Fix #5054: Remove the dead code generated by the Labeled-based pattern matcher

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

@@ -457,6 +457,12 @@ object PatternMatcher {
             apply(initializer(plan.sym))
           plan
         }
+        override def apply(plan: SeqPlan): Plan = {
+          apply(plan.head)
+          if (canFallThrough(plan.head))
+            apply(plan.tail)
+          plan
+        }
       }
       refCounter(plan)
       refCounter.count
@@ -564,8 +570,10 @@ object PatternMatcher {
       new MergeTests()(plan)
     }
 
-    /** Inline let-bound trees that are referenced only once.
-     *  Drop all variables that are not referenced anymore after this.
+    /** Inline let-bound trees that are referenced only once and eliminate dead code.
+     *
+     *  - Drop all variables that are not referenced anymore after inlining.
+     *  - Drop the `tail` of `SeqPlan`s whose `head` cannot fall through.
      */
     private def inlineVars(plan: Plan): Plan = {
       val refCount = varRefCount(plan)
@@ -597,6 +605,18 @@ object PatternMatcher {
             plan
           }
         }
+        override def apply(plan: SeqPlan): Plan = {
+          val newHead = apply(plan.head)
+          if (!canFallThrough(newHead)) {
+            // If the head cannot fall through, the tail is dead code
+            newHead
+          }
+          else {
+            plan.head = newHead
+            plan.tail = apply(plan.tail)
+            plan
+          }
+        }
       }
       Inliner(plan)
     }

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -50,7 +50,7 @@ class TestBCode extends DottyBytecodeTest {
   /** This test verifies that simple matches with `@switch` annotations are
    *  indeed transformed to a switch
    */
-  @Test def basicTransfromAnnotated = {
+  @Test def basicSwitch = {
     val source = """
                  |object Foo {
                  |  import scala.annotation.switch
@@ -69,6 +69,71 @@ class TestBCode extends DottyBytecodeTest {
     }
   }
 
+  @Test def switchWithAlternatives = {
+    val source =
+      """
+        |object Foo {
+        |  import scala.annotation.switch
+        |  def foo(i: Int) = (i: @switch) match {
+        |    case 2 => println(2)
+        |    case 1 | 3 | 5 => println(1)
+        |    case 0 => println(0)
+        |  }
+        |}
+      """.stripMargin
+
+    checkBCode(source) { dir =>
+      val moduleIn   = dir.lookupName("Foo$.class", directory = false)
+      val moduleNode = loadClassNode(moduleIn.input)
+      val methodNode = getMethod(moduleNode, "foo")
+      assert(verifySwitch(methodNode))
+    }
+  }
+
+  @Test def switchWithGuards = {
+    val source =
+      """
+        |object Foo {
+        |  import scala.annotation.switch
+        |  def foo(i: Int, b: Boolean) = (i: @switch) match {
+        |    case 2 => println(3)
+        |    case 1 if b => println(2)
+        |    case 1 => println(1)
+        |    case 0 => println(0)
+        |  }
+        |}
+      """.stripMargin
+
+    checkBCode(source) { dir =>
+      val moduleIn   = dir.lookupName("Foo$.class", directory = false)
+      val moduleNode = loadClassNode(moduleIn.input)
+      val methodNode = getMethod(moduleNode, "foo")
+      assert(verifySwitch(methodNode))
+    }
+  }
+
+  @Test def matchWithDefaultNoThrowMatchError = {
+    val source =
+      """class Test {
+        |  def test(s: String) = s match {
+        |    case "Hello" => 1
+        |    case _       => 2
+        |  }
+        |}
+      """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn = dir.lookupName("Test.class", directory = false)
+      val clsNode = loadClassNode(clsIn.input)
+      val method = getMethod(clsNode, "test")
+      val throwMatchError = instructionsFromMethod(method).exists {
+        case Op(Opcodes.ATHROW) => true
+        case _ => false
+      }
+      assertFalse(throwMatchError)
+    }
+  }
+
   @Test def failTransform = {
     val source = """
                  |object Foo {