UnsafeOverlap -> 36705

[PPKFS]

[BinderDavid]

Example from the GHC testsuite:

{-# LANGUAGE Haskell2010 #-}
{-# OPTIONS_GHC -fwarn-unsafe #-}
{-# LANGUAGE FlexibleInstances #-}
module SH_Overlap6_A (
    C(..)
  ) where

import SH_Overlap6_B

instance
  {-# OVERLAPS #-}
  C [Int] where
    f _ = "[Int]"

and

{-# LANGUAGE Haskell2010 #-}
{-# OPTIONS_GHC -fwarn-unsafe #-}
module SH_Overlap6_B (
    C(..)
  ) where

class C a where
  f :: a -> String

and

{-# LANGUAGE Haskell2010 #-}
{-# LANGUAGE Safe #-}
{-# LANGUAGE FlexibleInstances #-}

-- | Same as `SH_Overlap5` but dependencies are now inferred-safe, not
-- explicitly marked. Compilation should still fail.
module SH_Overlap6 where

import safe SH_Overlap6_A

instance C [a] where
  f _ = "[a]"

test :: String
test = f ([1,2,3,4] :: [Int])

creates the following message:

SH_Overlap6.hs:15:8: error: [GHC-36705]
    • 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_Overlap6_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 [safe] C [a] -- Defined at SH_Overlap6.hs:11:10
    • In the expression: f ([1, 2, 3, 4] :: [Int])
      In an equation for ‘test’: test = f ([1, 2, 3, 4] :: [Int])