SI-8479 Fix constructor default args under scaladoc

The `DocDef` node hid the `DefDef` constructor from the scrutinee
of the namer when determining if the class had constructor defaults
or not.

The current pattern for fixing these bugs is to delegate the check
to `TreeInfo`, and account for the wrapper `DocDef` node. I've
followed that pattern, but expressed my feelings about this approach
in a TODO comment.

Before this patch, the enclosed test failed with:

     error: not enough arguments for constructor SparkContext: (master: String, appName: String)SparkContext

1 files changed, 10 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/ast/TreeInfo.scala b/src/compiler/scala/tools/nsc/ast/TreeInfo.scala
index cbbb4c8ba8..1005cd1ccf 100644
--- a/src/compiler/scala/tools/nsc/ast/TreeInfo.scala
+++ b/src/compiler/scala/tools/nsc/ast/TreeInfo.scala
@@ -21,6 +21,11 @@ abstract class TreeInfo extends scala.reflect.internal.TreeInfo {
 
   import definitions.ThrowableClass
 
+  // TODO these overrides, and the slow trickle of bugs that they solve (e.g. SI-8479),
+  //      suggest that we should pursue an alternative design in which the DocDef nodes
+  //      are eliminated from the tree before typer, and instead are modelled as tree
+  //      attachments.
+
   /** Is tree legal as a member definition of an interface?
    */
   override def isInterfaceMember(tree: Tree): Boolean = tree match {
@@ -28,6 +33,11 @@ abstract class TreeInfo extends scala.reflect.internal.TreeInfo {
     case _ => super.isInterfaceMember(tree)
   }
 
+  override def isConstructorWithDefault(t: Tree) = t match {
+    case DocDef(_, definition) => isConstructorWithDefault(definition)
+    case _ => super.isConstructorWithDefault(t)
+  }
+
   /** Is tree a pure (i.e. non-side-effecting) definition?
    */
   override def isPureDef(tree: Tree): Boolean = tree match {