Fix bootstrap: pass null explicitly as run-time

Author: liufengyun

compiler/src/dotty/tools/dotc/util/SourcePosition.scala

@@ -70,7 +70,7 @@ extends interfaces.SourcePosition with Showable {
 }
 
 /** A sentinel for a non-existing source position */
-@sharable object NoSourcePosition extends SourcePosition(NoSource, NoSpan) {
+@sharable object NoSourcePosition extends SourcePosition(NoSource, NoSpan, null) {
   override def toString: String = "?"
   override def withOuter(outer: SourcePosition): SourcePosition = outer
 }

tests/neg/i4659b.scala

@@ -0,0 +1,5 @@
+case class SourcePosition(outer: SourcePosition = NoSourcePosition) {
+  assert(outer != null)  // crash
+}
+
+object NoSourcePosition extends SourcePosition() // error

tests/run/i4659b.scala

@@ -0,0 +1,7 @@
+case class SourcePosition(outer: SourcePosition = (NoSourcePosition: SourcePosition))
+
+object NoSourcePosition extends SourcePosition()
+
+object Test extends App {
+  assert(NoSourcePosition.outer == null)
+}