id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
2203	TFs in class instances heads	chak	chak	"Ganesh posted the following example on haskell-cafe:
{{{
{-# LANGUAGE ScopedTypeVariables, TypeFamilies, FlexibleInstances #-}

module Test1a where

class Foo a where
   type TheFoo a
   foo :: TheFoo a -> a
   foo' :: a -> Int

class Bar b where
   bar :: b -> Int

instance Foo a => Bar (Either a (TheFoo a)) where
   bar (Left a) = foo' a
   bar (Right b) = foo' (foo b :: a)

instance Foo Int where
   type TheFoo Int = Int
   foo = id
   foo' = id

val :: Either Int Int
val = Left 5

res :: Int
res = bar val
}}}
It fails to type check as the type of `bar` cannot be inferred.  However, GHC should reject the instance due to the TF in the head despite `FlexibleInstances`.

Moreover, the corrected code
{{{
{-# LANGUAGE ScopedTypeVariables, TypeFamilies, UndecidableInstances #-}

module Test1a where

class Foo a where
   type TheFoo a
   foo :: TheFoo a -> a
   foo' :: a -> Int

class Bar b where
   bar :: b -> Int

instance (b ~ TheFoo a, Foo a) => Bar (Either a b) where
   bar (Left a) = foo' a
   bar (Right b) = foo' (foo b :: a)

instance Foo Int where
   type TheFoo Int = Int
   foo = id
   foo' = id

val :: Either Int Int
val = Left 5

res :: Int
res = bar val
}}}
requires `UndecidableInstances`, although it shouldn't.

We should be able to allow equalities of the form `tv ~ F tv1 .. tvn` with tv and tvi being distinct type variables without requiring `UndecidableInstances`."	bug	closed	normal	6.12 branch	Compiler (Type checker)	6.9	fixed		ganesh@… claus	Unknown/Multiple	Unknown/Multiple		Unknown	T2203a & T2203b