Fix conversion of this.fld capture refs under separate compilation (#20238)

Author: odersky

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

@@ -135,5 +135,14 @@ class TypeUtils {
       case _ =>
         val cls = self.underlyingClassRef(refinementOK = false).typeSymbol
         cls.isTransparentClass && (!traitOnly || cls.is(Trait))
+
+    /** Is this type the ThisType of class `cls?`. Note we can't use `self eq cls.thisType` for this,
+     *  since ThisTypes take TermRef parameters and semantically equal TermRefs could have different
+     *  forms (for instance one could use as a prefix the ThisType of an enclosing static module or package,
+     *  and the other could select it from something further out)
+     */
+    def isThisTypeOf(cls: Symbol)(using Context) = self match
+      case self: Types.ThisType => self.cls == cls
+      case _ => false
   }
 }

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

@@ -2991,7 +2991,7 @@ object Types extends TypeUtils {
      */
     override def isTrackableRef(using Context) =
       ((prefix eq NoPrefix)
-      || symbol.is(ParamAccessor) && (prefix eq symbol.owner.thisType)
+      || symbol.is(ParamAccessor) && prefix.isThisTypeOf(symbol.owner)
       || isRootCapability
       ) && !symbol.isOneOf(UnstableValueFlags)
 

tests/pos-custom-args/captures/captureRef-separate-compilation/Future_1.scala

@@ -0,0 +1,5 @@
+class Future[T]
+object Future:
+  class Collector[T](fs: (Future[T]^)*)
+  class MutableCollector[T](val futures: (Future[T]^)*) extends Collector[T](futures*):
+    def add(future: Future[T]^{futures*}) = ???

tests/pos-custom-args/captures/captureRef-separate-compilation/test_2.scala

@@ -0,0 +1,5 @@
+def test =
+  val collector: Future.MutableCollector[Int] = Future.MutableCollector()
+  collector.add(???)
+
+