[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
      Resolution:                    |             Keywords:  TypeInType,
                                     |  TypeFamilies, Typeable
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):

 Interestingly, GHC isn't actually so great about //using// the `Typeable`
 instance for `MkT`, regardless of whether it's GHC 8.0 or 8.2. If you
 tweak `Bug` slightly:

 {{{#!hs
 {-# LANGUAGE TypeInType #-}
 module Bug where

 import Data.Typeable (Proxy(..), typeRep)
 import Foo

 main :: IO ()
 main = print $ typeRep (Proxy :: Proxy MkT)
 }}}

 Then it fails on both GHC 8.0 and 8.2 with a similar panic:

 GHC 8.0:

 {{{
 $ /opt/ghc/8.0.2/bin/runghc Bug.hs
 GHC error in desugarer lookup in Bug:
   Can't find interface-file declaration for variable $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
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.2 for x86_64-unknown-linux):
         initDs IOEnv failure
 }}}

 GHC 8.2:

 {{{
 $ /opt/ghc/8.2.1/bin/runghc Bug.hs
 GHC error in desugarer lookup in Bug:
   Can't find interface-file declaration for variable $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
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.0.20170623 for x86_64-unknown-linux):
         initDs
 }}}

 So to be honest, I'm not sure how the original program typechecks on GHC
 8.0, given the fragility of `MkT`'s `Typeable` instance.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13915#comment:2>
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