More details on Vararg Patterns

Changes to `unapply` are dropped and will be reverted in the
future. #3248 is reopened.

Author: allanrenucci

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

@@ -3,45 +3,38 @@ layout: doc-page
 title: "Vararg Patterns"
 ---
 
-The syntax of vararg patterns has changed. In the new syntax one
-writes varargs in patterns exactly like one writes them in
-expressions, using a `: _*` type annotation:
-
-    xs match {
-      case List(1, 2, xs: _*) => println(xs)    // binds xs
-      case List(1, _ : _*) =>                   // wildcard pattern
-    }
-
-The old syntax, which is shorter but less regular, is no longer
-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
-    }
-
-
-
+The syntax of vararg patterns has changed. In the new syntax one writes varargs in patterns exactly
+like one writes them in expressions, using a `: _*` type annotation:
+
+```scala
+xs match {
+  case List(1, 2, xs: _*) => println(xs)    // binds xs
+  case List(1, _ : _*) =>                   // wildcard pattern
+}
+```
+
+The old syntax, which is shorter but less regular, is no longer supported.
+
+```scala
+/*!*/ case List(1, 2, xs @ _*)       // syntax error
+/*!*/ case List(1, 2, _*) => ...     // syntax error
+```
+
+The change to the grammar is:
+
+```diff
+ SimplePattern ::=  ‘_’
+                 |  varid
+                 |  Literal
+                 |  StableId
+                 |  StableId ‘(’ [Patterns ‘)’
+-                |  StableId ‘(’ [Patterns ‘,’] [varid ‘@’] ‘_’ ‘*’ ‘)’
++                |  StableId ‘(’ [Patterns ‘,’] (varid | ‘_’) ‘:’ ‘_’ ‘*’ ‘)’
+                 |  ‘(’ [Patterns] ‘)’
+                 |  XmlPattern
+```
+
+## Compatibility considerations
+
+Under the `-language:Scala2` option, Dotty will accept both the old and the new syntax.
+A migration warning will be emitted when the old syntax is encountered.