Fix i19859 (#19860)

Fixes #19859.

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -405,19 +405,21 @@ class TreePickler(pickler: TastyPickler, attributes: Attributes) {
               pickleType(tp)
           }
         case This(qual) =>
+          // This may be needed when pickling a `This` inside a capture set. See #19662 and #19859.
+          // In this case, we pickle the tree as null.asInstanceOf[tree.tpe].
+          // Since the pickled tree is not the same as the input, special handling is needed
+          // in the tree printer when testing the pickler. See [[PlainPrinter#homogenize]].
+          inline def pickleCapturedThis =
+            pickleTree(Literal(Constant(null)).cast(tree.tpe).withSpan(tree.span))
           if (qual.isEmpty)
             if tree.tpe.isSingleton then pickleType(tree.tpe)
-            else
-              // This may happen when pickling a `This` inside a capture set. See #19662.
-              // In this case, we pickle the tree as null.asInstanceOf[tree.tpe].
-              // Since the pickled tree is not the same as the input, special handling is needed
-              // in the tree printer when testing the pickler. See [[PlainPrinter#homogenize]].
-              pickleTree(Literal(Constant(null)).cast(tree.tpe).withSpan(tree.span))
-          else {
-            writeByte(QUALTHIS)
-            val ThisType(tref) = tree.tpe: @unchecked
-            pickleTree(qual.withType(tref))
-          }
+            else pickleCapturedThis
+          else
+            tree.tpe match
+              case ThisType(tref) =>
+                writeByte(QUALTHIS)
+                pickleTree(qual.withType(tref))
+              case _ => pickleCapturedThis
         case Select(qual, name) =>
           name match {
             case OuterSelectName(_, levels) =>