# Pastebin D5CfI3xJ
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}

import Data.Proxy

data Alice

class C x where
  silly :: String

instance C Alice where
  silly = "Silly Alice"

data Foo where
  Foo :: C a => Proxy a -> Foo

f :: Foo -> String
f foo = case foo of
  Foo (Proxy :: Proxy a) -> silly @a