id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
2855	Surprising type (family) type derived	guest		"Consider the following module
{{{
{-# LANGUAGE TypeFamilies #-}
module Bug5 where

class C a where
    type T a
    f :: T a -> T a -> T a

data D a = D { t :: T a }

g r = f (t r) (t r)
}}}
Now ask for the type of g
{{{
*Bug5> :t g
g :: (T a ~ T a1, C a1) => D a -> T a1
}}}
Why isn't the type
{{{
g :: (C a) => D a -> T a
}}}
?

The strange type is a minor nuisance, but it gets worse
{{{
{-# LANGUAGE TypeFamilies #-}
module Bug6 where

class C a where
    type T a
    type U a
    f :: T a -> T a -> T a
    x :: U a -> T a

data D a = D { t :: T a, u :: U a }

g r = f (t r) (x (u r))
}}}
This doesn't type check at all.

An even simpler example that fails:
{{{
{-# LANGUAGE TypeFamilies, ScopedTypeVariables #-}
module Bug7 where

class C a where
    type T a
    type U a
    x :: U a -> T a

data D a = D (U a)

g :: (C a) => D a -> T a
g (D u) = x u
}}}
"	feature request	closed	normal		Compiler	6.10.1	duplicate		lennart@…	Unknown/Multiple	Unknown/Multiple		Unknown