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consider

{-# LANGUAGE NoExtendedDefaultRules #-}
{-# OPTIONS_GHC -fwarn-type-defaults #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ExistentialQuantification #-}

data EShow = forall a. Show a => EShow a

smallPrint t = concatMap (\f-> case f t of EShow a -> show a) [EShow . foo, EShow . bar, EShow . baz]

data ES = forall a. Show a => ES {unES:: a}

smallPrintES t = concatMap (\f-> show $ unES $ f t) [ES . foo, ES . bar, ES . baz]

data Test = Test { foo :: Int, bar :: Char, baz :: Bool }

main = print $ smallPrintES $ Test 1 'x' False

for which

$ /cygdrive/c/ghc/ghc-6.11.20090320/bin/ghc -e "main" exists.hs
"()()()"

while

$ /cygdrive/c/ghc/ghc-6.8.3/bin/ghc -e "main" exists.hs

exists.hs:12:40:
    Cannot use record selector `unES' as a function due to escaped type variables
    Probably fix: use pattern-matching syntax instead
    In the first argument of `($)', namely `unES'
    In the second argument of `($)', namely `unES $ f t'
    In the expression: show $ unES $ f t

The expected results were error message or this

$ /cygdrive/c/ghc/ghc-6.11.20090320/bin/ghc -e "smallPrint $ Test 1 'x' False" exists.hs
"1'x'False"

There seem to be two issues:

• shouldn't ghc head report the use of unES as an error?

• how does ghc head arrive at that result, with these flags, without any warnings?

Trac metadata

Trac field	Value
Version	6.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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