Usually that's because the IntelliJ Scala plugin fails to grok it otherwise. In this case it looks like the curlies are actually necessary:

[error] /Users/benjy/src/sbt/compile/inc/src/main/scala/sbt/CompileSetup.scala:30: not found: value og
[error] 		implicit val outputGroupsOrdering = Ordering.by(og: MultipleOutput.OutputGroup => og.sourceDirectory)
[error] 		                                                                                  ^

I can remove the parens if you think it's important. I like them for clarity.