@Test
  void rWildReturnType() {
    assertThatExceptionOfType(IllegalArgumentException.class)
        .isThrownBy(
            () -> {
              AsyncFeign.builder().target(WildApi.class, "http://localhost");
            });
  }
when you must parse a 2xx status to determine if the operation succeeded or not.
core/src/test/java/feign/AsyncFeignTest.java