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GHC panic while forcing the thunk for TyThing IsFile (regression)

    • Closed Issue created by inaki @trac-inaki

    Consider the following set of files:

    -- A.hs
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeFamilies #-}

module A (AI(..)) where

import GHC.Exts (Constraint)

class AI (info :: *) where
    type S info :: * -> Constraint
    -- C.hs
module C () where

import {-# SOURCE #-} qualified OC as OC
import {-# SOURCE #-} qualified OV as OV
    -- IF.hs
module IF where

import qualified C as C
import {-# SOURCE #-} qualified OF as OF

class IsF o
    -- IF.hs-boot
module IF where

class IsF o
    -- OC.hs
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE EmptyDataDecls #-}

module OC () where

import A (AI(..))

data I
instance AI I where
    type S I = (~) ()
    -- OC.hs-boot
module OC where
    -- OF.hs
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE EmptyDataDecls #-}

module OF () where

import A (AI(..))

import {-# SOURCE #-} qualified IF as IF

data P
instance AI P where
    type S P = IF.IsF
    -- OF.hs-boot
module OF where
    -- OV.hs
module OV () where
    -- OV.hs-boot
module OV where

    This works with ghc-8.0.2 and earlier versions, but fails with ghc-8.2.1. When I run ghc IF for 8.2.1 I get

    [ 1 of 10] Compiling A                ( A.hs, A.o )
[ 2 of 10] Compiling IF[boot]         ( IF.hs-boot, IF.o-boot )
[ 3 of 10] Compiling OC[boot]         ( OC.hs-boot, OC.o-boot )
[ 4 of 10] Compiling OC               ( OC.hs, OC.o )
[ 5 of 10] Compiling OF[boot]         ( OF.hs-boot, OF.o-boot )
[ 6 of 10] Compiling OF               ( OF.hs, OF.o )
[ 7 of 10] Compiling OV[boot]         ( OV.hs-boot, OV.o-boot )
[ 8 of 10] Compiling C                ( C.hs, C.o )
[ 9 of 10] Compiling IF               ( IF.hs, IF.o )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.2.1 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 IsF
  which was lazily initialized by initIfaceCheck typecheckLoop,
  I tried to tie the knot, but I couldn't find IsF
  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:1696:23 in ghc:TcIface

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

    For context, this is a minimal testcase of the panic reported in #13803 (closed) for gi-gio, which persists after fixing the panic in the minimal testcase in ticket:13803#comment:137812. (It seems like the original code was hitting more than one issue.)

    Trac metadata
    Trac field Value
    Version 8.2.1
    Type Bug
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
    Component Compiler
    Test case
    Differential revisions
    BlockedBy
    Related #13803 (closed), #13981 (closed)
    Blocking
    CC
    Operating system
    Architecture
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