Detection of TF instance conflict depends on instance order

[reinerp]

With 6.10 RC1, the following code compiles without complaint:

{-# OPTIONS -fglasgow-exts #-}
module OverlapTest where

type family A x
type instance A a = Bool
type instance A Int = Char

even though the overlapping instances "A a" and "A Int" conflict. Reordering the instances to

{-# OPTIONS -fglasgow-exts #-}
module OverlapTest where

type family A x
type instance A Int = Char
type instance A a = Bool

correctly doesn't compile, complaining about conflicting instances.

[igloo]

Thanks for the report.

Manuel, sounds like one for you?

[igloo]

Type-family-related bugs aren't release critical

[dorchard]

Just discovered this bug today but glad to see it has already been picked up. The bug applies to data families too. You can do some pretty whacked out stuff with this bug, like launching missiles ;)

{-# LANGUAGE TypeFamilies #-}

-- MkFoo is a constructor on pure functions thereore "safe" :)

data family Foo a
data instance Foo a = MkFoo (Int -> Int)

unBox :: Foo a -> (Int -> Int)
unBox (MkFoo x) = x

-- Nasty person adds this code in a module

launchMissiles :: IO ()
launchMissiles = print ("Launching missiles...")

data instance Foo Int = MakeLauncher (IO())
launch = (unBox (MakeLauncher launchMissiles)) 42

launch segfaults, but about 1 in 3 times (depending on the memory layout) the missiles are launched!