Update: Google says that cloud services should be returning to normal for the majority of users. Things appear to be getting back to normal for GO and Discord.
I've got halved hatching time and double candy in Pokémon GO right now, but I can't seem to play. As a Google user this doesn't happen all that often, but whenever I try to sign in I get an "unable to authenticate" message or I get stuck on the loading screen. Plenty of others seem to be experiencing the same thing, and the outages appear to be affecting a lot more than just Pokémon GO. Discord, Spotify and Snapchat are also experiencing server problems. I've heard reports of problems with Blizzard, Fortnite and Steam, but all three of those appear to be working fine for me.
According to Discord's status page, the problem is originating from issues relating to the Google Cloud Platform, which would explain why it's affecting so many apparently unconnected apps. We've seen similar widespread outages with different services before, like when problems with Amazon Web Services took out Netflix and other apps on Christmas Eve in 2012.
Here's the text from the Google Cloud status page:
We are investigating a problem with Google Cloud Global Loadbalancers returning 502s for many services including AppEngine, Stackdriver, Dialogflow, as well as customer Global Load Balancers. We will provide another update by Tuesday, 2018-07-17 13:00 US/Pacific.
Hopefully, we'll hear more about this soon: outages stemming from larger services like Google are sometimes resolved more quickly than those with individual services, but major problems can be just as vexing for the massive teams at Google as they can at those of individual apps. A larger and larger portion of the video game world uses Discord for communication, and so this is a fairly major outage for the industry.