Difficulties around inferring exotic contexts

In the following code:

{-# LANGUAGE TypeFamilies, FlexibleContexts, UndecidableInstances #-}

class C r
type family F r

newtype Foo r = Foo r
instance (C (F r), Eq r) => Eq (Foo r)

newtype Bar r = Bar (Foo r) deriving (Eq)

GHC produces the error

    No instance for (C (F r))
      arising from the 'deriving' clause of a data type declaration
    Possible fix:
      use a standalone 'deriving instance' declaration,
        so you can specify the instance context yourself
    When deriving the instance for (Eq (Bar r))

If I enable StandaloneDeriving, I am able to derive the instance: deriving instance (Eq (Foo r)) => Eq (Bar r)

The docs indicate that this is exactly the instance that should be generated by GND for

newtype T v1..vn = MkT (t vk+1..vn) deriving (C t1..tj),

where my code uses the following parameters:

  T = Bar
  v1 = r
  n = 1
  MkT = Bar
  t = Foo r
  k= 1 
  C = Eq
  j = 0

and all of the conditions enumerated in the docs seem to be met. Furthermore, Bar seems to fit squarely into the format of data T1 in section 7.5.1 of the same documentation, which indicates it should not need StandaloneDeriving. It seems that the type family constraint on the Eq instance for Foo is causing the problem, but it's not clear why I'm forced to write a standalone instance that is identical to the one that should be generated by GND.

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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