SI-8731 warning if @switch is ignored

For matches with two or fewer cases, @switch is ignored. This should
not happen silently.

2 files changed, 42 insertions, 0 deletions

diff --git a/test/junit/scala/issues/BytecodeTests.scala b/test/junit/scala/issues/BytecodeTests.scala
new file mode 100644
index 0000000000..7a05472277
--- /dev/null
+++ b/test/junit/scala/issues/BytecodeTests.scala
@@ -0,0 +1,39 @@
+package scala.issues
+
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+import org.junit.Test
+import scala.tools.asm.Opcodes
+import scala.tools.nsc.backend.jvm.CodeGenTools._
+import org.junit.Assert._
+import scala.collection.JavaConverters._
+import scala.tools.partest.ASMConverters._
+
+@RunWith(classOf[JUnit4])
+class BytecodeTests {
+  val compiler = newCompiler()
+
+  @Test
+  def t8731(): Unit = {
+    val code =
+      """class C {
+        |  def f(x: Int) = (x: @annotation.switch) match {
+        |    case 1 => 0
+        |    case 2 => 1
+        |    case 3 => 2
+        |  }
+        |  final val K = 10
+        |  def g(x: Int) = (x: @annotation.switch) match {
+        |    case K => 0
+        |    case 1 => 10
+        |    case 2 => 20
+        |  }
+        |}
+      """.stripMargin
+
+    val List(c) = compileClasses(compiler)(code)
+
+    assertTrue(getSingleMethod(c, "f").instructions.count(_.isInstanceOf[TableSwitch]) == 1)
+    assertTrue(getSingleMethod(c, "g").instructions.count(_.isInstanceOf[LookupSwitch]) == 1)
+  }
+}
diff --git a/test/junit/scala/tools/nsc/backend/jvm/CodeGenTools.scala b/test/junit/scala/tools/nsc/backend/jvm/CodeGenTools.scala
index 15bc1f427d..b892eb36cf 100644
--- a/test/junit/scala/tools/nsc/backend/jvm/CodeGenTools.scala
+++ b/test/junit/scala/tools/nsc/backend/jvm/CodeGenTools.scala
@@ -76,4 +76,7 @@ object CodeGenTools {
   def assertSameCode(actual: List[Instruction], expected: List[Instruction]): Unit = {
     assertTrue(s"\nExpected: $expected\nActual  : $actual", actual === expected)
   }
+
+  def getSingleMethod(classNode: ClassNode, name: String): Method =
+    convertMethod(classNode.methods.asScala.toList.find(_.name == name).get)
 }