Merge pull request #4024 from dotty-staging/fix-#3747

Fix #3861: Spec changes to extractors implemented in #3747

Author: odersky

docs/docs/reference/changed/vararg-patterns.md

@@ -18,4 +18,30 @@ supported:
     /*!*/ case List(1, 2, xs @ _*)       // syntax error
     /*!*/ case List(1, 2, _*) => ...     // syntax error
 
+Another change relates to extractors producing values with `T*` types, which can then be
+matched by vararg patterns. Previously, such extractors used an `unapplySeq` whereas now they use an `unapply` (in the long term, we plan to get rid of `unapplySeq` altogether, replacing all its usages with `unapply`).
+
+Example: Previously, this was correct:
+
+    class Person(val name: String, val children: Person *)
+    object Person {
+      def unapplySeq(p: Person) = Some((p.name, p.children))
+    }
+
+    def childCount(p: Person) = p match {
+      case Person(_, ns @ _*) => ns.length
+    }
+
+Now, the equivalent program is written as follows:
+
+    class Person(val name: String, val children: Person *)
+    object Person {
+      def unapply(p: Person) = Some((p.name, p.children))
+    }
+
+    def childCount(p: Person) = p match {
+      case Person(_, ns : _*) => ns.length
+    }
+
+