Example of unexpected ambiguity.

Main.hs

#! /usr/bin/env stack
-- stack --resolver ghc-9.0.1 script

-- https://github.com/tfausak/witch/issues/19

{-# language AllowAmbiguousTypes #-}
{-# language ScopedTypeVariables #-}
{-# language TypeApplications #-}
{-# language TypeFamilies #-}

data Break

type family Ambiguous a where
  Ambiguous Break = ()
  Ambiguous a = a

as :: forall a b . Ambiguous a ~ b => b -> b
as = id

identity :: forall a . a -> a
identity = as @a

{-
• Couldn't match type ‘a’ with ‘Ambiguous a’
    arising from a use of ‘as’
  ‘a’ is a rigid type variable bound by
    the type signature for:
      identity :: forall a. a -> a
    at /tmp/tmp.vkOEgTlyHU/Main.hs:17:1-29
• In the expression: as @a
  In an equation for ‘identity’: identity = as @a
• Relevant bindings include
    identity :: a -> a (bound at /tmp/tmp.vkOEgTlyHU/Main.hs:18:1)
-}

main :: IO ()
main = pure $ identity ()