Re: [GHC] #13803: Panic while forcing the thunk for TyThing IsFile (regression)

8 Jun 2017

      #13803: Panic while forcing the thunk for TyThing IsFile (regression)
-------------------------------------+-------------------------------------
        Reporter:  inaki             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:  hs-boot
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 OK, here is as small as I can possibly make this: a "meager" five files:

 {{{#!hs
 -- D.hs
 {-# LANGUAGE TypeFamilies #-}
 module D (D) where

 type family D a
 type instance D Int = Int
 }}}

 {{{#!hs
 -- E.hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 module E where

 import                GHC.Exts (Constraint)
 import {-# SOURCE #-} Y

 class C i where
   type CF i :: * -> Constraint

 data E
 instance C E where
     type CF E = Y
 }}}

 {{{#!hs
 -- E.hs-boot
 module E where
 }}}

 {{{#!hs
 -- Y.hs
 module Y where

 import                D ()
 import {-# SOURCE #-} E

 class Y o
 }}}

 {{{#!hs
 -- Y.hs-boot
 module Y where

 class Y o
 }}}

 {{{
 $ /opt/ghc/8.2.1/bin/ghci Y.hs
 GHCi, version 8.2.0.20170522: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 5] Compiling D                ( D.hs, interpreted )
 [2 of 5] Compiling E[boot]          ( E.hs-boot, interpreted )
 [3 of 5] Compiling Y[boot]          ( Y.hs-boot, interpreted )
 [4 of 5] Compiling E                ( E.hs, interpreted )
 [5 of 5] Compiling Y                ( Y.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.0.20170522 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 Y
   which was lazily initialized by initIfaceCheck typecheckLoop,
   I tried to tie the knot, but I couldn't find Y
   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: []
   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
 }}}

 Note that the name of `D.hs` actually matters here. For instance, if you
 rename `D.hs` to `Z.hs`, it'll actually compile!

 {{{
 $ /opt/ghc/8.2.1/bin/ghci Y.hs
 GHCi, version 8.2.0.20170522: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 5] Compiling E[boot]          ( E.hs-boot, interpreted )
 [2 of 5] Compiling Y[boot]          ( Y.hs-boot, interpreted )
 [3 of 5] Compiling E                ( E.hs, interpreted )
 [4 of 5] Compiling Z                ( Z.hs, interpreted )
 [5 of 5] Compiling Y                ( Y.hs, interpreted )
 Ok, modules loaded: E, E, Y, Y, Z.
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13803#comment:9
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