To reproduce, create the files:

Main.hs:

module Main ( main, Baz ) where

import Foo ( Bar )

type Spqr = Bar
data Baz = Baz Spqr

main = putStrLn "Hello"

Main.hs-boot:

module Main where

data Baz

Foo.hs:

module Foo where

import {-# SOURCE #-} Main ( Baz )

type Bar = Baz

Then run this command sequence:

$ ghc Main.hs-boot
...
$ ghc Foo.hs
...
$ ghc Main.hs
ghc-6.8.2: panic! (the 'impossible' happened)
  (GHC version 6.8.2 for i386-apple-darwin):
	tcIfaceGlobal (local): not found:
    main:Main.Baz{tc r2T}
    []

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

Note the problem does not seem to manifest itself outside of one-shot mode.

The full trace is:

$ ghc Main.hs -ddump-if-trace -ddump-tc-trace
FYI: cannot read old interface file:
    Main.hi: openBinaryFile: does not exist (No such file or directory)
Considering whether to load base:Prelude
Reading interface for base:Prelude; reason: Prelude is directly imported
readIFace /usr/local/lib/ghc-6.8.2/lib/base-3.0.1.0/Prelude.hi
updating EPS_
Considering whether to load main:Foo
Reading interface for main:Foo; reason: Foo is directly imported
readIFace Foo.hi
updating EPS_
updating EPS_
Loading orphan modules: base:GHC.Base
Considering whether to load base:GHC.Base {- SYSTEM -}
Reading interface for base:GHC.Base;
    reason: base:GHC.Base is a orphan-instance module
readIFace /usr/local/lib/ghc-6.8.2/lib/base-3.0.1.0/GHC/Base.hi
updating EPS_
loadHiBootInterface main:Main
Reading [boot] interface for main:Main;
    reason: Need the hi-boot interface for main:Main to compare against the Real Thing
readIFace Main.hi-boot
Finished typechecking interface for main:MainStarting fork { Declaration for Baz
Loading decl for Main.Baz
updating EPS_
tcIfaceDecl4 Main.Baz
} ending fork Declaration for Baz

Type envt: [(r2T, Type constructor `Main.Baz')]
Main.hs:1:0: Tc2
Main.hs:1:0: tcTyAndCl Main
Main.hs:1:0:
    kcd1 Spqr [[]] [[]]
    In the type synonym declaration for `Spqr'
Main.hs:5:12:
    lk1 Bar
    In the type synonym declaration for `Spqr'
Main.hs:5:12:Starting fork { Declaration for Bar
Loading decl for Foo.Bar
updating EPS_
ghc-6.8.2: panic! (the 'impossible' happened)
  (GHC version 6.8.2 for i386-apple-darwin):
	tcIfaceGlobal (local): not found:
    main:Main.Baz{tc r2T}
    []

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

This happens because when typechecking the type synonym Main.Spqr, kcTyVar calls tcLookup. This causes GHC to pull on the declaration for Foo.Bar. However, this declaration makes use of Main.Baz via a SOURCE import and we haven't yet got around to type checking Main.Baz!

Hence tcIfaceGlobal dies, unable to find Baz in the type environment.

Note that reversing the order of the declarations of Main.Baz and Main.Spqr doesn't seem to make a difference. I think this is because Baz in turn uses Spqr.

I only have limited knowledge of the type checker, but I think a possible fix to this would be to type check the right hand sides of type synonym declarations lazily so we put Main.Baz into the environment before we go off exploring Foo.Bar. We seem to use a trick to deal with a similar type checking interface data constructors?

Trac metadata

Trac field	Value
Version	6.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown