Fix(#16459) xml parse regression (#19531)

close #16459 

The parser could not parse `if expr` that contains single-quoted text(s)
inside XML literal with newline(s) because `followedByToken`, which is
used to detect `do` or `then` token after `if`, unintentionally consumed
XMLSTART symbol, which prevented the parser from delegating parse to XML
parser.

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -906,6 +906,7 @@ object Parsers {
       var braces = 0
       while (true) {
         val token = lookahead.token
+        if (query != LARROW && token == XMLSTART) return false
         if (braces == 0) {
           if (token == query) return true
           if (stopScanTokens.contains(token) || lookahead.isNestedEnd) return false
@@ -927,6 +928,7 @@ object Parsers {
       lookahead.nextToken()
       while (parens != 0 && lookahead.token != EOF) {
         val token = lookahead.token
+        if (token == XMLSTART) return true
         if (token == LPAREN) parens += 1
         else if (token == RPAREN) parens -= 1
         lookahead.nextToken()

tests/run/i16459.scala

@@ -0,0 +1,107 @@
+object Test {
+  import scala.xml.*
+  def main(args: Array[String]): Unit = {
+
+  val xml = if(true) {
+    <script type="text/javascript">
+      'location.reload()'
+      'foo bar'
+     </script>
+  } else <div>empty</div>
+
+  assert(
+    xml match
+      case elm: Elem if
+        elm.label == "script"
+        && elm.child.length == 1
+        && elm.child(0) == Atom(Text("\n      'location.reload()'\n      'foo bar'\n     "))
+          => true
+      case _ => false
+      ,
+      xml
+    )
+  // Scala 3 syntax
+  val auxiliary0 = if true then {
+    <script type="text/javascript">
+      'location.reload()'
+      'foo bar'
+     </script>
+  } else <div>empty</div>
+
+  val auxiliary1 = if true then
+    <script type="text/javascript">
+      'location.reload()'
+      'foo bar'
+     </script>
+  else <div>empty</div>
+  
+  val auxiliary2 = if true then <div>A</div>else <div>B</div>
+
+  // Note:
+  // This does not pass in Scala 2.12.18 and 2.13.12
+  // due to "Sequence argument type annotation `: _*` cannot be used here:"
+  val auxiliary3 = if(true) <div>A</div>else <div>B</div>
+  
+  // Note: This passes in Scala  2.12.18 and 2.13.12 too.
+  val auxiliary4 = if(true) <div attr="...">A</div>else <div attr="...">B</div>
+
+  // Pattern match without guard.
+  // Note: This passes in Scala  2.12.18 and 2.13.12 too.
+  val auxiliary5 = for (case _ @ <div>empty</div> <- Seq(xml)) yield ()
+  // Note: These pass in Scala  2.12.18 and 2.13.12.
+  val auxiliary6 = for (case _ @ <div>empty</div><- Seq(xml)) yield ()
+  val auxiliary7 = for (case _ @ <div>empty</div><-Seq(xml)) yield ()
+  // Pattern match with if guard.
+  // Note: This passes in Scala  2.12.18 and 2.13.12 too.
+  val auxiliary8 = for (case _ @ <foo>FooBar</foo> <- Seq(xml) if true)
+    yield ()
+  // Note: These pass in Scala  2.12.18 and 2.13.12.
+  val auxiliary9 = for (case _ @ <foo>FooBar</foo><- Seq(xml) if true)
+    yield ()
+  val auxiliary10 = for (case _ @ <foo>FooBar</foo><-Seq(xml) if true)
+    yield ()
+  
+  }
+
+}
+
+package scala.xml {
+  type MetaData = AnyRef
+
+  class UnprefixedAttribute(
+    val key: String,
+    val value: Text,
+    next1: MetaData
+  ) extends MetaData
+
+  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 Elem(prefix: String, val label: String, attributes1: MetaData, scope: NamespaceBinding, minimizeEmpty: Boolean, val child: Node*) extends Node
+  object Elem {
+    def unapply(e:Elem):Option[(String,String,Any,Text,Any)] = Some(("dummy","dummy",null,null,null))
+  }
+  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
+  }
+}