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This causes a panic:

{-# LANGUAGE TypeFamilies, GADTs #-}
module GHCBug (
  PVR(..),
  Core(..),
  analyseCore
)
where

data Core a where Ctr :: Core Double
data PVR a = PVR a deriving (Eq, Show)


class Sub a where
    type AssocSyn a :: * -> *

instance Sub Double where
    type AssocSyn Double = PVR


analyseCore :: Core a -> ((AssocSyn a) a)
analyseCore Ctr = pvr
  where
    -- GHC panics if we use the below as the type sig for 'pvr'
    pvr :: PVR ~ AssocSyn a => (AssocSyn a) a
    -- pvr :: (AssocSyn a) a
    pvr = undefined

main :: IO ()
main = print "ok"

The basic compiler panic is:

c:/ws/main/depot/QA/EDG/EDG_priv/FPF_Dev.br/src $ ghc -main-is GHCBug ~/GHCBug.hs
ghc.exe: panic! (the 'impossible' happened)
  (GHC version 6.10.1 for i386-unknown-mingw32):
	initC: srt_lbl

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

The core lint failure part is:

*** Checking old interface for main:GHCBug:
*** Parser:
*** Renamer/typechecker:
*** Desugar:
    Result size = 199
*** Core Lint Errors: in result of Desugar ***
{-# LINE 21 "F:\ME\GHCBug.hs #-}:
    [RHS of pvr_awa :: GHCBug.AssocSyn
                         GHC.Types.Double GHC.Types.Double]
    pvr_afN is out of scope
*** Offending Program ***

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Windows
Architecture