Change method signatures to be explicitly nullable where applicable

build.sbt

@@ -110,6 +110,16 @@ lazy val xml = crossProject(JSPlatform, JVMPlatform, NativePlatform)
         ProblemFilters.exclude[IncompatibleSignatureProblem]("scala.xml.Null.get"),
         ProblemFilters.exclude[IncompatibleSignatureProblem]("scala.xml.Node.attribute"),
 
+        // Nullable types result in a raw Seq, is fixed in 3.8
+        ProblemFilters.exclude[IncompatibleSignatureProblem]("scala.xml.Attribute.unapply"),
+        ProblemFilters.exclude[IncompatibleSignatureProblem]("scala.xml.Attribute.apply"),
+        ProblemFilters.exclude[IncompatibleSignatureProblem]("scala.xml.Attribute.value"),
+        ProblemFilters.exclude[IncompatibleSignatureProblem]("scala.xml.MetaData.value"),
+        ProblemFilters.exclude[IncompatibleSignatureProblem]("scala.xml.PrefixedAttribute.unapply"),
+        ProblemFilters.exclude[IncompatibleSignatureProblem]("scala.xml.PrefixedAttribute.this"),
+        ProblemFilters.exclude[IncompatibleSignatureProblem]("scala.xml.UnprefixedAttribute.unapply"),
+        ProblemFilters.exclude[IncompatibleSignatureProblem]("scala.xml.UnprefixedAttribute.this"),
+
         // trait Attribute now extends trait ScalaVersionSpecificMetaData to ensure the previous signatures
         // with return type `collection.Seq` remain valid.
         // (trait Attribute extends MetaData, but that parent is not present in bytecode because it's a class.)

jvm/src/test/scala/scala/xml/XMLTest.scala

@@ -8,6 +8,7 @@ import java.net.URL
 import scala.xml.dtd.{DocType, PublicID}
 import scala.xml.parsing.ConstructingParser
 import scala.xml.Utility.sort
+import xml.Nullables._
 
 object XMLTestJVM {
   val e: MetaData = Null //Node.NoAttributes
@@ -17,7 +18,7 @@ object XMLTestJVM {
 class XMLTestJVM {
   import XMLTestJVM.{e, sc}
 
-  def Elem(prefix: String, label: String, attributes: MetaData, scope: NamespaceBinding, child: Node*): Elem =
+  def Elem(prefix: Nullable[String], label: String, attributes: MetaData, scope: NamespaceBinding, child: Node*): Elem =
     scala.xml.Elem.apply(prefix, label, attributes, scope, minimizeEmpty = true, child: _*)
 
   lazy val parsedxml1: Elem = XML.loadString("<hello><world/></hello>")
@@ -385,9 +386,9 @@ class XMLTestJVM {
 
   @UnitTest
   def t5052(): Unit = {
-    assertTrue(<elem attr={ null: String }/> xml_== <elem/>)
+    assertTrue(<elem attr={ null: Nullable[String] }/> xml_== <elem/>)
     assertTrue(<elem attr={ None }/> xml_== <elem/>)
-    assertTrue(<elem/> xml_== <elem attr={ null: String }/>)
+    assertTrue(<elem/> xml_== <elem attr={ null: Nullable[String] }/>)
     assertTrue(<elem/> xml_== <elem attr={ None }/>)
   }
 
@@ -400,9 +401,9 @@ class XMLTestJVM {
     assertHonorsIterableContract(<a y={ None }/>.attributes)
     assertHonorsIterableContract(<a y={ None } x=""/>.attributes)
     assertHonorsIterableContract(<a a="" y={ None }/>.attributes)
-    assertHonorsIterableContract(<a y={ null: String }/>.attributes)
-    assertHonorsIterableContract(<a y={ null: String } x=""/>.attributes)
-    assertHonorsIterableContract(<a a="" y={ null: String }/>.attributes)
+    assertHonorsIterableContract(<a y={ null: Nullable[String] }/>.attributes)
+    assertHonorsIterableContract(<a y={ null: Nullable[String] } x=""/>.attributes)
+    assertHonorsIterableContract(<a a="" y={ null: Nullable[String] }/>.attributes)
   }
 
   @UnitTest
@@ -459,18 +460,18 @@ class XMLTestJVM {
   @UnitTest
   def attributes(): Unit = {
     val noAttr: Elem = <t/>
-    val attrNull: Elem = <t a={ null: String }/>
+    val attrNull: Elem = <t a={ null: Nullable[String] }/>
     val attrNone: Elem = <t a={ None: Option[Seq[Node]] }/>
-    val preAttrNull: Elem = <t p:a={ null: String }/>
+    val preAttrNull: Elem = <t p:a={ null: Nullable[String] }/>
     val preAttrNone: Elem = <t p:a={ None: Option[Seq[Node]] }/>
     assertEquals(noAttr, attrNull)
     assertEquals(noAttr, attrNone)
     assertEquals(noAttr, preAttrNull)
     assertEquals(noAttr, preAttrNone)
 
     val xml1: Elem = <t b="1" d="2"/>
-    val xml2: Elem = <t a={ null: String } p:a={ null: String } b="1" c={ null: String } d="2"/>
-    val xml3: Elem = <t b="1" c={ null: String } d="2" a={ null: String } p:a={ null: String }/>
+    val xml2: Elem = <t a={ null: Nullable[String] } p:a={ null: Nullable[String] } b="1" c={ null: Nullable[String] } d="2"/>
+    val xml3: Elem = <t b="1" c={ null: Nullable[String] } d="2" a={ null: Nullable[String] } p:a={ null: Nullable[String] }/>
     assertEquals(xml1, xml2)
     assertEquals(xml1, xml3)
 
@@ -759,8 +760,8 @@ class XMLTestJVM {
   def documentBaseURI(): Unit = {
     val url: URL = resourceUrl("site")
     // XMLLoader returns the document's baseURI:
-    assert(XML.withSAXParser(xercesInternal.newSAXParser).loadDocument(url).baseURI.endsWith("/test-classes/scala/xml/site.xml"))
-    assert(XML.withSAXParser(xercesExternal.newSAXParser).loadDocument(url).baseURI.endsWith("/test-classes/scala/xml/site.xml"))
+    assert(XML.withSAXParser(xercesInternal.newSAXParser).loadDocument(url).baseURI.nn.endsWith("/test-classes/scala/xml/site.xml"))
+    assert(XML.withSAXParser(xercesExternal.newSAXParser).loadDocument(url).baseURI.nn.endsWith("/test-classes/scala/xml/site.xml"))
     // ConstructingParser does not return it of course: since it uses scala.io.Source it has no idea where is the XML coming from:
     assertNull(ConstructingParser.fromSource(scala.io.Source.fromURI(url.toURI), preserveWS = false).document().baseURI)
   }

shared/src/main/scala-2.13+/scala/xml/ScalaVersionSpecific.scala

@@ -12,6 +12,8 @@
 
 package scala.xml
 
+
+import xml.Nullables._
 import scala.collection.immutable.StrictOptimizedSeqOps
 import scala.collection.{View, SeqOps, IterableOnce, immutable, mutable}
 import scala.collection.BuildFrom
@@ -65,11 +67,11 @@ private[xml] trait ScalaVersionSpecificNode { self: Node =>
 }
 
 private[xml] trait ScalaVersionSpecificMetaData { self: MetaData =>
-  def apply(key: String): scala.collection.Seq[Node]
-  def apply(namespace_uri: String, owner: Node, key: String): scala.collection.Seq[Node]
-  def apply(namespace_uri: String, scp: NamespaceBinding, k: String): scala.collection.Seq[Node]
+  def apply(key: String): Nullable[scala.collection.Seq[Node]]
+  def apply(namespace_uri: String, owner: Node, key: Nullable[String]): Nullable[scala.collection.Seq[Node]]
+  def apply(namespace_uri: Nullable[String], scp: NamespaceBinding, k: Nullable[String]): Nullable[scala.collection.Seq[Node]]
 
-  def value: scala.collection.Seq[Node]
+  def value: Nullable[scala.collection.Seq[Node]]
 }
 
 private[xml] trait ScalaVersionSpecificTextBuffer { self: TextBuffer =>

shared/src/main/scala-2/scala/xml/Nullables.scala

@@ -0,0 +1,21 @@
+/*
+ * Scala (https://www.scala-lang.org)
+ *
+ * Copyright EPFL and Lightbend, Inc.
+ *
+ * Licensed under Apache License 2.0
+ * (http://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package scala.xml
+object Nullables {
+    type Nullable[T] = T
+    implicit class NonNullOps[T](private val x: T) extends AnyVal {
+        def nn: T = {
+            x
+        }
+    }
+}

shared/src/main/scala-3/scala/xml/Nullables.scala

@@ -0,0 +1,16 @@
+/*
+ * Scala (https://www.scala-lang.org)
+ *
+ * Copyright EPFL and Lightbend, Inc.
+ *
+ * Licensed under Apache License 2.0
+ * (http://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package scala.xml
+object Nullables {
+  type Nullable[T] = T | Null
+}

shared/src/main/scala/scala/xml/Attribute.scala

@@ -13,6 +13,7 @@
 package scala
 package xml
 
+import xml.Nullables._
 import scala.collection.Seq
 
 /**
@@ -22,21 +23,21 @@ import scala.collection.Seq
  *  @author  Burak Emir
  */
 object Attribute {
-  def unapply(x: Attribute): Option[(String, Seq[Node], MetaData)] = x match {
+  def unapply(x: Attribute): Option[(Nullable[String], Nullable[Seq[Node]], MetaData)] = x match {
     case PrefixedAttribute(_, key, value, next) => Some((key, value, next))
     case UnprefixedAttribute(key, value, next)  => Some((key, value, next))
     case _                                      => None
   }
 
   /** Convenience functions which choose Un/Prefixedness appropriately */
-  def apply(key: String, value: Seq[Node], next: MetaData): Attribute =
+  def apply(key: Nullable[String], value: Seq[Node], next: MetaData): Attribute =
     new UnprefixedAttribute(key, value, next)
 
-  def apply(pre: String, key: String, value: String, next: MetaData): Attribute =
+  def apply(pre: Nullable[String], key: String, value: String, next: MetaData): Attribute =
     if (pre == null || pre == "") new UnprefixedAttribute(key, value, next)
     else new PrefixedAttribute(pre, key, value, next)
 
-  def apply(pre: String, key: String, value: Seq[Node], next: MetaData): Attribute =
+  def apply(pre: Nullable[String], key: String, value: Seq[Node], next: MetaData): Attribute =
     if (pre == null || pre == "") new UnprefixedAttribute(key, value, next)
     else new PrefixedAttribute(pre, key, value, next)
 
@@ -54,29 +55,29 @@ object Attribute {
  *  @author  Burak Emir
  */
 trait Attribute extends MetaData with ScalaVersionSpecificMetaData {
-  def pre: String // will be null if unprefixed
-  override val key: String
-  override val value: ScalaVersionSpecific.SeqOfNode
+  def pre: Nullable[String] // will be null if unprefixed
+  override val key: Nullable[String]
+  override val value: Nullable[ScalaVersionSpecific.SeqOfNode]
   override val next: MetaData
 
-  override def apply(key: String): ScalaVersionSpecific.SeqOfNode
-  override def apply(namespace: String, scope: NamespaceBinding, key: String): ScalaVersionSpecific.SeqOfNode
+  override def apply(key: String): Nullable[ScalaVersionSpecific.SeqOfNode]
+  override def apply(namespace: Nullable[String], scope: NamespaceBinding, key: Nullable[String]): Nullable[ScalaVersionSpecific.SeqOfNode]
   override def copy(next: MetaData): Attribute
 
-  override def remove(key: String): MetaData =
+  override def remove(key: Nullable[String]): MetaData =
     if (!isPrefixed && this.key == key) next
     else copy(next.remove(key))
 
-  override def remove(namespace: String, scope: NamespaceBinding, key: String): MetaData =
+  override def remove(namespace: Nullable[String], scope: NamespaceBinding, key: String): MetaData =
     if (this.key == key && scope.getURI(pre) == namespace) next
     else copy(next.remove(namespace, scope, key))
 
   override def isPrefixed: Boolean = pre != null
 
-  override def getNamespace(owner: Node): String
+  override def getNamespace(owner: Node): Nullable[String]
 
   override def wellformed(scope: NamespaceBinding): Boolean = {
-    val arg: String = if (isPrefixed) scope.getURI(pre) else null
+    val arg: Nullable[String] = if (isPrefixed) scope.getURI(pre) else null
     (next(arg, scope, key) == null) && next.wellformed(scope)
   }
 

shared/src/main/scala/scala/xml/Document.scala

@@ -14,6 +14,7 @@ package scala
 package xml
 
 import scala.collection.Seq
+import xml.Nullables._
 
 /**
  * A document information item (according to InfoSet spec). The comments
@@ -42,25 +43,25 @@ class Document extends NodeSeq with Serializable {
   var docElem: Node = _
 
   /** The dtd that comes with the document, if any */
-  var dtd: scala.xml.dtd.DTD = _
+  var dtd: Nullable[scala.xml.dtd.DTD] = _
 
   /**
    * An unordered set of notation information items, one for each notation
    *  declared in the DTD. If any notation is multiply declared, this property
    *  has no value.
    */
   def notations: Seq[scala.xml.dtd.NotationDecl] =
-    dtd.notations
+    dtd.nn.notations
 
   /**
    * An unordered set of unparsed entity information items, one for each
    *  unparsed entity declared in the DTD.
    */
   def unparsedEntities: Seq[scala.xml.dtd.EntityDecl] =
-    dtd.unparsedEntities
+    dtd.nn.unparsedEntities
 
   /** The base URI of the document entity. */
-  var baseURI: String = _
+  var baseURI: Nullable[String] = _
 
   /**
    * The name of the character encoding scheme in which the document entity

shared/src/main/scala/scala/xml/Elem.scala

@@ -14,6 +14,7 @@ package scala
 package xml
 
 import scala.collection.Seq
+import xml.Nullables._
 
 /**
  * This singleton object contains the `apply` and `unapplySeq` methods for
@@ -24,10 +25,10 @@ import scala.collection.Seq
 // Note: used by the Scala compiler.
 object Elem {
 
-  def apply(prefix: String, label: String, attributes: MetaData, scope: NamespaceBinding, minimizeEmpty: Boolean, child: Node*): Elem =
+  def apply(prefix: Nullable[String], label: Nullable[String], attributes: MetaData, scope: NamespaceBinding, minimizeEmpty: Boolean, child: Node*): Elem =
     new Elem(prefix, label, attributes, scope, minimizeEmpty, child: _*)
 
-  def unapplySeq(n: Node): Option[(String, String, MetaData, NamespaceBinding, ScalaVersionSpecific.SeqOfNode)] =
+  def unapplySeq(n: Node): Option[(Nullable[String], Nullable[String], MetaData, NamespaceBinding, ScalaVersionSpecific.SeqOfNode)] =
     n match {
       case _: SpecialNode | _: Group => None
       case _                         => Some((n.prefix, n.label, n.attributes, n.scope, n.child))
@@ -55,8 +56,8 @@ object Elem {
  */
 // Note: used by the Scala compiler.
 class Elem(
-  override val prefix: String,
-  override val label: String,
+  override val prefix: Nullable[String],
+  override val label: Nullable[String],
   attributes1: MetaData,
   override val scope: NamespaceBinding,
   val minimizeEmpty: Boolean,
@@ -98,8 +99,8 @@ class Elem(
    *  @return a new symbol with updated attributes
    */
   def copy(
-    prefix: String = this.prefix,
-    label: String = this.label,
+    prefix: Nullable[String] = this.prefix,
+    label: Nullable[String] = this.label,
     attributes: MetaData = this.attributes,
     scope: NamespaceBinding = this.scope,
     minimizeEmpty: Boolean = this.minimizeEmpty,