id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
4093	compiler hangs (type checking?)	dias	simonpj	"The compiler hangs while compiling the following program, probably during type checking. Perhaps it has something to do with type functions?

Note: I can give you several variations of the same program that also hang. Let me know if you want them.


{{{
{-# LANGUAGE GADTs, EmptyDataDecls, ScopedTypeVariables, TypeFamilies #-}

module Test () where

data C
data O

type family   EitherCO e a b :: *
type instance EitherCO C a b = a
type instance EitherCO O a b = b

data MaybeC ex t where
  JustC    :: t -> MaybeC C t
  NothingC ::      MaybeC O t

data Block (n :: * -> * -> *) e x


blockToNodeList ::
  forall n e x. (EitherCO e (A C O n) (A O O n) ~ A e O n,
                 EitherCO x (A C C n) (A C O n) ~ A C x n) =>
    Block n e x -> A e x n

type A e x n = (MaybeC e (n C O), MaybeC x (n O C))
blockToNodeList b = foldBlockNodesF (f, l) b z 
  where
    z :: EitherCO e (EitherCO e (A C O n) (A O O n)) (EitherCO e (A C O n) (A O O n))
    z = undefined

    f :: n C O -> EitherCO e (A C O n) (A O O n) -> EitherCO e (A C O n) (A O O n)
    f n _ = (JustC n, NothingC)

    l :: n O C -> EitherCO e (A C O n) (A O O n) -> EitherCO e (A C C n) (A O C n)
    l _ = undefined

foldBlockNodesF  :: forall n a b c e x .
                   ( n C O       -> a -> b
                   , n O C       -> b -> c)
                 -> (Block n e x -> EitherCO e a b -> EitherCO x c b)
foldBlockNodesF _ = undefined
}}}"	bug	closed	high	7.0.1	Compiler	6.13	fixed			Linux	x86	Compile-time crash		indexed-types/should_fail/T4093a, T4093b