scala $ skala -Xfatal-warnings

     ________ ___   / /  ___
    / __/ __// _ | / /  / _ |
  __\ \/ /__/ __ |/ /__/ __ |
 /____/\___/_/ |_/____/_/ | |
                          |/  version 2.13.0-20181127-135407-b76530e

scala> def f() { }
               ^
       warning: procedure syntax is deprecated: instead, add `: Unit =` to explicitly declare `f`'s return type
f: ()Unit

scala> 

scala> @deprecated("","") def g() = 42
g: ()Int

scala> g()
       ^
       warning: method g is deprecated:
error: No warnings can be incurred under -Xfatal-warnings.