@Override
  public ExtensionContainer declareExtension(@Nullable PluginInfo pluginInfo, Object extension) {
    return this;
  }
@Test
  public void declareExtension_does_nothing() {
    ListContainer container = new ListContainer();
    assertThatNoException().isThrownBy(() -> container
      .declareExtension("A", A.class)
      .declareExtension(mock(PluginInfo.class), B.class));
  }