tcfail142.hs:18:8: error:
    • Could not deduce (Bar a0 r)
      from the context: Bar a r
        bound by the type signature for:
                   bar :: forall a r. Bar a r => r -> ()
        at tcfail142.hs:18:8-25
      The type variable ‘a0’ is ambiguous
    • In the ambiguity check for ‘bar’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      In the type signature: bar :: Bar a r => r -> ()