InstanceSigs doesn't work with ambigous types

Why this code:

{-# LANGUAGE ScopedTypeVariables, InstanceSigs, AllowAmbiguousTypes #-}

module Bug where

class Foo a 

class Bar a where
    bar :: forall b. (Foo b) => a

instance Bar Int where
    bar :: forall b. (Foo b) => Int  -- error here
    bar = undefined
        where
            x :: b
            x = undefined

is rejected with message:

Error: * Could not deduce (Foo b0)
    from the context: Foo b
      bound by the type signature for:
                 bar :: Foo b => Int
      at Bug.hs:11:12-35
    The type variable `b0' is ambiguous
  * When checking that: forall b. Foo b => Int
      is more polymorphic than: forall b. Foo b => Int
    When checking that instance signature for `bar'
      is more general than its signature in the class
      Instance sig: forall b. Foo b => Int
         Class sig: forall b. Foo b => Int
    In the instance declaration for `Bar Int'

? Where does b0 type var come from?

Trac metadata

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