GHC doesn't add constraint when deriving

I can't remember if this was intended, but GHC cannot derive newtype Functor or derive stock Functor for WGeneric even if it knows the constraint to add (I have FlexibleInstances enabled)

import GHC.Generic

-- • Could not deduce (Functor (Rep1 f))
--     arising from the 'deriving' clause of a data type declaration
--   from the context: Functor f
--     bound by the deriving clause for ‘Functor (WGeneric f)’
--     at /home/baldur/hs/074.hs:63:5-11
--   Possible fix:
--     use a standalone 'deriving instance' declaration,
--       so you can specify the instance context yourself
newtype WGeneric f a = WGeneric (Sum (Rep1 f) f a)
  deriving newtype
    Functor

but either of these works

deriving newtype instance (Functor f, Functor (Rep1 f)) => Functor (WGeneric f)

deriving stock   instance (Functor f, Functor (Rep1 f)) => Functor (WGeneric f)

Could GHC add the inferred Functor (Rep1 f)?

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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