Constraint order must match with RankNTypes

In the following code, GHC will not compile unless the constraints on the higher-rank function have the same *order* as the method declaration in ApPair.

{-# LANGUAGE RankNTypes #-}

class ApPair r where
  apPair :: 
    (forall a . (ApPair a, Num a) => Maybe a)
    -> Maybe r 

instance (ApPair a, ApPair b) 
 => ApPair (a,b) where
  apPair = apPair'

apPair' :: (ApPair b, ApPair c)
  => (forall a . (ApPair a, Num a) => Maybe a) -> Maybe (b,c) 
apPair' f = 
  let (Just a) = apPair f
      (Just b) = apPair f
  in Just $ (a, b)

That is, the following does *not* compile:

apPair' :: (ApPair b, ApPair c)
  => (forall a . (Num a, ApPair a) => Maybe a) -> Maybe (b,c) 
apPair' f = 
  let (Just a) = apPair f
      (Just b) = apPair f
  in Just $ (a, b)

GHC probably shouldn't care about lexical matching when checking constraints.

Trac metadata

Trac field	Value
Version	7.8.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
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