Check This references in refersToParamOf (#20005)

fixes #19990

See the
[comment](#19990 (comment))
of that issue for an explanation of this issue.

Author: odersky

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

@@ -72,12 +72,11 @@ object Annotations {
     def refersToParamOf(tl: TermLambda)(using Context): Boolean =
       val args = arguments
       if args.isEmpty then false
-      else tree.existsSubTree {
-        case id: Ident => id.tpe.stripped match
+      else tree.existsSubTree:
+        case id: (Ident | This) => id.tpe.stripped match
           case TermParamRef(tl1, _) => tl eq tl1
           case _ => false
         case _ => false
-      }
 
     /** A string representation of the annotation. Overridden in BodyAnnotation.
      */

tests/pos-custom-args/captures/i19990.scala

@@ -0,0 +1,13 @@
+import language.experimental.captureChecking
+
+trait Iterable[T] { self: Iterable[T]^ =>
+  def map[U](f: T => U): Iterable[U]^{this, f}
+}
+
+object Test {
+  def indentLines(level: Int, lines: Iterable[String]) =
+    lines.map(line => line.split("\n").map("  " + _).mkString("\n"))
+
+  def indentErrorMessages(messages: Iterable[String]) =
+    indentLines(1, messages)
+}