Basically what's happening is, the Minecraft Launcher, for whatever reason, can't access the main class in the jar. When I click "play", it downloads all the necessary files (jars, etc.), closes the launcher as if it were about to start, and then immediately opens back up with an error message under the Game Output tab.

The error message is this: "OpenJDK 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release
Error: Could not find or load main class net.minecraft.client.main.Main"

I have tried deleting the jar file, forcing it to re-download, but it still happens. I have also observed an identical error occurring on a separate computer, running the same operating system, java version, etc.

I don't think the jar is corrupt, because inside the jar, there is, actually, a net.minecraft.client.main.Main class.

UPDATE: I tried again with Minecraft 1.0, and the same thing happened but it said "Error: Could not find or load main class net.minecraft.launchwrapper.Launch". I'm assuming this is because the older versions of Minecraft had a different main class. This shows that the problem is with the launcher and not the actual Minecraft jar.

THIS IS HAPPENING AGAIN AS OF TODAY, 11/19/18.

_{Honestly, you'd think Mojang would be careful enough to not let a bug like this into production...}