Reports usages of advanced language features that are not enabled in the current file.

Advanced language features need to be enabled either by the corresponding compiler option or by importing an explicit feature flag in the file where they are used.

Postfix operator notation:
val s = 123 toString

Reflective call:
def foo(v: {def bar()}) = v.bar()

Dynamic member selection:
class Foo extends Dynamic

Implicit conversion:
implicit def toInt(s: String) = s.toInt

Higher-kinded type (enabled by default since Scala 2.13.1):
class Bar[M[A]]

Existential type:
def foo(v: Seq[T] forSome { type T })

Macro definition:
def assert(s: String) = macro Asserts.assertImpl

Scala compiler may also emit these warnings when "-feature" option is provided.

For compiler options see: Project Settings | Compiler | Scala Compiler | Features