Vulpix: Introduce WarnTest/checkWarnings

[Cherry-picked 4740017]

Author: dwijnand

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -108,6 +108,15 @@ class CompilationTests {
     ).times(2).checkCompile()
   }
 
+  // Warning tests ------------------------------------------------------------
+
+  @Test def warn: Unit = {
+    implicit val testGroup: TestGroup = TestGroup("compileWarn")
+    aggregateTests(
+      compileFilesInDir("tests/warn", defaultOptions),
+    ).checkWarnings()
+  }
+
   // Negative tests ------------------------------------------------------------
 
   @Test def negAll: Unit = {

compiler/test/dotty/tools/vulpix/ParallelTesting.scala

@@ -723,6 +723,12 @@ trait ParallelTesting extends RunnerOrchestration { self =>
   private final class PosTest(testSources: List[TestSource], times: Int, threadLimit: Option[Int], suppressAllOutput: Boolean)(implicit summaryReport: SummaryReporting)
   extends Test(testSources, times, threadLimit, suppressAllOutput)
 
+  private final class WarnTest(testSources: List[TestSource], times: Int, threadLimit: Option[Int], suppressAllOutput: Boolean)(implicit summaryReport: SummaryReporting)
+  extends Test(testSources, times, threadLimit, suppressAllOutput):
+    override def suppressErrors = true
+    override def onSuccess(testSource: TestSource, reporters: Seq[TestReporter], logger: LoggedRunnable): Unit =
+      diffCheckfile(testSource, reporters, logger)
+
   private final class RewriteTest(testSources: List[TestSource], checkFiles: Map[JFile, JFile], times: Int, threadLimit: Option[Int], suppressAllOutput: Boolean)(implicit summaryReport: SummaryReporting)
   extends Test(testSources, times, threadLimit, suppressAllOutput) {
     private def verifyOutput(testSource: TestSource, reporters: Seq[TestReporter], logger: LoggedRunnable) = {
@@ -1020,6 +1026,9 @@ trait ParallelTesting extends RunnerOrchestration { self =>
     def checkCompile()(implicit summaryReport: SummaryReporting): this.type =
       checkPass(new PosTest(targets, times, threadLimit, shouldFail || shouldSuppressOutput), "Pos")
 
+    def checkWarnings()(implicit summaryReport: SummaryReporting): this.type =
+      checkPass(new WarnTest(targets, times, threadLimit, shouldFail || shouldSuppressOutput), "Warn")
+
     /** Creates a "neg" test run, which makes sure that each test generates the
      *  correct number of errors at the correct positions. It also makes sure
      *  that none of these tests crashes the compiler.