Bad Beta Expand and the Impossible

Bad things happen with the following program, in
combination with -O2 -funfolding-use-threshold30
-funbox-strict-fields

import Parsec

data GMLToken = TBody ()

token :: Parser GMLToken
token = do { return (TBody undefined) }

GHC 5.0 sez:

Bad eta expand
    ParsecError.zdwParseError{-rj0-}
      ww1{-rls-} (PrelBase.zdwZMZN{-6n-} @
ParsecError.Message{-rj8-})
    ParsecError.ParseError{-rgd-}
Bad eta expand
    Parsec.zdwState{-rcs-} ww{-rlt-} ww1{-rls-}
    Parsec.State{-rct-}
Bad eta expand
    Parsec.zdwOk{-rcz-}
      @ Parse.GMLToken{-r2-} w{-rlr-} tpl{-rlq-}
a1{-rlp-}
    Parsec.Reply{-rcB-} Parse.GMLToken{-r2-}
ghc-5.00: panic! (the `impossible' happened, GHC
version 5.00):
	/tmp/ghc8815.lpp:1469: Non-exhaustive patterns in
function cat_evals

Deleting any of the options makes the problem go away.

Trac metadata

Trac field	Value
Version	None
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	ResolvedFixed
Component	Compiler
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