Fix #4382: Fix checking that hk types cannot take wildcard arguments

Four existing test cases had to be reclassified to require -language:Scala2.

Author: odersky

compiler/src/dotty/tools/dotc/core/StdNames.scala

@@ -9,7 +9,6 @@ import Symbols._
 import Contexts._
 import Decorators.PreNamedString
 import util.NameTransformer
-import scala.collection.breakOut
 
 object StdNames {
 

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -29,7 +29,7 @@ import dotty.tools.dotc.transform.Erasure
 import printing.Printer
 import Hashable._
 import Uniques._
-import collection.{mutable, Seq, breakOut}
+import collection.{mutable, Seq}
 import config.Config
 import annotation.tailrec
 import Flags.FlagSet

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

@@ -86,15 +86,11 @@ object Checking {
     if (boundsCheck) checkBounds(orderedArgs, bounds, instantiate)
 
     def checkWildcardApply(tp: Type, pos: Position): Unit = tp match {
-      case tp @ AppliedType(tycon, args) if args.exists(_.isInstanceOf[TypeBounds]) =>
-        tycon match {
-          case tycon: TypeLambda =>
-            ctx.errorOrMigrationWarning(
-              ex"unreducible application of higher-kinded type $tycon to wildcard arguments",
-              pos)
-          case _ =>
-            checkWildcardApply(tp.superType, pos)
-        }
+      case tp @ AppliedType(tycon, args) =>
+        if (tycon.isLambdaSub && args.exists(_.isInstanceOf[TypeBounds]))
+          ctx.errorOrMigrationWarning(
+            ex"unreducible application of higher-kinded type $tycon to wildcard arguments",
+            pos)
       case _ =>
     }
     def checkValidIfApply(implicit ctx: Context): Unit =

tests/neg/i4382.scala

@@ -0,0 +1,13 @@
+object App {
+  type Id[A] >: A <: A
+  def v1: Id[_] = ??? // error
+
+  type HkL[A] >: A
+  def v2: HkL[_] = ??? // error
+
+  type HkU[A] <: A
+  def v3: HkU[_] = ???  // error
+
+  type HkAbs[A]
+  def v4: HkAbs[_] = ??? // error
+}