Technically, USB-C and fast-charging methods like Qualcomm's Quick Charge 3.0 aren't supposed to mix. As Google's Benson Leung (a frequent crusader for good USB implementations) noted last fall, the USB-C spec explicitly forbids the variable voltage tricks that QC3.0 and others use to top up your battery in a hurry. Does that mean that your HTC 10 or LG G5 is at risk of catching fire, then? Not if you ask Qualcomm. It tells Android Central that you can make its fast-charge solution fit USB-C's voltage demands, and there have been "no reports" of problems so far. In short, you shouldn't have to worry when you plug in. So it's all settled, right?
Well, not quite. The Qualcomm statement doesn't address whether or not the USB-C phones using QC3.0 really are honoring the port standard. It's notable that Google's own USB-C phones, the Nexus 5X and 6P, don't use Quick Charge precisely because of concerns that it's out of spec. This doesn't mean that companies like HTC or LG are living dangerously by implementing speedy charging tech, but you shouldn't be surprised if they're the exceptions rather than the rule.