Quick charge adapter on not supported devices The adapter which comes with my new phone has Qualcomm Quick Charge 3.0 label on it. The output reads: 5V-3A / 9V-2A / 12V-1.5A. Is it safe to use it with my old phones? For example, if I have a very dated phone which only takes 5V-0.5A, will the adapter outputs 12V and fries it? And what would happen if I use that adapter with devices supporting fast charging technologies from other companies: USB-PD from USB-IF, Pump Express from Mediatek, VOOC from Oppo, and SuperCharge from Huawei?
Is it safe to use it with my old phones? 



It is safe. See what happens if you plug a non QC 2.0 device into a QC 2.0 charger?



  And what would happen if I use that adapter with devices supporting fast charging technologies from other companies.



They all are proprietary technologies which vary in terms of what is changed to charge fast - Voltage, Current or both, apart from other factors. Details obviously are not in public domain. Therefore, they are not guaranteed to work on devices using other technologies, so one cannot predict whether it works or not. 

OEM's don't recommend such practices. YMMV