Add support for user-defined annotation subclasses

This adds infrastructure so that the compiler or compiler plugins
can support user-defined annotation subclasses.

Author: lrytz

library/src/scala/annotation/meta/superArg.scala

@@ -0,0 +1,34 @@
+/*
+ * Scala (https://www.scala-lang.org)
+ *
+ * Copyright EPFL and Lightbend, Inc. dba Akka
+ *
+ * Licensed under Apache License 2.0
+ * (http://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package scala.annotation
+package meta
+
+/**
+ * This internal annotation encodes arguments passed to annotation superclasses. Example:
+ *
+ * {{{
+ *   class a(x: Int) extends Annotation
+ *   class b extends a(42) // the compiler adds `@superArg("x", 42)` to class b
+ * }}}
+ */
+class superArg(p: String, v: Any) extends StaticAnnotation
+
+/**
+ * This internal annotation encodes arguments passed to annotation superclasses. Example:
+ *
+ * {{{
+ *   class a(x: Int) extends Annotation
+ *   class b(y: Int) extends a(y) // the compiler adds `@superFwdArg("x", "y")` to class b
+ * }}}
+ */
+class superFwdArg(p: String, n: String) extends StaticAnnotation