Add migration warnings

The idea is that whenever Dotty detects a migration problem under -language:Scala2,
it should issue a migration warning, so we know what needs to be rewritten.

3 files changed, 10 insertions, 1 deletions

diff --git a/src/dotty/tools/dotc/config/ScalaSettings.scala b/src/dotty/tools/dotc/config/ScalaSettings.scala
index 05fefc8b4..62b071372 100644
--- a/src/dotty/tools/dotc/config/ScalaSettings.scala
+++ b/src/dotty/tools/dotc/config/ScalaSettings.scala
@@ -18,6 +18,7 @@ class ScalaSettings extends Settings.SettingGroup {
    */
   val dependencyfile = StringSetting("-dependencyfile", "file", "Set dependency tracking file.", ".scala_dependencies")
   val deprecation = BooleanSetting("-deprecation", "Emit warning and location for usages of deprecated APIs.")
+  val migration = BooleanSetting("-migration", "Emit warning and location for migration issues from Scala 2.")
   val encoding = StringSetting("-encoding", "encoding", "Specify character encoding used by source files.", Properties.sourceEncoding)
   val explaintypes = BooleanSetting("-explaintypes", "Explain type errors in more detail.")
   val feature = BooleanSetting("-feature", "Emit warning and location for usages of features that should be imported explicitly.")
diff --git a/src/dotty/tools/dotc/core/TypeComparer.scala b/src/dotty/tools/dotc/core/TypeComparer.scala
index 684e9cbfd..d60828a21 100644
--- a/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -500,7 +500,9 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     projection.name == tpnme.hkApply &&
     !other.isHKApply &&
     other.testLifted(projection.prefix.LambdaClass(forcing = true).typeParams,
-      if (inOrder) isSubType(projection.prefix, _) else isSubType(_, projection.prefix),
+        { xx => println(i"test lifted with $xx")
+          if (inOrder) isSubType(projection.prefix, xx) else isSubType(xx, projection.prefix)
+        },
       if (inOrder) Nil else classBounds(projection.prefix))
 
   /** The class symbols bounding the type of the `Apply` member of `tp` */
diff --git a/src/dotty/tools/dotc/reporting/Reporter.scala b/src/dotty/tools/dotc/reporting/Reporter.scala
index 0358f71f6..4a72c2066 100644
--- a/src/dotty/tools/dotc/reporting/Reporter.scala
+++ b/src/dotty/tools/dotc/reporting/Reporter.scala
@@ -66,6 +66,9 @@ object Reporter {
   class DeprecationWarning(msgFn: => String, pos: SourcePosition) extends ConditionalWarning(msgFn, pos) {
     def enablingOption(implicit ctx: Context) = ctx.settings.deprecation
   }
+  class MigrationWarning(msgFn: => String, pos: SourcePosition) extends ConditionalWarning(msgFn, pos) {
+    def enablingOption(implicit ctx: Context) = ctx.settings.migration
+  }
 }
 
 import Reporter._
@@ -82,6 +85,9 @@ trait Reporting { this: Context =>
   def deprecationWarning(msg: => String, pos: SourcePosition = NoSourcePosition): Unit =
     reporter.report(new DeprecationWarning(msg, pos))
 
+  def migrationWarning(msg: => String, pos: SourcePosition = NoSourcePosition): Unit =
+    reporter.report(new MigrationWarning(msg, pos))
+
   def uncheckedWarning(msg: => String, pos: SourcePosition = NoSourcePosition): Unit =
     reporter.report(new UncheckedWarning(msg, pos))