Bring back RefiningVar

We might want to treat it specially since a RefiningVar should ideally be closed for
further additions when the constructor has been analyzed.

Author: odersky

compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -634,6 +634,13 @@ object CaptureSet:
     override def toString = s"Var$id$elems"
   end Var
 
+  /** Variables that represent refinements of class parameters can have the universal
+   *  capture set, since they represent only what is the result of the constructor.
+   *  Test case: Without that tweak, logger.scala would not compile.
+   */
+  class RefiningVar(owner: Symbol)(using Context) extends Var(owner):
+    override def disallowRootCapability(handler: () => Context ?=> Unit)(using Context) = this
+
   /** A variable that is derived from some other variable via a map or filter. */
   abstract class DerivedVar(owner: Symbol, initialElems: Refs)(using @constructorOnly ctx: Context)
   extends Var(owner, initialElems):

compiler/src/dotty/tools/dotc/cc/Setup.scala

@@ -193,10 +193,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
                   val getterType =
                     mapInferred(refine = false)(tp.memberInfo(getter)).strippedDealias
                   RefinedType(core, getter.name,
-                      CapturingType(getterType,
-                      new CaptureSet.Var(ctx.owner):
-                        override def disallowRootCapability(handler: () => Context ?=> Unit)(using Context) = this
-                      ))
+                      CapturingType(getterType, new CaptureSet.RefiningVar(ctx.owner)))
                     .showing(i"add capture refinement $tp --> $result", capt)
                 else
                   core