` {-# MINIMAL f, g | () #-}` pragma doesn't work as expected

Summary

import Data.Proxy (Proxy)
import Data.Kind (Type)

-- law: \a -> poly a (root a) ≡ 0
class C (a :: Type) where
  poly :: Proxy a -> Double -> Double
  poly _ x = 2 * x + 5
  root :: Proxy a -> Double
  root _ = -2.5
  {-# MINIMAL poly, root | () #-}

data A
instance C A where
  poly _ x = x + 5
  -- gives warning correctly because of missing `root`
  -- root _ = -5

data B
--- shouldn't give warning
instance C B

The () should mean that giving no definitions should also give no warnings. This is necessary in cases like this where you have laws that involve multiple members of the class.

Steps to reproduce

Compile the above code. You get

Test.hs:20:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        either (‘poly’ and ‘root’) or ‘()’
    • In the instance declaration for ‘C B’
   |
20 | instance C B

Expected behavior

There should be no warning on B.

Environment

• GHC version used: 9.2.2

Optional:

• Operating System: NixOS
• System Architecture: aarch64-linux

On Discourse: https://discourse.haskell.org/t/minimal-f-g/4697