@Test
public void singlePlaceholderResolverReturnsPlaceholderIfNotResolvable() {

    final PipelineFunctionParameterResolverFactory.SingleParameterResolver parameterResolver =
            PipelineFunctionParameterResolverFactory.forStringOrPlaceholderParameter();
    final String stringPlaceholder = "(    " + KNOWN_PLACEHOLDER + "   )";
    when(expressionResolver.resolveAsPipelineElement(anyString())).thenReturn(PipelineElement.unresolved());
    assertThat(parameterResolver.apply(stringPlaceholder, expressionResolver, DUMMY)).isEmpty();
    verify(expressionResolver).resolveAsPipelineElement(KNOWN_PLACEHOLDER);
}