If you're online and have an active CC subscription you will be alerted that it was created in a later version of InDesign. You will also be given the opportunity to allow the file to be uploaded to Adobe servers where it will be converted and downloaded for you.
In effect this simply converts to IDML for you. There is no way to directly open a CC2017 file in CC2015.
Thanks, Bob, that's all I wanted to know, I appreciate!
Happy New Year! :-)