Refinement for knownHK for PolyParams

pos test t2082.scala shows that knownHK can be constructed before
the binder PolyType of a PolyParam is initialized.

Author: odersky

src/dotty/tools/dotc/core/TypeApplications.scala

@@ -494,7 +494,9 @@ class TypeApplications(val self: Type) extends AnyVal {
     case self: SingletonType => -1
     case self: TypeVar => self.origin.knownHK
     case self: WildcardType => self.optBounds.knownHK
+    case self: PolyParam => self.underlying.knownHK
     case self: TypeProxy => self.underlying.knownHK
+    case NoType => 0
     case _ => -1
   }
 

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

@@ -2765,7 +2765,11 @@ object Types {
 
     def paramName = binder.paramNames(paramNum)
 
-    override def underlying(implicit ctx: Context): Type = binder.paramBounds(paramNum)
+    override def underlying(implicit ctx: Context): Type = {
+      val bounds = binder.paramBounds
+      if (bounds == null) NoType // this can happen if the PolyType is not initialized yet
+      else bounds(paramNum)
+    }
     // no customized hashCode/equals needed because cycle is broken in PolyType
     override def toString = s"PolyParam($paramName)"