[1 of 3] Compiling SH_Overlap2_B    ( SH_Overlap2_B.hs, SH_Overlap2_B.o )
[2 of 3] Compiling SH_Overlap2_A    ( SH_Overlap2_A.hs, SH_Overlap2_A.o )
[3 of 3] Compiling SH_Overlap2      ( SH_Overlap2.hs, SH_Overlap2.o )

SH_Overlap2.hs:19:8: error:
    • Unsafe overlapping instances for C [Int]
        arising from a use of ‘f’
      The matching instance is:
        instance [overlap ok] [safe] C [Int]
          -- Defined at SH_Overlap2_A.hs:12:3
      It is compiled in a Safe module and as such can only
      overlap instances from the same module, however it
      overlaps the following instances from different modules:
        instance C [a] -- Defined at SH_Overlap2.hs:15:3
    • In the expression: f ([1, 2, 3, 4] :: [Int])
      In an equation for ‘test’: test = f ([1, 2, 3, 4] :: [Int])