ambig2.hs

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}

type family Ambiguous (a :: k) :: b where
  Ambiguous x = x

all' :: Foldable (Ambiguous t) => (a -> Bool) -> Ambiguous t a -> Bool
all' = all

x :: Bool
x = all' (const True) []

-- ambig.hs:13:23: error:
--     • Couldn't match type ‘Ambiguous t0’ with ‘[]’
--       Expected type: Ambiguous t0 b0
--         Actual type: [b0]
--       The type variable ‘t0’ is ambiguous
--     • In the second argument of ‘all'’, namely ‘[]’
--       In the expression: all' (const True) []
--       In an equation for ‘x’: x = all' (const True) []
--    |
-- 13 | x = all' (const True) []
--    |                       ^^