Document the fix and add the test

Author: Linyxus

compiler/src/dotty/tools/dotc/core/Annotations.scala

@@ -64,9 +64,13 @@ object Annotations {
             else
               val tp1 = tm(tree.tpe)
               foldOver(if tp1 frozen_=:= tree.tpe then x else tp1, tree)
-        val diff = findDiff(NoType, args)
+        val diff = if symbol.isRetainsLike then defn.AnyType else findDiff(NoType, args)
+          // If this is a @retains annotation, the diff check seems to be unreliable.
+          // We work around this issue by always mapping @retains annotations. See #20035.
+          //
+          // FIXME: investigate why the diff check is incorrect for @retains
         if tm.isRange(diff) then EmptyAnnotation
-        else if diff.exists || symbol.isRetainsLike then derivedAnnotation(tm.mapOver(tree))
+        else if diff.exists then derivedAnnotation(tm.mapOver(tree))
         else this
 
     /** Does this annotation refer to a parameter of `tl`? */
@@ -79,7 +83,10 @@ object Annotations {
           case _ => false
         case ref: This => ref.tpe match
           case TermParamRef(tl1, _) => tl eq tl1
-          case AnnotatedType(tp1, annot1) => annot1.refersToParamOf(tl)
+          case AnnotatedType(tp1, annot1) =>
+            // The type of captured elements in @retains annotation could themself
+            // refer to term parameters.  See #20035.
+            annot1.refersToParamOf(tl)
           case _ => false
         case _ => false
 

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

@@ -0,0 +1,11 @@
+import language.experimental.captureChecking
+
+trait Seq[+A]:
+  def zipAll[A1 >: A, B](that: Seq[B]^, thisElem: A1, thatElem: B): Seq[(A1, B)]^{this, that}
+  def map[B](f: A => B): Seq[B]^{this, f}
+
+def zipAllOption[X](left: Seq[X], right: Seq[X]) =
+  left.map(Option(_)).zipAll(right.map(Option(_)), None, None)
+
+def fillRow[T](headRow: Seq[T], tailRow: Seq[T]) =
+  val paddedZip = zipAllOption(headRow, tailRow)