Fix the pickling of This inside capture sets (#19797)

Fixes #19662

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

@@ -405,7 +405,14 @@ class TreePickler(pickler: TastyPickler, attributes: Attributes) {
               pickleType(tp)
           }
         case This(qual) =>
-          if (qual.isEmpty) pickleType(tree.tpe)
+          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

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -15,7 +15,7 @@ import util.SourcePosition
 import scala.util.control.NonFatal
 import scala.annotation.switch
 import config.{Config, Feature}
-import cc.{CapturingType, RetainingType, CaptureSet, ReachCapability, MaybeCapability, isBoxed, levelOwner, retainedElems}
+import cc.{CapturingType, RetainingType, CaptureSet, ReachCapability, MaybeCapability, isBoxed, levelOwner, retainedElems, isRetainsLike}
 
 class PlainPrinter(_ctx: Context) extends Printer {
 
@@ -60,7 +60,8 @@ class PlainPrinter(_ctx: Context) extends Printer {
         case OrType(tp1, tp2) =>
           homogenize(tp1) | homogenize(tp2)
         case AnnotatedType(parent, annot)
-        if !ctx.mode.is(Mode.Type) && annot.symbol == defn.UncheckedVarianceAnnot =>
+        if !ctx.mode.is(Mode.Type) && annot.symbol == defn.UncheckedVarianceAnnot
+           || annot.symbol.isRetainsLike =>
           homogenize(parent)
         case tp: SkolemType =>
           homogenize(tp.info)

tests/pos-custom-args/captures/i19662.scala

@@ -0,0 +1,9 @@
+// Modified from #19662 to make it independent of the cc stdlib.
+import language.experimental.captureChecking
+trait MySeq[+A] { self: MySeq[A]^ =>
+  def map[B](f: A => B): MySeq[B]^{this, f}
+}
+object MySeq:
+  def apply(x: Int): MySeq[Int] = ???
+class C:
+  private def foo = MySeq(5).map { i => i }