Error

GHC/Foo.hs:6:12: error:
    • Overlapping instances for MyEq Bool arising from a use of ‘==’
      Matching instances:
        instance MyEq Bool -- Defined in ‘GHC.Base’
        instance MyEq Bool -- Defined in ‘GHC.Base’
    • In the expression: True == True
      In an equation for ‘foo’: foo = True == True

GHC.Base.hs

module GHC.Base (module GHC.Base, Bool(..)) where

import GHC.Prim
import GHC.Types

default ()

class MyEq a where
    (==), (/=) :: a -> a -> Bool

    x /= y = not (x == y)
    x == y = not (x /= y)

instance MyEq Bool where
    False == False = True
    True == True = True
    _ == _ = False

not :: Bool -> Bool
not True  = False
not False = True

inline x = x

GHC.Foo.hs

module GHC.Foo where

import GHC.Base

foo = True == True

Makefile

TOP=../../..
include $(TOP)/mk/boilerplate.mk
include $(TOP)/mk/test.mk

clean:
	rm -f GHC/*.o
	rm -f GHC/*.hi

base01:
	rm -f GHC/*.o
	rm -f GHC/*.hi
	'$(TEST_HC)' $(TEST_HC_OPTS) -XNoImplicitPrelude -this-unit-id base -c GHC/Base.hs
	'$(TEST_HC)' $(TEST_HC_OPTS) -XNoImplicitPrelude -this-unit-id base --make GHC.Foo