chore: add support for *unqualified selectors* in patterns

Author: hamzaremmal

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3291,7 +3291,7 @@ object Parsers {
      *                    |  SimplePattern1 [TypeArgs] [ArgumentPatterns]
      *                    |  ‘given’ RefinedType
      *  SimplePattern1   ::= SimpleRef
-     *                    |  SimplePattern1 `.' id
+     *                    |  [SimplePattern1] `.' id
      *  PatVar           ::= id
      *                    |  `_'
      */
@@ -3308,6 +3308,9 @@ object Parsers {
         simpleExpr(Location.InPattern)
       case XMLSTART =>
         xmlLiteralPattern()
+      case DOT =>
+        accept(DOT)
+        simplePatternRest(selector(EmptyTree))
       case GIVEN =>
         atSpan(in.offset) {
           val givenMod = atSpan(in.skipToken())(Mod.Given())

tests/pos/unqualified-selector-enum.scala

@@ -2,6 +2,11 @@ enum Opt[+T]:
   case none extends Opt[Nothing]
   case some[T](value: T) extends Opt[T]
 
+  def map[U](f: T => U): Opt[U] = this match
+    case .none => .none
+    case Opt.some(v) => .some(f(v)) // TODO: This should be `case .some(v) => .some(f(v))`
+  end map
+
 object Proxy:
   def opt(o: Opt[Int]): Opt[Int] = o