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The Huawei P9 phone doesn't support USB Power Delivery or any other fast charging technology over its USB Type-C connector. In case the phone desperately needs a charge when the Samsung 25W USB-C Fast Charging Wall Charger is the only charging option available, we see how charging happens over Samsung’s proprietary Adaptive Fast Charging scheme.

An easy way is by using the GRL-USB-PD-A1 analyzer by Granite River Labs to determine the charging behavior between the phone and charger. We first plug in the Huawei P9 with its battery fully depleted to the Samsung 25W USB-C Fast Charging Wall Charger using the USB Type-A to Type-C cable accessory supplied with the charger. 

Note that the Samsung 25W USB-C Fast Charging Wall Charger advertises on its casing to support 25 watts (12 volts @ 2.1 amps), 15 watts (9 volts @ 1.67 amps), and 10 watts (5 volts @ 2 amps) power output through its USB Type-A connector.

When initially connected from zero charge, the Huawei P9 first pulls about 0.5 amps current while at 5 volts when the phone is off. As the current pulled rises up quickly when reaching 30 seconds, it changes rapidly to cause the phone to trickle charge for the next 27 seconds. The current then finally starts to stabilize for the phone to get around 9.5 watts (5 volts @ 1.9 amps) of charging with the Samsung 25W charger. 

Using the Samsung 25W USB-C Fast Charging Wall Charger with the Huawei P9 is just like using any other 10W charger including the phone’s own 10 watts (5 volts @ 2 amps) power adapter. The Samsung charger offers a decent power option for the Huawei phone just in case it's missing the Huawei charger and needs a quick charge.