writing .phpt tests against IS_INDIRECT values

I'm contributing to an extension -- https://github.com/jbboehr/php-mustache -- and part of that has been little fixes for PHP7. I posted a PR to fix how it was handling zvals that came in as IS_INDIRECT. The maintainer asked if I could extend the .phpt coverage accordingly. The reading I've done has made it sound like this is an implementation detail that would never surface in userland. Is that true, or is there something I can do from tests to make sure these code paths get used? Thanks, Adam