Panic with self-import

A test in the singletons package leads to a panic when run:

ghc: panic! (the 'impossible' happened)
  (GHC version 7.8.2 for x86_64-unknown-linux):
        tcIfaceGlobal (local): not found:
    singletons-1.0:Singletons.Star.TFCo:R:DemoteRep*KProxy{tc r0}
    []

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

Edit: see ticket:9032#comment:83149 for a testcase.

mentioned in issue #1012 (closed)

changed weight to 5

added Tbug Trac import labels

Here's a test case with no external dependencies, obtained from singletons and th-desugar; crashes 7.8.2 and HEAD. Put two files

!CustomStar.hs

{-# LANGUAGE TypeFamilies, TemplateHaskell #-}

module CustomStar ( singletonStar ) where

import Language.Haskell.TH

data family Sing a

singFamilyName :: Name
singFamilyName = ''Sing

singletonStar :: Q [Dec]
singletonStar = do
  aName <- newName "z"
  return $ [DataInstD [] singFamilyName [SigT (VarT aName) StarT] [] []]

Star.hs

{-# LANGUAGE CPP, TemplateHaskell, TypeFamilies #-}

module Star where

import CustomStar

#ifdef ERR
import Star
#endif

$(singletonStar)

and execute

rm -f *.dyn_hi *.dyn_o *.hi *.o
ghc CustomStar.hs
ghc -c Star.hs
ghc -c Star.hs -DERR

Trac metadata

Trac field	Value
Related	→ #1012 (closed)

changed the description

changed milestone to %7.10.1

Still present in 7.9. I have cleaned up the description a bit.

Trac metadata

Trac field	Value
Component	Compiler → Template Haskell

added TemplateHaskell label

changed weight to 10

I think a module should simply never import itself. I propose to make this an error. Does anyone object?

I'm increasing the priority to highest to make sure that we attend to this, not because it's terribly important. It's very easy to implement if we agree that it should be illegal.

Simon

Trac metadata

Trac field	Value
Priority	normal → highest

It's already an error (Module imports form a cycle: module 'X' imports itself), but in this case the self-import somehow gives a panic.

Replying to [ticket:9032#comment:92593 monoidal]:

It's already an error (Module imports form a cycle: module 'X' imports itself), but in this case the self-import somehow gives a panic.

Good point. So it's already an error; albeit one that is only reported with --make. We should get a decent error in one-shot mode too.

Simon

mentioned in commit edd233ac

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed
Test case	→ rename/should_fail/T9032

reopened

added 1 deleted label

closed

removed 1 deleted label

has been cherry-picked into ghc-7.10 branch

mentioned in issue #9997 (closed)

added compiler crash label

added Phighest label