I'm finding that https://github.com/akka/akka/tree/release-2.3-dev (the branch for the upcoming version of Akka) does not compile in IntelliJ because it now uses https://github.com/sbt/sbt-boilerplate.
https://github.com/sbt/sbt-boilerplate generates code to be compiled, so if the plugin doesn't run, there are missing classes in the build.
1) Checkout https://github.com/akka/akka/tree/release-2.3-dev
2) Import into IDEA using SBT plugin
3) Find a runnable class such as akka.http.server.TestServer and run it.
The build will fail while trying to resolve AppendOneInstances or other classes that reside in akka/akka-http/src/main/boilerplate/akka/http/server/util.
Build the project once from the command line, then try step three again.
Please let me know if there's a better place for me to post this, thanks!