assert_raise…

https://hexdocs.pm/ex_unit/ExUnit.Assertions.html#assert_raise/2

This was a good idea! I’ve added it to the new version (0.15.0) and just published it to hex: https://hex.pm/packages/assertions
https://hexdocs.pm/assertions/0.15.0/Assertions.html#assert_raise/1

My suggestion was not entirely right. You could write a expect_failure macro like this:

defmodule ExpectFailure do
  defmacro expect_failure(ast) do
    quote do
      try do
        unquote(ast)
        flunk("This should has crashed: " <> unquote(Macro.to_string(ast)))
      rescue
        e in ExUnit.AssertionError ->
          raise e

        _ ->
          :ok
      end
    end
  end
end

defmodule AsdfTest do
  use ExUnit.Case
  doctest Asdf
  import ExpectFailure

  def foo(x) when is_number(x) do
    x + 1
  end

  test "expect_failure catches errors" do
    expect_failure(foo("a"))
  end

  test "expect_failure should crash" do
    assert_raise ExUnit.AssertionError, fn ->
      expect_failure(foo(1))
    end
  end
end