Also handle switches with only alternatives.

Author: sjrd

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -901,14 +901,18 @@ object PatternMatcher {
           Return(Literal(Constant(())), ref(label))
         case plan: SeqPlan =>
           def default = seq(emit(plan.head) :: Nil, emit(plan.tail))
+          def maybeEmitSwitch(scrutinee: Tree): Tree = {
+            val switchCases = collectSwitchCases(scrutinee, plan)
+            if (hasEnoughSwitchCases(switchCases, MinSwitchCases)) // at least 3 cases + default
+              Match(scrutinee, emitSwitchCases(switchCases))
+            else
+              default
+          }
           plan.head match {
             case testPlan: TestPlan =>
-              val scrutinee = testPlan.scrutinee
-              val switchCases = collectSwitchCases(scrutinee, plan)
-              if (hasEnoughSwitchCases(switchCases, MinSwitchCases)) // at least 3 cases + default
-                Match(scrutinee, emitSwitchCases(switchCases))
-              else
-                default
+              maybeEmitSwitch(testPlan.scrutinee)
+            case LabeledPlan(_, SeqPlan(LabeledPlan(_, SeqPlan(testPlan: TestPlan, _)), _)) =>
+              maybeEmitSwitch(testPlan.scrutinee)
             case _ =>
               default
           }

tests/pos-special/fatal-warnings/switches.scala

@@ -1,8 +1,24 @@
+import scala.annotation.switch
+
 class Test {
-  def test1(x: Int): Int = (x: @annotation.switch) match {
+  import Test._
+
+  def test1(x: Int): Int = (x: @switch) match {
     case 1 => 1
     case 2 | 3 | 4 => 2
     case 65 => 3
     case 72 => 4
   }
+
+  def test2(c: Char): Boolean = (c: @switch) match {
+    case LF | CR | FF | SU => true
+    case _ => false
+  }
+}
+
+object Test {
+  final val LF = '\u000A'
+  final val CR = '\u000D'
+  final val FF = '\u000C'
+  final val SU = '\u001A'
 }