Simplify box adaptation (#16845)

This PR simplifies the box adaptation implementation.

1. Instead of adding a `destructCapturingType` function, use the
existing methods to analyze and decompose the input type. It should work
well with annotated types.
2. Add explanation on the implementation.

Author: odersky

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

@@ -656,7 +656,7 @@ class CheckCaptures extends Recheck, SymTransformer:
           expected
 
     /** Adapt `actual` type to `expected` type by inserting boxing and unboxing conversions
-     * 
+     *
      *  @param alwaysConst  always make capture set variables constant after adaptation
      */
     def adaptBoxed(actual: Type, expected: Type, pos: SrcPos, alwaysConst: Boolean = false)(using Context): Type =
@@ -721,61 +721,52 @@ class CheckCaptures extends Recheck, SymTransformer:
         val arrow = if covariant then "~~>" else "<~~"
         i"adapting $actual $arrow $expected"
 
-      /** Destruct a capturing type `tp` to a tuple (cs, tp0, boxed),
-       *  where `tp0` is not a capturing type.
-       *
-       *  If `tp` is a nested capturing type, the return tuple always represents
-       *  the innermost capturing type. The outer capture annotations can be
-       *  reconstructed with the returned function.
-       */
-      def destructCapturingType(tp: Type, reconstruct: Type => Type = x => x): ((Type, CaptureSet, Boolean), Type => Type) =
-        tp.dealias match
-          case tp @ CapturingType(parent, cs) =>
-            if parent.dealias.isCapturingType then
-              destructCapturingType(parent, res => reconstruct(tp.derivedCapturingType(res, cs)))
-            else
-              ((parent, cs, tp.isBoxed), reconstruct)
-          case actual =>
-            val res = if tp.isFromJavaObject then tp else actual
-            ((res, CaptureSet(), false), reconstruct)
-
       def adapt(actual: Type, expected: Type, covariant: Boolean): Type = trace(adaptInfo(actual, expected, covariant), recheckr, show = true) {
         if expected.isInstanceOf[WildcardType] then actual
         else
-          val ((parent, cs, actualIsBoxed), recon) = destructCapturingType(actual)
-
-          val needsAdaptation = actualIsBoxed != expected.isBoxedCapturing
-          val insertBox = needsAdaptation && covariant != actualIsBoxed
-
-          val (parent1, cs1) = parent match {
+          // Decompose the actual type into the inner shape type, the capture set and the box status
+          val styp = if actual.isFromJavaObject then actual else actual.stripCapturing
+          val cs = actual.captureSet
+          val boxed = actual.isBoxedCapturing
+
+          // A box/unbox should be inserted, if the actual box status mismatches with the expectation
+          val needsAdaptation = boxed != expected.isBoxedCapturing
+          // Whether to insert a box or an unbox?
+          val insertBox = needsAdaptation && covariant != boxed
+
+          // Adapt the inner shape type: get the adapted shape type, and the capture set leaked during adaptation
+          val (styp1, leaked) = styp match {
             case actual @ AppliedType(tycon, args) if defn.isNonRefinedFunction(actual) =>
-              val (parent1, leaked) = adaptFun(parent, args.init, args.last, expected, covariant, insertBox,
+              adaptFun(actual, args.init, args.last, expected, covariant, insertBox,
                   (aargs1, ares1) => actual.derivedAppliedType(tycon, aargs1 :+ ares1))
-              (parent1, leaked ++ cs)
             case actual @ RefinedType(_, _, rinfo: MethodType) if defn.isFunctionType(actual) =>
               // TODO Find a way to combine handling of generic and dependent function types (here and elsewhere)
-              val (parent1, leaked) = adaptFun(parent, rinfo.paramInfos, rinfo.resType, expected, covariant, insertBox,
+              adaptFun(actual, rinfo.paramInfos, rinfo.resType, expected, covariant, insertBox,
                 (aargs1, ares1) =>
                   rinfo.derivedLambdaType(paramInfos = aargs1, resType = ares1)
                     .toFunctionType(isJava = false, alwaysDependent = true))
-              (parent1, leaked ++ cs)
             case actual: MethodType =>
-              val (parent1, leaked) = adaptFun(parent, actual.paramInfos, actual.resType, expected, covariant, insertBox,
+              adaptFun(actual, actual.paramInfos, actual.resType, expected, covariant, insertBox,
                 (aargs1, ares1) =>
                   actual.derivedLambdaType(paramInfos = aargs1, resType = ares1))
-              (parent1, leaked ++ cs)
             case actual @ RefinedType(p, nme, rinfo: PolyType) if defn.isFunctionOrPolyType(actual) =>
-              val (parent1, leaked) = adaptTypeFun(parent, rinfo.resType, expected, covariant, insertBox,
+              adaptTypeFun(actual, rinfo.resType, expected, covariant, insertBox,
                 ares1 =>
                   val rinfo1 = rinfo.derivedLambdaType(rinfo.paramNames, rinfo.paramInfos, ares1)
                   val actual1 = actual.derivedRefinedType(p, nme, rinfo1)
                   actual1
               )
-              (parent1, leaked ++ cs)
             case _ =>
-              (parent, cs)
+              (styp, CaptureSet())
           }
 
+          // Capture set of the term after adaptation
+          val cs1 = cs ++ leaked
+
+          // Compute the adapted type
+          def adaptedType(resultBoxed: Boolean) =
+            styp1.capturing(if alwaysConst then CaptureSet(cs1.elems) else cs1).forceBoxStatus(resultBoxed)
+
           if needsAdaptation then
             val criticalSet =          // the set which is not allowed to have `*`
               if covariant then cs1    // can't box with `*`
@@ -797,9 +788,9 @@ class CheckCaptures extends Recheck, SymTransformer:
               }
               if !insertBox then  // unboxing
                 markFree(criticalSet, pos)
-              recon(CapturingType(parent1, if alwaysConst then CaptureSet(cs1.elems) else cs1, !actualIsBoxed))
+              adaptedType(!boxed)
           else
-            recon(CapturingType(parent1, if alwaysConst then CaptureSet(cs1.elems) else cs1, actualIsBoxed))
+            adaptedType(boxed)
       }
 
       var actualw = actual.widenDealias