Encode default args of annotations in a meta annotation

... and use the defaults at callsites

Author: lrytz

library/src/scala/annotation/meta/defaultArg.scala

@@ -0,0 +1,30 @@
+/*
+ * 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 meta annotation is used by the compiler to support default annotation arguments.
+ *
+ * For an annotation definition `class ann(x: Int = defaultExpr) extends Annotation`, the compiler adds
+ * `@defaultArg(defaultExpr)` to the parameter `x`. This causes the syntax tree of `defaultExpr` to be
+ * stored in the classfile.
+ *
+ * When using a default annotation argument, the compiler can recover the syntax tree and insert it in the
+ * `AnnotationInfo`.
+ *
+ * For details, see `scala.reflect.internal.AnnotationInfos.AnnotationInfo`.
+ */
+@meta.param class defaultArg(arg: Any) extends StaticAnnotation {
+  def this() = this(null)
+}