So, there have been some discussions and angry tweets recently about irritating Scala "features" (like value discarding and auto-tupling) that can actually be turned off by selecting the right compiler flag in conjunction with -Xfatal-warnings. I highly recommend a set of options something like those below.
scalacOptions ++= Seq(
"-deprecation",
"-encoding", "UTF-8", // yes, this is 2 args
"-feature",
"-language:existentials",