[GHC] #14382: The 'impossible' happened whilst installing gi-gtk via cabal

[GHC]

#14382: The 'impossible' happened whilst installing gi-gtk via cabal
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        Reporter:  maartenjacobs  |                Owner:  (none)
            Type:  bug            |               Status:  new
        Priority:  normal         |            Milestone:
       Component:  Compiler       |              Version:  8.2.1
      Resolution:                 |             Keywords:  gtk, pango
Operating System:  Linux          |         Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown   |            Test Case:
      Blocked By:                 |             Blocking:
 Related Tickets:                 |  Differential Rev(s):
       Wiki Page:                 |
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Comment (by bgamari):

 Alright, so I managed to reproduce this on `HEAD` as well. The call stack
 looks like,
 {{{
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.3.20171026 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 IsFile
   which was lazily initialized by initIfaceCheck typecheckLoop,
   I tried to tie the knot, but I couldn't find IsFile
   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.
   This little catastrophe occurred while compiling GI.Gio.Interfaces.File
   Contents of current type environment: []
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1147:37 in
 ghc:Outputable
         pprPanic, called at compiler/iface/TcIface.hs:1717:23 in
 ghc:TcIface
         tcIfaceGlobal, called at compiler/iface/TcIface.hs:1775:19 in
 ghc:TcIface
         tcIfaceTyCon, called at compiler/iface/TcIface.hs:1275:21 in
 ghc:TcIface
         tcIfaceType, called at compiler/iface/TcIface.hs:878:17 in
 ghc:TcIface
         tc_ax_branch, called at compiler/iface/TcIface.hs:866:37 in
 ghc:TcIface
         tc_ax_branches, called at compiler/iface/TcIface.hs:819:25 in
 ghc:TcIface
         tc_iface_decl, called at compiler/iface/TcIface.hs:640:15 in
 ghc:TcIface
         tcIfaceDecl, called at compiler/iface/LoadIface.hs:681:37 in
 ghc:LoadIface

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 Moreover, I was able to pare down the affected module quite far. Replacing
 `GI.Gio.Interfaces.File` from `gi-gio-2.0.14` with the following still
 reproduces the issue,

 {{{#!hs
 module GI.Gio.Interfaces.File where

 import qualified GI.GObject.Objects.Object as GObject.Object
 import qualified GI.Gio.Callbacks as Gio.Callbacks
 import {-# SOURCE #-} qualified GI.Gio.Objects.FileEnumerator as
 Gio.FileEnumerator
 import {-# SOURCE #-} qualified GI.Gio.Structs.FileAttributeInfoList as
 Gio.FileAttributeInfoList

 class GObject o => IsFile o
 }}}

 Every line of this seems to be important; removing any one eliminates the
 panic.

 I have a repro but it's quite involved. I'm going to try to reduce it
 further before trying to explain it.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14382#comment:5>
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