Just a couple of days after we sent word that the Galaxy S6 — and a few other popular phones haven't been working properly with Android Auto, a fix has arrived. It's not a huge shock that the latest version of Google Play Services (which manages a variety of Google apps on your phone) rolling out now enables Android Auto on these popular devices, and we're happy to say that indeed the GS6 is working.
So put the GS6 in the good column, along with the HTC One M8 and M9, the Moto X, Nexus phones and others.
The version numbering of the Play Services app (which really shouldn't be user-facing in the first place) varies by device, so there's not a whole lot you can do here — just wait for the update that's pushing now, and once it arrives you'll be all good for a friendly in-car experience on your shiny new phone. A good number of folks in our forums are tracking down what's working currently.