Ambiguous type var with DefaultSignatures and FunctionalDependencies

Consider:

{-# LANGUAGE DefaultSignatures, FunctionalDependencies, ScopedTypeVariables #-}
class Foo a b where
  foo :: a -> b -> Int
class Bar a b | b -> a where
  get :: b -> a
  bar :: b -> Int
  default bar :: Foo a b => b -> Int
  bar b = foo (get b :: a) b

This works fine with ghc 7.10.3 (even without the explicit :: a type spec), but with 8.0.1, produces:

    - Could not deduce (Foo a0 b)
      from the context: Foo a b
        bound by the type signature for:
                   bar :: Foo a b => b -> Int
        at t.hs:6:3-17
      The type variable `a0'€™ is ambiguous
    - In the ambiguity check for `bar'™
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the class method:
        bar :: forall a b. Bar a b => b -> Int
      In the class declaration for `Bar'€™

Not sure if this is intentional or some good way to get around it, but I couldn't find anything related to it in the release notes or any similar looking bugs. (Real-life case: https://github.com/dylex/postgresql-typed/blob/master/Database/PostgreSQL/Typed/Dynamic.hs#L48 )

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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