[GHC] #8449: Core lint error with data families and promotion across modules

[GHC]

#8449: Core lint error with data families and promotion across modules
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       Reporter:  goldfire          |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 I have these files:

 {{{
 {-# LANGUAGE GADTs, PolyKinds, DataKinds, TypeFamilies #-}

 module A where

 data family SList (a :: k)
 data instance SList (a :: [k]) where
   SNil :: SList '[]
 }}}

 {{{
 module B where

 import A

 foo = SNil
 }}}

 If I compile them together, with `-dcore-lint` on, all is well. But, if I
 compile `A` and then `B` (both with `-dcore-lint`), I get this:

 {{{
 *** Core Lint errors : in result of Desugar (after optimization) ***
 <no location info>: Warning:
     In the type ‛A.SList '[]’
     Kind application error in type ‛[*]’
       Function kind = * -> *
       Arg kinds = [(*, BOX)]
 <no location info>: Warning:
     In the type ‛A.SList '[]’
     Kind application error in type ‛A.SList '[]’
       Function kind = k_izi -> *
       Arg kinds = [([*], *), ('[], [*])]
 *** Offending Program ***
 B.foo :: A.SList '[]
 [LclIdX, Str=DmdType]
 B.foo = A.$WSNil @ *

 *** End of Offense ***


 <no location info>:
 Compilation had errors
 }}}

 This was tested on a very recent HEAD (Oct 11). The error does not happen
 if `SList` is a normal datatype, as opposed to a data family.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8449>
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