QuantifiedConstraints fails to deduce trivial constraint

While experimenting with quantified constraints, I find I need to explicitly specify a quantified constraint in an instance declaration that I think should be deduced automatically. When I leave it out, GHC complains that it cannot deduce Num (f a) in the code below.

The full source code is attached. The relevant lines are

instance ( Functor f, Dom f ~ NFun, Cod f ~ NFun
         , forall a. Ok NFun a => Num a -- I don't want to write this constraint
         ) => Functor (NIdF f) where
  type Dom (NIdF f) = Dom f
  type Cod (NIdF f) = Cod f
  fmap f = NFun \(NIdF xs) -> NIdF (neval (fmap f) xs)

where the line marked "I don't want to write ..." points to the constraint I don't want to have to write manually. The definition of the NFun is

instance Category NFun where
  type Ok NFun a = Num a
  id = NFun P.id
  NFun g . NFun f = NFun (g P.. f)

Note that the constraint in question does not involve the type NIdF for which the instance is declared – it is a generic condition that is always true, given the instance declaration of NFun (see the definition of Ok there).

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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