Deriving instances for SOP functors using phantom types

[fpringle]

Hi, not sure if this is the right place to ask, but I have an issue regarding Show and Eq instances for NP and NS types of functors that don't use their type parameter. The functors in my specific use-case are {Field,Constructor}Info, but the simplest example I can think of is Proxy:

newtype Proxies xs = Proxies (NP Proxy xs)
  deriving (Show)

This gives the following type error, which of course makes sense based on the superclass for Show (NS f xs) using All rather than QuantifiedConstraints:

src/Diff.hs:36:13-16: error: [GHC-05617]
    • Could not solve: (AllF (Compose Show Proxy) xs, AllF Top xs)
        arising from a superclass required to satisfy ‘All (Compose Show Proxy) xs’,
        arising from the first field of ‘Proxies’ (type ‘NP Proxy xs’)
    • When deriving the instance for (Show (Proxies xs))
   |
36 |   deriving (Show)
   |             ^^^^
src/Diff.hs:36:13-16: error: [GHC-39999]
    • No instance for ‘All (Compose Show Proxy) xs’
        arising from the first field of ‘Proxies’ (type ‘NP Proxy xs’)
      Possible fix:
        use a standalone 'deriving instance' declaration,
          so you can specify the instance context yourself
    • When deriving the instance for (Show (Proxies xs))
   |
36 |   deriving (Show)

Is there any way I can convince GHC that Proxies has a valid Show instance regardless of what xs is, while still getting a derived instance for free? Something like

deriving instance (forall x. Show (Proxy x)) => Show (Proxies xs)