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Given the program:

{-# LANGUAGE GADTs, ExistentialQuantification #-}
module GadtBug(bug) where

data Existential = forall a . Existential (Gadt a)

data Gadt a where Value :: Gadt Double


bug = [ match undefined | ps <- undefined, _ <- ps ]
  where
        match (Existential _) = undefined
        match (Existential _) = undefined

Using GHC Head from yesterday, compiling without optimisations I get:

C:\Neil>ghc --make GadtBug.hs
[1 of 1] Compiling GadtBug          ( GadtBug.hs, GadtBug.o )

GadtBug.hs:11:8:
    Warning: Pattern match(es) are overlapped
             In the definition of `match': match (Existential _) = ...

With optimisations (-O or -O2) I get:

C:\Neil\paradise-bug-csdg\alone>ghc --make GadtBug.hs -O
[1 of 1] Compiling GadtBug          ( GadtBug.hs, GadtBug.o )

GadtBug.hs:11:8:
    Warning: Pattern match(es) are overlapped
             In the definition of `match': match (Existential _) = ...
ghc: panic! (the 'impossible' happened)
  (GHC version 6.9.20080905 for i386-unknown-mingw32):
        idInfo a{tv ag4} [sk]

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

-- Neil

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