id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
3406	'impossible' happened while messing around with ScopedTypeVariables	RyanN	simonpj	"The attached bug.hs was an (incorrect) intermediate step on my way to figuring out how to make a form of heterogeneous collections using existential types.

I had just switched on ScopedTypeVariables when I ran into this problem:  
{{{
ghc: panic! (the 'impossible' happened)
  (GHC version 6.10.3 for x86_64-unknown-linux):
        readFilledBox a{tv ary} [box]
}}}
I tested it on 6.10.1 and 6.10.3 (both got the same error).
I ran it on various flavors of Redhat Enterprise/CentOs.

I also ran into the same crash using 6.8.2 on an 32 bit ubuntu machine, as well as 6.8.2 on a 64 bit ubuntu machine.

-Ryan
"	bug	closed	normal	6.12.1	Compiler	6.10.3	fixed	ScopedTypeVariabes		Linux	x86_64 (amd64)		Unknown	typecheck/should_fail/T3406