Simple enhancement for pattern matching with capturing types

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -14,9 +14,11 @@ import Flags.*, Constants.*
 import Decorators.*
 import NameKinds.{PatMatStdBinderName, PatMatAltsName, PatMatResultName}
 import config.Printers.patmatch
+import config.Feature
 import reporting.*
 import ast.*
 import util.Property.*
+import cc.{CapturingType, Capabilities}
 
 import scala.annotation.tailrec
 import scala.collection.mutable
@@ -418,8 +420,11 @@ object PatternMatcher {
               && !hasExplicitTypeArgs(extractor)
             case _ => false
           }
+          val castTp = if Feature.ccEnabled
+            then CapturingType(tpt.tpe, scrutinee.termRef.singletonCaptureSet)
+            else tpt.tpe
           TestPlan(TypeTest(tpt, isTrusted), scrutinee, tree.span,
-            letAbstract(ref(scrutinee).cast(tpt.tpe)) { casted =>
+            letAbstract(ref(scrutinee).cast(castTp)) { casted =>
               nonNull += casted
               patternPlan(casted, pat, onSuccess)
             })

scala2-library-cc/src/scala/collection/IndexedSeqView.scala

@@ -160,7 +160,7 @@ object IndexedSeqView {
 
   @SerialVersionUID(3L)
   class Reverse[A](underlying: SomeIndexedSeqOps[A]^) extends SeqView.Reverse[A](underlying) with IndexedSeqView[A] {
-    override def reverse: IndexedSeqView[A] = underlying match {
+    override def reverse: IndexedSeqView[A]^{underlying} = underlying match {
       case x: IndexedSeqView[A] => x
       case _ => super.reverse
     }

scala2-library-cc/src/scala/collection/mutable/CheckedIndexedSeqView.scala

@@ -101,7 +101,7 @@ private[mutable] object CheckedIndexedSeqView {
   @SerialVersionUID(3L)
   class Reverse[A](underlying: SomeIndexedSeqOps[A]^)(protected val mutationCount: () ->{cap.rd} Int)
     extends IndexedSeqView.Reverse[A](underlying) with CheckedIndexedSeqView[A] {
-    override def reverse: IndexedSeqView[A] = underlying match {
+    override def reverse: IndexedSeqView[A]^{underlying} = underlying match {
       case x: IndexedSeqView[A] => x
       case _ => super.reverse
     }

tests/neg-custom-args/captures/match.scala

@@ -0,0 +1,21 @@
+import language.experimental.captureChecking
+
+trait A
+
+case class B(x: AnyRef^) extends A
+
+def test =
+  val x: AnyRef^ = new AnyRef
+  val a: A^{x} = B(x)
+
+  val y1: A = a match
+    case b: B => b // error: (b: B) becomes B^{x} implicitly
+
+  val y2: A^{x} = a match
+    case b: B => b // ok
+
+  val x3: AnyRef = a match
+    case B(x2: AnyRef) => x2 // error: we lose some information about field x, but it still cannot be pure
+
+  val x4: AnyRef = a match
+    case b: B => b.x // error