Overlapping instances

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}

module Main where

main :: IO ()
main = pure ()

data Key a where
  KeyInt :: Key Int

instance Show (Key a) where
  show KeyInt = "KeyInt"

class KnownKey a where
  knownKey :: Maybe (Key a)
  knownKey = Nothing

instance {-# INCOHERENT #-} KnownKey Int where
  knownKey = Just KeyInt

instance {-# OVERLAPPING #-} KnownKey a where
  knownKey = Nothing

myFunc :: forall a. a -> Maybe (Key a)
myFunc val = (knownKey :: Maybe (Key a))
-- hopefully
-- >>> myFunc (3 :: Int) = Just KeyInt