Associated type defaults + MultiParamTypeClasses error

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.

Trac metadata

Trac field	Value
Version	7.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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