Handle variance unsoundness in scalac

The included test

     pos-special/variances-constr.scala

demonstrates an unsoundness in the variance checking of scalac.
Scalac excludes symbols owned by constructors from the
checking. This is unsound, as can be demonstrated by compiling the test
and observing output of the program run:

Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
	at Test$.main(variances-constr.scala:17)
	at Test.main(variances-constr.scala)

Dotty allows this code only under -language:Scala2 and issues a migration warning.

1 files changed, 8 insertions, 4 deletions

diff --git a/src/dotty/tools/dotc/typer/VarianceChecker.scala b/src/dotty/tools/dotc/typer/VarianceChecker.scala
index 1d3ceaa57..5d3bd4f20 100644
--- a/src/dotty/tools/dotc/typer/VarianceChecker.scala
+++ b/src/dotty/tools/dotc/typer/VarianceChecker.scala
@@ -110,16 +110,20 @@ class VarianceChecker()(implicit ctx: Context) {
   private object Traverser extends TreeTraverser {
     def checkVariance(sym: Symbol) = Validator.validateDefinition(sym) match {
       case Some(VarianceError(tvar, required)) =>
-        ctx.error(
-          i"${varianceString(tvar.flags)} $tvar occurs in ${varianceString(required)} position in type ${sym.info} of $sym",
-          sym.pos)
+        def msg = i"${varianceString(tvar.flags)} $tvar occurs in ${varianceString(required)} position in type ${sym.info} of $sym"
+        if (ctx.scala2Mode && sym.owner.isConstructor)
+          ctx.migrationWarning(s"According to new variance rules, this is no longer accepted; need to annotate with @uncheckedVariance:\n$msg", sym.pos)
+        else ctx.error(msg, sym.pos)
       case None =>
     }
 
     override def traverse(tree: Tree)(implicit ctx: Context) = {
       def sym = tree.symbol
       // No variance check for private/protected[this] methods/values.
-      def skip = !sym.exists || sym.is(Local)
+      def skip =
+        !sym.exists ||
+        sym.is(Local) || // !!! watch out for protected local!
+        sym.is(TypeParam) && sym.owner.isClass // already taken care of in primary constructor of class
       tree match {
         case defn: MemberDef if skip =>
           ctx.debuglog(s"Skipping variance check of ${sym.showDcl}")