
        dt_aXp `cast` (Sym (N:R:T[0]) :: Int# ~R# R:T)
        })
       `cast` (Sym (D:R:T0[0]) :: R:T ~R# T)

 <elided>
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16141#comment:2>
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