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{-# LANGUAGE UndecidableInstances, KindSignatures, TypeApplications, FlexibleInstances #-}
 
class A (m :: * -> *)
 
class A1 (m :: * -> *)
 
class B (m :: * -> *)
 
instance A m => B m
 
instance {-# OVERLAPPING #-} A1 m => B m
 
data C a
 
instance A C
 
data D a
 
instance A1 D
 
implicitly :: a => ()
implicitly = ()
 
test = implicitly @(B C)
test1 = implicitly @(B D)
 
main = print "ok"

[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:9:10: error:
    Duplicate instance declarations:
      instance A m => B m -- Defined at prog.hs:9:10
      instance [overlapping] A1 m => B m -- Defined at prog.hs:11:30
  |
9 | instance A m => B m
  |          ^^^^^^^^^^

Standard output is empty