use UncheckedWarning for unchecked warnings. enable -unchecked by default.

Author: lrytz

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -51,7 +51,7 @@ trait AllScalaSettings extends CommonScalaSettings, VerboseSettings, WarningSett
   val explain: Setting[Boolean] = BooleanSetting("-explain", "Explain errors in more detail.", aliases = List("--explain"))
   val feature: Setting[Boolean] = BooleanSetting("-feature", "Emit warning and location for usages of features that should be imported explicitly.", aliases = List("--feature"))
   val source: Setting[String] = ChoiceSetting("-source", "source version", "source version", List("3.0", "future", "3.0-migration", "future-migration"), "3.0", aliases = List("--source"))
-  val unchecked: Setting[Boolean] = BooleanSetting("-unchecked", "Enable additional warnings where generated code depends on assumptions.", aliases = List("--unchecked"))
+  val unchecked: Setting[Boolean] = BooleanSetting("-unchecked", "Enable additional warnings where generated code depends on assumptions.", initialValue = true, aliases = List("--unchecked"))
   val uniqid: Setting[Boolean] = BooleanSetting("-uniqid", "Uniquely tag all identifiers in debugging output.", aliases = List("--unique-id"))
   val language: Setting[List[String]] = MultiStringSetting("-language", "feature", "Enable one or more language features.", aliases = List("--language"))
   val rewrite: Setting[Option[Rewrites]] = OptionSetting[Rewrites]("-rewrite", "When used in conjunction with a `...-migration` source version, rewrites sources to migrate to new version.", aliases = List("--rewrite"))

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

@@ -353,7 +353,7 @@ object TypeTestsCasts {
           val argType = tree.args.head.tpe
           val isTrusted = tree.hasAttachment(PatternMatcher.TrustedTypeTestKey)
           if (!isTrusted && !checkable(expr.tpe, argType, tree.span))
-            report.warning(i"the type test for $argType cannot be checked at runtime", expr.srcPos)
+            report.uncheckedWarning(i"the type test for $argType cannot be checked at runtime", expr.srcPos)
           transformTypeTest(expr, tree.args.head.tpe, flagUnrelated = true)
         }
         else if (sym.isTypeCast)