foo.ex

defmodule Foo do
  @type t :: %__MODULE__{x: non_neg_integer}
  defstruct x: 0

  @spec test(t) :: t
  def test(%__MODULE__{} = foo), do: test2(foo)

  @spec test2(t) :: t
  defp test2(%__MODULE__{} = foo), do: %__MODULE__{foo | x: 5}
end

# fails with
# foo.ex:9: The variable _@1 can never match since previous clauses completely covered the type #{'__struct__':='Elixir.Foo', _=>_}