Advanced language features, that require either explicitly imported feature flag or explicit compiler option to be enabled (SIP-18).

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:
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.

See: Project Settings / Compiler / Scala Compiler / Features

Since: Scala 2.10