Haskell program to test whether ScopedTypeVariables is enabled

DetectScopedTypeVariables.hs

{-# LANGUAGE GHC2021, NoScopedTypeVariables, RequiredTypeArguments, AllowAmbiguousTypes  #-}

import Data.Proxy

hasScopedTypeVariables :: Bool
hasScopedTypeVariables = f Char

f :: forall a -> C a => Bool
f a = g @Int
  where
    g :: forall a . C a => Bool
    g = thing (Proxy @a)  -- Refers to the inner a iff ScopedTypeVariables is on

class C a where
    thing :: Proxy a -> Bool

instance C Int where
    thing _ = True

instance C Char where
    thing _ = False