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

2 Jul 2017

      #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:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 If you try using `MkT`'s `Typeable` instance in the same module it's
 defined in, it gets even crazier. This program:

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

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

 data family T a
 data instance T Int = MkT

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

 Gives the same panic on GHC 8.0.2 and 8.2.1:

 {{{
 $ /opt/ghc/8.2.1/bin/runghc Foo.hs
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.0.20170623 for x86_64-unknown-linux):
         tcIfaceGlobal (local): not found
   You are in a maze of twisty little passages, all alike.
   While forcing the thunk for TyThing $tc'MkT
   which was lazily initialized by mkDsEnvs,
   I tried to tie the knot, but I couldn't find $tc'MkT
   in the current type environment.
   If you are developing GHC, please read Note [Tying the knot]
   and Note [Type-checking inside the knot].
   Consider rebuilding GHC with profiling for a better stack trace.
   Contents of current type environment:
     [a1Vr :-> Identifier ‘$dShow_a1Vr’,
      a218 :-> Identifier ‘$dTypeable_a218’,
      r1qQ :-> Type constructor ‘T’, r1rU :-> Data constructor ‘MkT’,
      r1rV :-> Identifier ‘main’, r1uL :-> Identifier ‘$tcT’,
      r1v7 :-> Type constructor ‘R:TInt’,
      r1vc :-> Coercion axiom ‘D:R:TInt0’, r1vg :-> Identifier ‘$WMkT’,
      r1vh :-> Identifier ‘MkT’, r21e :-> Identifier ‘$trModule’]
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in
 ghc:Outputable
         pprPanic, called at compiler/iface/TcIface.hs:1689:23 in
 ghc:TcIface
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13915#comment:3
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