fix(#16458): regression in xml syntax parsing

compiler/src/dotty/tools/dotc/parsing/xml/MarkupParserCommon.scala

@@ -202,6 +202,14 @@ private[dotty] trait MarkupParserCommon {
   /** skip optional space S? */
   def xSpaceOpt(): Unit = while (isSpace(ch) && !eof) nextch()
 
+  /** skip optional space S? and return the number of consumed characters */
+  def xSpaceOptN(): Int =
+    var i = 0
+    while (isSpace(ch) && !eof) do
+      nextch()
+      i += 1
+    i
+
   /** scan [3] S ::= (#x20 | #x9 | #xD | #xA)+ */
   def xSpace(): Unit =
     if (isSpace(ch)) { nextch(); xSpaceOpt() }

compiler/src/dotty/tools/dotc/parsing/xml/MarkupParsers.scala

@@ -371,10 +371,17 @@ object MarkupParsers {
         // parse more XML?
         if (charComingAfter(xSpaceOpt()) == '<') {
           while {
-            xSpaceOpt()
-            nextch()
-            ts.append(element)
-            charComingAfter(xSpaceOpt()) == '<'
+            if xSpaceOptN() == 0 then
+              nextch()
+              if content_LT(ts) then // Is `</>` valid xml?
+                xToken("/>")
+              charComingAfter(xSpaceOpt()) == '<'
+            else
+              // this is surely not a special node as any special node 
+              // should start with `<{special symbol}` without space.
+              nextch()
+              ts.append(element)
+              charComingAfter(xSpaceOpt()) == '<'
           } do ()
           handle.makeXMLseq(Span(start, curOffset, start), ts)
         }

tests/pos/i16458.scala

@@ -0,0 +1,37 @@
+def x = <div>FooBar</div><!-- /.modal-content -->
+
+package scala.xml {
+  type MetaData = AnyRef
+
+  trait NamespaceBinding
+  object TopScope extends NamespaceBinding
+  object Null
+  abstract class Node {
+    def label: String
+    def child: Seq[Node]
+    override def toString = label + child.mkString
+  }
+  class Comment(commentText: String) extends Node{
+    def label = commentText
+    def child = Nil
+  }
+  class Elem(prefix: String, val label: String, attributes1: MetaData, scope: NamespaceBinding, minimizeEmpty: Boolean, val child: Node*) extends Node
+  class NodeBuffer extends Seq[Node] {
+    val nodes = scala.collection.mutable.ArrayBuffer.empty[Node]
+    def &+(o: Any): NodeBuffer =
+      o match {
+        case n: Node => nodes.addOne(n) ; this
+        case t: Text => nodes.addOne(Atom(t)) ; this
+      }
+    // Members declared in scala.collection.IterableOnce
+    def iterator: Iterator[scala.xml.Node] = nodes.iterator
+    // Members declared in scala.collection.SeqOps
+    def apply(i: Int): scala.xml.Node = nodes(i)
+    def length: Int = nodes.length
+  }
+  case class Text(text: String)
+  case class Atom(t: Text) extends Node {
+    def label = t.text
+    def child = Nil
+  }
+}