[GHC] #13915: GHC 8.2 regression: "Can't find interface-file declaration" for promoted data family instance

[GHC]

#13915: GHC 8.2 regression: "Can't find interface-file declaration" for promoted
data family instance
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.2.1-rc2
           Keywords:  TypeInType,    |  Operating System:  Unknown/Multiple
  TypeFamilies                       |
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Due to #12088, you can't define a data family instance and promote it in
 the same module. One could, up until GHC 8.2, work around this using
 (somewhat obscure) wisdom: define the data family instance in a separate
 module from where it's promoted. For example, `Bug` typechecks in GHC
 8.0.1 and 8.0.2:

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 module Foo where

 data family T a
 data instance T Int = MkT
 }}}
 {{{#!hs
 {-# LANGUAGE TypeInType #-}
 module Bug where

 import Foo

 data Proxy (a :: k)
 data S = MkS (Proxy 'MkT)
 }}}

 However, this stopped typechecking in GHC 8.2:

 {{{
 $ /opt/ghc/8.2.1/bin/ghci Bug.hs
 GHCi, version 8.2.0.20170623: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 2] Compiling Foo              ( Foo.hs, interpreted )
 [2 of 2] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:1:1: error:
     Can't find interface-file declaration for variable Foo.$tc'MkT
       Probable cause: bug in .hi-boot file, or inconsistent .hi file
       Use -ddump-if-trace to get an idea of which file caused the error
   |
 1 | {-# LANGUAGE TypeInType #-}
   | ^
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13915>
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