Fixed equality and string representation of xml attributes with null value

Prior to this patch <t a={ null: String }/> was not equal to <t/> and
it's string representation was "<t ></t>" instead of "<t></t>"

This includes changing MetaData.normalize() so that it doesn't reverse
the chain. On the downside, the iterate function in MetaData.normalize()
is not tail-recursive now.

Author: khernyo

src/library/scala/xml/Elem.scala

@@ -41,14 +41,16 @@ object Elem {
 class Elem(
   override val prefix: String,
   val label: String,
-  override val attributes: MetaData,
+  attributes1: MetaData,
   override val scope: NamespaceBinding,
   val child: Node*)
 extends Node with Serializable
 {
   final override def doCollectNamespaces = true
   final override def doTransform         = true
 
+  override val attributes = MetaData.normalize(attributes1, scope)
+
   if (prefix == "")
     throw new IllegalArgumentException("prefix of zero length, use null instead")
 

src/library/scala/xml/MetaData.scala

@@ -38,8 +38,8 @@ object MetaData {
     def iterate(md: MetaData, normalized_attribs: MetaData, set: Set[String]): MetaData = {
       lazy val key = getUniversalKey(md, scope)
       if (md eq Null) normalized_attribs
-      else if (set(key)) iterate(md.next, normalized_attribs, set)
-      else iterate(md.next, md copy normalized_attribs, set + key)
+      else if ((md.value eq null) || set(key)) iterate(md.next, normalized_attribs, set)
+      else md copy iterate(md.next, normalized_attribs, set + key)
     }
     iterate(attribs, Null, Set())
   }

src/library/scala/xml/UnprefixedAttribute.scala

@@ -22,7 +22,7 @@ extends Attribute
   final val pre = null
   val next = if (value ne null) next1 else next1.remove(key)
 
-  /** same as this(key, Text(value), next) */
+  /** same as this(key, Text(value), next), or no attribute if value is null */
   def this(key: String, value: String, next: MetaData) =
     this(key, if (value ne null) Text(value) else null: NodeSeq, next)
 

src/library/scala/xml/Utility.scala

@@ -61,7 +61,7 @@ object Utility extends AnyRef with parsing.TokenTests {
     val key = md.key
     val smaller = sort(md.filter { m => m.key < key })
     val greater = sort(md.filter { m => m.key > key })
-    smaller.append( Null ).append(md.copy ( greater ))
+    smaller.copy(md.copy ( greater ))
   }
 
   /** Return the node with its attribute list sorted alphabetically

test/files/jvm/xml03syntax.check

@@ -23,4 +23,4 @@ true
 4
 
 node=<elem key="<b>hello</b>"></elem>, key=Some(<b>hello</b>)
-node=<elem ></elem>, key=None
+node=<elem></elem>, key=None

test/files/run/xml-attribute.scala

@@ -0,0 +1,14 @@
+import xml.Node
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val noAttr = <t/>
+    val attrNull = <t a={ null: String }/>
+    val attrNone = <t a={ None: Option[Seq[Node]] }/>
+    assert(noAttr == attrNull)
+    assert(noAttr == attrNone)
+    assert(noAttr.toString() == "<t></t>")
+    assert(attrNull.toString() == "<t></t>")
+    assert(attrNone.toString() == "<t></t>")
+  }
+}