Type synonyms in class associated types behave strangely

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeFamilies #-}

type Syn a = Associated a

class Eq (Associated a) => Foo a where
    type Associated a :: *
    foo :: a -> Syn a -> Bool

instance Foo () where
    type Associated () = Int
    foo _ x = x == x

In 7.6.3 and 7.8.1-rc2, this file compiles. In 7.8.1 release, GHC produces this error:

    No instance for (Eq (Syn ())) arising from a use of ‘==’
    In the expression: x == x
    In an equation for ‘foo’: foo _ x = x == x
    In the instance declaration for ‘Foo ()’

even though there is an Eq instance for Associated (), and Syn is just a type synonym.

Trac metadata

Trac field	Value
Version	7.8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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