'impossible' happened while messing around with ScopedTypeVariables
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