[Haskell-cafe] data kinds and exhaustiveness of typeclass instances

Hello, the following program doesn't typecheck in GHC 9: data Tag = A | B data Foo (a :: Tag) = Foo class C a where f :: a -> Int instance C (Foo A) where f x = 1 instance C (Foo B) where f x = 2 g :: Foo a -> Int g = f Yet one could argue that for all a :: Tag, C (Foo a) holds because a :: Tag can only take two values: A or B, and C (Foo A) and C (Foo B) hold. Is there a way to somehow convince GHC of that fact so that g typechecks? Cheers, Paul