id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
3176	erroneous defaulting? behaviour for existentials	claus		"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?
"	bug	closed	normal		Compiler	6.11	fixed			Unknown/Multiple	Unknown/Multiple		Unknown	typecheck/should_fail/T3176