Fix #23194: Try to handle SkolemTypes in SingletonTypeTree during pickling (#23236)

Fix #23194

Author: Linyxus

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

@@ -716,8 +716,13 @@ class TreePickler(pickler: TastyPickler, attributes: Attributes) {
           if passesConditionForErroringBestEffortCode(tree.hasType) then pickleType(tree.tpe)
           else pickleErrorType()
         case SingletonTypeTree(ref) =>
-          writeByte(SINGLETONtpt)
-          pickleTree(ref)
+          val tp = ref.tpe
+          val tp1 = tp.deskolemized
+          if tp1 ne tp then
+            pickleType(tp1)
+          else
+            writeByte(SINGLETONtpt)
+            pickleTree(ref)
         case RefinedTypeTree(parent, refinements) =>
           if (refinements.isEmpty) pickleTree(parent)
           else {

tests/pos/i23194.scala

@@ -0,0 +1,14 @@
+class R[T] extends annotation.StaticAnnotation
+
+class A[T]:
+  val next: A[T] = null
+  val self: this.type = this
+  val selfnext: this.next.type = this.next
+  def f: (A[T] @R[this.type], A[T] @R[this.next.type]) = ???
+  def g: (A[T] @R[self.type], A[T] @R[selfnext.type]) = ???
+
+class Test:
+  def test =
+    val (a, b) = A[String]().f
+    val (a2, b2) = A[String]().g
+