[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       |           Version:  8.6.3
  (Type checker)                     |
           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:
-------------------------------------+-------------------------------------
 The credit goes to wuzzeb for originally discovering this bug
 [https://www.reddit.com/r/haskell/comments/ad9a7k/strictdata_typefamilies_and_ghc_86/
 here]. I've minimized their test case slightly below:

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

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

 With optimization enabled, this program compiles with GHC 8.0.2 through
 8.4.4, but not with 8.6.3 or HEAD:

 {{{
 $ /opt/ghc/8.4.4/bin/ghc -fforce-recomp -O Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 $ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:6:39: error:
     • Couldn't match a lifted type with an unlifted type
         arising from the coercion of the method ‘==’
           from type ‘GHC.Prim.Int# -> GHC.Prim.Int# -> Bool’
             to type ‘T -> T -> Bool’
     • When deriving the instance for (Eq T)
   |
 6 | newtype instance T = MkT Int deriving Eq
   |                                       ^^
 }}}

 Based on the error message, it appears as if GHC mistakenly believes that
 the representation type of the `T` instance is `Int#`, rather than `Int`.

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