[GHC] #14075: GHC panic with parallel make

1 Aug 2017

      #14075: GHC panic with parallel make
-------------------------------------+-------------------------------------
           Reporter:  inaki          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #13803, #13981
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Consider the following modules:
 {{{#!hs
 -- F.hs

 module F () where
 }}}

 {{{#!hs
 -- F.hs-boot

 module F where

 import O (O)

 newtype F = F ()
 instance O F where
 }}}

 {{{#!hs
 -- O.hs

 module O (O) where

 class O a where
 }}}

 {{{#!hs
 -- V.hs

 module V () where

 import {-# SOURCE #-} F ()
 }}}

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

 If I try to compile this with
 {{{
 ghc -j2 F O V
 }}}

 I get
 {{{
 [1 of 4] Compiling O                ( O.hs, O.o )
 [2 of 4] Compiling F[boot]          ( F.hs-boot, F.o-boot )
 [3 of 4] Compiling F                ( F.hs, F.o )

 <no location info>: error:
     ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.2.0.20170720 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 F
   which was lazily initialized by initIfaceCheck typecheckLoop,
   I tried to tie the knot, but I couldn't find F
   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:
     [r1hL :-> 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:1696:23 in
 ghc:TcIface

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

 On the other hand
 {{{
 ghc -j1 F O V
 }}}
 works just fine, and gives the expected
 {{{
 [2 of 5] Compiling O                ( O.hs, O.o )
 [3 of 5] Compiling F[boot]          ( F.hs-boot, F.o-boot )
 [4 of 5] Compiling F                ( F.hs, F.o )

 F.hs-boot:7:1: error:
     ‘F.F’ is exported by the hs-boot file, but not exported by the module
   |
 7 | newtype F = F ()
   | ^^^^^^^^^^^^^^^^

 F.hs:1:1: error:
     instance O.O F.F -- Defined at F.hs-boot:8:10
       is defined in the hs-boot file, but not in the module itself
   |
 1 | -- F.hs
   | ^
 }}}

 The example is a little bit sick, in that the original code is not
 expected to compile. I run into this by accident when trying to construct
 a minimal example of the issue reported in #13803 (note that that bug is
 marked as closed, but the original issue reported there remains unfixed, I
 am trying to construct a minimal testcase for the original issue there).

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14075
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