Roll the isAlwaysEmpty optimisation into capturing

Author: Linyxus

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

@@ -2613,11 +2613,8 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       val refs2 = tp2.captureSet
       if subCaptures(refs2, refs1, frozen = true).isOK
         && tp1.isBoxedCapturing == tp2.isBoxedCapturing
-      then
-        if refs2.isAlwaysEmpty then parent1 & tp2
-        else (parent1 & tp2).capturing(refs2)
-      else
-        tp1.derivedCapturingType(parent1 & tp2, refs1)
+      then (parent1 & tp2).capturing(refs2)
+      else tp1.derivedCapturingType(parent1 & tp2, refs1)
     case tp1: AnnotatedType if !tp1.isRefining =>
       tp1.underlying & tp2
     case _ =>

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

@@ -1940,7 +1940,7 @@ object Types {
      *  the two capture sets are combined.
      */
     def capturing(cs: CaptureSet)(using Context): Type =
-      if cs.isConst && cs.subCaptures(captureSet, frozen = true).isOK then this
+      if cs.isAlwaysEmpty || cs.isConst && cs.subCaptures(captureSet, frozen = true).isOK then this
       else this match
         case CapturingType(parent, cs1) => parent.capturing(cs1 ++ cs)
         case _ => CapturingType(this, cs)