Can you charge a Qualcomm Quick Charge device without a supported charger? I have a laptop with a 20V 2.25A charger and a phone that supports QC 3.0. In theory, 3.0 has support for up to 20V, but I am not sure if I can use the laptop charger with the phone given that the charger is not necessarily QC approved. Both devices are USB C.

Phone

Charger
Quick charging is not just about Voltage or current in isolation. It is about varying both to deliver the right amount of power ( = product of voltage and current ) as required. Read more here Fast Charging by Qualcomm 3.0- impact on battery?

Besides, the device detects if the charger is quick charge or not. If it is not, then it acts as of it is connected to a normal charger, which is rated 5V, 2A , far lower than the one you want to try

To answer your question - NO, not advised