Fix false negatives

Special provisions need to be made if the type argument is a wildcard
or of type Nothing.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -110,7 +110,7 @@ object Checking {
    *  Test cases are neg/i2771.scala and neg/i2771b.scala.
    */
   def preCheckKind(arg: Tree, paramBounds: TypeBounds)(implicit ctx: Context): Tree =
-    if (arg.tpe.hasSameKindAs(paramBounds.hi)) arg
+    if (arg.tpe.widen.isRef(defn.NothingClass) || arg.tpe.hasSameKindAs(paramBounds.hi)) arg
     else errorTree(arg, em"Type argument ${arg.tpe} has not the same kind as its bound $paramBounds")
 
   def preCheckKinds(args: List[Tree], paramBoundss: List[TypeBounds])(implicit ctx: Context): List[Tree] = {

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1131,16 +1131,23 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
           if (tpt1.symbol.isClass)
             tparam match {
               case tparam: Symbol =>
-                // This is needed to get the test `compileParSetSubset` to work
-                tparam.ensureCompleted()
+                tparam.ensureCompleted() // This is needed to get the test `compileParSetSubset` to work
               case _ =>
             }
           if (desugaredArg.isType) typed(desugaredArg, argPt)
           else desugaredArg.withType(UnspecifiedErrorType)
         }
         args.zipWithConserve(tparams)(typedArg(_, _)).asInstanceOf[List[Tree]]
       }
-      val args2 = preCheckKinds(args1, tparams.map(_.paramInfo.bounds))
+      val paramBounds = (tparams, args).zipped.map {
+        case (tparam, TypeBoundsTree(EmptyTree, EmptyTree)) =>
+          // if type argument is a wildcard, suppress kind checking since
+          // there is no real argument.
+          TypeBounds.empty
+        case (tparam, _) =>
+          tparam.paramInfo.bounds
+      }
+      val args2 = preCheckKinds(args1, paramBounds)
       // check that arguments conform to bounds is done in phase PostTyper
       assignType(cpy.AppliedTypeTree(tree)(tpt1, args2), tpt1, args2)
     }