TypeFamilies and ScopedTypeVariables inconsistency in default method

Question

I'm confused about the behavior of the typechecker in the following code:

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

class (BarClass (Foo a) a)=> FooClass a where
    type Foo a
    foo :: Foo a -> a

    -- THIS WORKS:
    theBarFooOf :: a -> Bar (Foo a) a
    foo = bar $ theBarFooOf (undefined :: a)

    -- THIS DOES NOT:
 -- theBarFoo :: Bar (Foo a) a
 -- foo = bar (theBarFoo :: Bar (Foo a) a)

class BarClass t r where
    type Bar t r
    bar :: (Bar t r) -> t -> r

The second version (implementing foo via a slightly simpler method, theBarFoo) generates the following error:

Could not deduce (Bar (Foo a) a ~ Bar (Foo a0) a0)
from the context (FooClass a)
  bound by the class declaration for `FooClass'
  at demo.hs:(5,1)-(15,42)
NB: `Bar' is a type function, and may not be injective
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Expected type: Bar (Foo a) a
  Actual type: Bar (Foo a) a
In the first argument of `bar', namely
  `(theBarFoo :: Bar (Foo a) a)'
In the expression: bar (theBarFoo :: Bar (Foo a) a)
In an equation for `foo': foo = bar (theBarFoo :: Bar (Foo a) a)

Why should a value of Bar (Foo a) a be ambiguous when a is in scope from the instance head, yet this works fine: a -> Bar (Foo a) a? Is this a bug?

I'm using GHC 7.6.3.