id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
2734	Newtype deriving over phantom types broke between GHC 6.6.1 and GHC 6.8.1	DavidA		"Consider the following code:
{{{
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-}

data Lex

data Glex

newtype Monomial ord = M [Int] deriving (Eq,Show)

instance Ord (Monomial Lex) where
    compare (M xs) (M ys) = compare xs ys

instance Ord (Monomial Glex) where
    compare (M xs) (M ys) = compare (sum xs, xs) (sum ys, ys)

-- newtype Polynomial ord = P [Monomial ord] deriving (Eq,Show,Ord)

newtype Polynomial ord = P [Monomial ord] deriving (Eq,Show)

instance Ord (Monomial ord) => Ord (Polynomial ord) where
    compare (P ts) (P us) = compare ts us
}}}
In 6.6.1, it was permissible to derive the Ord instance for Polynomial ord from the ord instance for Monomial ord - the commented out code would compile. In 6.8.1-3, the commented out code doesn't compile, so you have to do the derivation by hand, as shown."	bug	closed	normal		Compiler	6.8.3	invalid	newtype deriving, phantom type		Windows	x86		Unknown