id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
5481	Associated type defaults + MultiParamTypeClasses error	illissius		"If I try this with GHC HEAD as of yesterday:

{{{
{-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}

class Foo a b where
    type X a
    type X a = b
    type Y b
    type Y b = a
}}}

I get this error:

{{{
test2.hs:7:5:
    Type indexes must match class instance head
    Found `b' but expected `a'
    In the type synonym instance declaration for `Y'
    In the class declaration for `Foo'
}}}

Notably, it doesn't complain about {{{type X a = b}}}, only {{{type Y b = a}}}. Unless I'm doing something dumb, it should probably work both ways."	bug	closed	normal		Compiler	7.3	fixed			Unknown/Multiple	Unknown/Multiple	GHC rejects valid program