A.hs

module A (A(..)) where

newtype A = A Int
  deriving (Show, Eq)

B.hs

{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE DerivingVia #-}
module B(test) where

import A
import qualified Data.Map as Map
import GHC.Exts

deriving via (Down A) instance Ord A 

test :: Map.Map A Int
test = Map.fromList [(A 1,1)]

C.hs

{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
module C(test) where

import A
import qualified Data.Map as Map

deriving newtype instance Ord A 

test :: Map.Map A Int
test = Map.fromList [(A 1,1)]

M.hs

import A
import qualified C
import qualified B

import qualified Data.Map as Map

test0 = C.test <> B.test
test1 = Map.insert (A 1) 1 C.test 
test2 = Map.insert (A 1) 1 B.test 

output.txt

M.hs:9:9: error:
    • Overlapping instances for Ord A
        arising from a use of ‘Map.insert’
      Matching instances:
        instance Ord A -- Defined at C.hs:9:1
        instance Ord A -- Defined at B.hs:10:1
    • In the expression: Map.insert (A 1) 1 B.test
      In an equation for ‘test2’: test2 = Map.insert (A 1) 1 B.test