SI-6939 Fix namespace binding (xmlns) not overriding outer binding

Given a nested XML literal to the compiler Elem instance is generated with
namespace binding of the inner element copying that of the outer element:

    val foo = <x:foo xmlns:x="http://foo.com/">
      <x:bar xmlns:x="http://bar.com/"><x:baz/></x:bar></x:foo>

With the above example, `foo.child.head.scope.toString` returns

    " xmlns:x="http://bar.com/" xmlns:x="http://foo.com/""

This is incorrect since the outer xmls:x should be overridden by the inner binding.

XML library also parses XML document in a similar manner:

    val foo2 = scala.xml.XML.loadString("""<x:foo xmlns:x="http://foo.com/"><x:bar xmlns:x="http://bar.com/"><x:baz/></x:bar></x:foo>""")

Despite this erroneous behavior, since the structure of NamespaceBinding class
is designed to be singly-linked list, the stacking of namespace bindings allows
constant-time creation with simple implementation.
Since the inner namespace binding comes before the outer one, query methods like `getURI` method behave correctly.

Because this bug is manifested when Elem is turned back into XML string, it could be fixed by shadowing the redefined namespace binding right when
buildString is called. With this change `foo.child.head.scope.toString` now returns:

    " xmlns:x="http://bar.com/""

Author: eed3si9n

src/library/scala/xml/NamespaceBinding.scala

@@ -38,6 +38,22 @@ case class NamespaceBinding(prefix: String, uri: String, parent: NamespaceBindin
 
   override def toString(): String = sbToString(buildString(_, TopScope))
 
+  private def shadowRedefined: NamespaceBinding = shadowRedefined(TopScope)
+
+  private def shadowRedefined(stop: NamespaceBinding): NamespaceBinding = {
+    def prefixList(x: NamespaceBinding): List[String] =
+      if ((x == null) || (x eq stop)) Nil
+      else x.prefix :: prefixList(x.parent)
+    def fromPrefixList(l: List[String]): NamespaceBinding = l match {
+      case Nil => stop
+      case x :: xs => new NamespaceBinding(x, this.getURI(x), fromPrefixList(xs))
+    }
+    val ps0 = prefixList(this).reverse
+    val ps = ps0.distinct
+    if (ps.size == ps0.size) this
+    else fromPrefixList(ps)
+  }
+
   override def canEqual(other: Any) = other match {
     case _: NamespaceBinding  => true
     case _                    => false
@@ -53,12 +69,16 @@ case class NamespaceBinding(prefix: String, uri: String, parent: NamespaceBindin
   def buildString(stop: NamespaceBinding): String = sbToString(buildString(_, stop))
 
   def buildString(sb: StringBuilder, stop: NamespaceBinding) {
-    if (this eq stop) return    // contains?
+    shadowRedefined(stop).doBuildString(sb, stop)
+  }
+
+  private def doBuildString(sb: StringBuilder, stop: NamespaceBinding) {
+    if ((this == null) || (this eq stop)) return    // contains?
 
     val s = " xmlns%s=\"%s\"".format(
       (if (prefix != null) ":" + prefix else ""),
       (if (uri != null) uri else "")
     )
-    parent.buildString(sb append s, stop) // copy(ignore)
+    parent.doBuildString(sb append s, stop) // copy(ignore)
   }
 }

test/files/run/t6939.scala

@@ -0,0 +1,13 @@
+object Test extends App {
+  val foo = <x:foo xmlns:x="http://foo.com/"><x:bar xmlns:x="http://bar.com/"><x:baz/></x:bar></x:foo>
+  assert(foo.child.head.scope.toString == """ xmlns:x="http://bar.com/"""")
+
+  val fooDefault = <foo xmlns="http://foo.com/"><bar xmlns="http://bar.com/"><baz/></bar></foo>
+  assert(fooDefault.child.head.scope.toString == """ xmlns="http://bar.com/"""")
+
+  val foo2 = scala.xml.XML.loadString("""<x:foo xmlns:x="http://foo.com/"><x:bar xmlns:x="http://bar.com/"><x:baz/></x:bar></x:foo>""")
+  assert(foo2.child.head.scope.toString == """ xmlns:x="http://bar.com/"""")
+
+  val foo2Default = scala.xml.XML.loadString("""<foo xmlns="http://foo.com/"><bar xmlns="http://bar.com/"><baz/></bar></foo>""")
+  assert(foo2Default.child.head.scope.toString == """ xmlns="http://bar.com/"""")
+}