[ghc-steering-committee] Mind the gap

[Chris Dornan]

Quick quiz: the below Haskell2010 ‘Phantoms' module (also in this Gist: https://gist.github.com/cdornan/f75cd8024434d998c87610cbb7fb6ab3) appears (for me on GHC 9.2.2 and 9.2.3) to not be a GHC2021 module, reporting this error under the latter configuration:

Phantoms.hs:22:10: error:
    • Could not deduce (Typeable k)
        arising from the superclasses of an instance declaration
      from the context: (C a, Typeable a)
        bound by the instance declaration
        at Phantoms.hs:22:10-36
    • In the instance declaration for ‘S (D a)’
   |
22 | instance (C a,Typeable a) => S (D a) where smethod = undefined
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Phantoms.hs:22:10: error:
    • Could not deduce (Typeable k)
        arising from the superclasses of an instance declaration
      from the context: (C a, Typeable a)
        bound by the instance declaration
        at Phantoms.hs:22:10-36
    • In the instance declaration for ‘S (D a)’
   |
22 | instance (C a,Typeable a) => S (D a) where smethod = undefined

The question is which language extension(s) are giving rise to these errors?

The ‘Phantoms' module is this:

{-# LANGUAGE DerivingVia #-}

module Phantoms where

import Data.Typeable


class C a where
  cmethod :: Proxy a -> ()

class (Show a, Typeable a) => S a where
  smethod :: a -> Int


newtype UsingD a = UsingD { getUsingD :: a }


newtype D a = D { getD :: Int }
    deriving (Show) via UsingD (D a)


instance (C a,Typeable a) => S (D a) where smethod = undefined

instance (C a,Typeable a) => Show (UsingD a) where showsPrec = undefined


instance (C a) => C (D a) where cmethod _ = undefined