public Adder apply(Adder adder) {
            return new FailsToPropagateException(adder);
          }
        },
        "add(",
        "exception");
  }

  public void testNotInterfaceType() {
    assertThrows(
        IllegalArgumentException.class,
        () ->
            new ForwardingWrapperTester()
                .testForwarding(String.class, Functions.<String>identity()));
  }