Address reviewer comments

Author: odersky

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

@@ -532,7 +532,7 @@ object Contexts {
     moreProperties = Map.empty
     typeComparer = new TypeComparer(this)
     searchHistory = new SearchHistory(0, Map())
-    gadt = new GADTMap(SimpleMap.Empty) // EmptyGADTMap
+    gadt = EmptyGADTMap
   }
 
   @sharable object NoContext extends Context {

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

@@ -256,7 +256,7 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisTrans
         case tree @ Annotated(annotated, annot) =>
           cpy.Annotated(tree)(transform(annotated), transformAnnot(annot))
         case tree: AppliedTypeTree =>
-          Checking.checkAppliedType(tree)
+          Checking.checkAppliedType(tree, boundsCheck = !ctx.mode.is(Mode.Pattern))
           super.transform(tree)
         case SingletonTypeTree(ref) =>
           Checking.checkRealizable(ref.tpe, ref.pos.focus)
@@ -282,7 +282,14 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisTrans
           }
           super.transform(tree)
         case Typed(Ident(nme.WILDCARD), _) =>
-          tree // skip checking pattern type
+          super.transform(tree)(ctx.addMode(Mode.Pattern))
+            // The added mode signals that bounds in a pattern need not
+            // conform to selector bounds. I.e. assume
+            //     type Tree[T >: Null <: Type]
+            // One is still allowed to write
+            //     case x: Tree[_]
+            // (which translates to)
+            //     case x: (_: Tree[_])
         case tree =>
           super.transform(tree)
       }

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

@@ -68,7 +68,7 @@ object Checking {
    *     Unreducible applications correspond to general existentials, and we
    *     cannot handle those.
    */
-  def checkAppliedType(tree: AppliedTypeTree)(implicit ctx: Context) = {
+  def checkAppliedType(tree: AppliedTypeTree, boundsCheck: Boolean)(implicit ctx: Context) = {
     val AppliedTypeTree(tycon, args) = tree
     // If `args` is a list of named arguments, return corresponding type parameters,
     // otherwise return type parameters unchanged
@@ -81,7 +81,7 @@ object Checking {
     val bounds = tparams.map(_.paramInfoAsSeenFrom(tycon.tpe).bounds)
     def instantiate(bound: Type, args: List[Type]) =
       HKTypeLambda.fromParams(tparams, bound).appliedTo(args)
-    checkBounds(orderedArgs, bounds, instantiate)
+    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]) =>