Fix #13626

Author: mbovel

compiler/src/dotty/tools/dotc/ast/Positioned.scala

@@ -23,6 +23,8 @@ abstract class Positioned(implicit @constructorOnly src: SourceFile) extends Src
 
   private var mySpan: Span = _
 
+  private var mySource: SourceFile = src
+
   /** A unique identifier in case -Yshow-tree-ids, or -Ydebug-tree-with-id
    *  is set, -1 otherwise.
    */
@@ -48,7 +50,8 @@ abstract class Positioned(implicit @constructorOnly src: SourceFile) extends Src
 
   span = envelope(src)
 
-  val source: SourceFile = src
+  def source: SourceFile = mySource
+
   def sourcePos(using Context): SourcePosition = source.atSpan(span)
 
   /** This positioned item, widened to `SrcPos`. Used to make clear we only need the
@@ -127,7 +130,7 @@ abstract class Positioned(implicit @constructorOnly src: SourceFile) extends Src
   def cloneIn(src: SourceFile): this.type = {
     val newpd: this.type = clone.asInstanceOf[this.type]
     newpd.allocateId()
-    // assert(newpd.uniqueId != 2208, s"source = $this, ${this.uniqueId}, ${this.span}")
+    newpd.mySource = src
     newpd
   }