Require array element types to be sealed

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

@@ -206,6 +206,12 @@ extension (tp: Type)
     case _: TypeRef | _: AppliedType => tp.typeSymbol.hasAnnotation(defn.CapabilityAnnot)
     case _ => false
 
+  def isSealed(using Context): Boolean = tp match
+    case tp: TypeParamRef => tp.underlying.isSealed
+    case tp: TypeBounds => tp.hi.hasAnnotation(defn.Caps_SealedAnnot)
+    case tp: TypeRef => tp.symbol.is(Sealed) || tp.info.isSealed // TODO: drop symbol flag?
+    case _ => false
+
   /** Drop @retains annotations everywhere */
   def dropAllRetains(using Context): Type = // TODO we should drop retains from inferred types before unpickling
     val tm = new TypeMap:

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

@@ -872,6 +872,7 @@ object CaptureSet:
       upper.isAlwaysEmpty || upper.isConst && upper.elems.size == 1 && upper.elems.contains(r1)
     if variance > 0 || isExact then upper
     else if variance < 0 then CaptureSet.empty
+    else if ctx.mode.is(Mode.Printing) then upper
     else assert(false, i"trying to add $upper from $r via ${tm.getClass} in a non-variant setting")
 
   /** Apply `f` to each element in `xs`, and join result sets with `++` */

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

@@ -148,7 +148,7 @@ object CheckCaptures:
     val check = new TypeTraverser:
 
       extension (tparam: Symbol) def isParametricIn(carrier: Symbol): Boolean =
-        val encl = carrier.owner.enclosingMethodOrClass
+        val encl = carrier.maybeOwner.enclosingMethodOrClass
         if encl.isClass then tparam.isParametricIn(encl)
         else
           def recur(encl: Symbol): Boolean =
@@ -160,11 +160,9 @@ object CheckCaptures:
       def traverse(t: Type) =
         t.dealiasKeepAnnots match
           case t: TypeRef =>
-            capt.println(i"disallow $t, $tp, $what, ${t.symbol.is(Sealed)}")
+            capt.println(i"disallow $t, $tp, $what, ${t.isSealed}")
             t.info match
-              case TypeBounds(_, hi)
-              if !t.symbol.is(Sealed) && !hi.hasAnnotation(defn.Caps_SealedAnnot)
-                  && !t.symbol.isParametricIn(carrier) =>
+              case TypeBounds(_, hi) if !t.isSealed && !t.symbol.isParametricIn(carrier) =>
                 if hi.isAny then
                   report.error(
                     em"""$what cannot $have $tp since
@@ -543,8 +541,8 @@ class CheckCaptures extends Recheck, SymTransformer:
         val TypeApply(fn, args) = tree
         val polyType = atPhase(thisPhase.prev):
           fn.tpe.widen.asInstanceOf[TypeLambda]
-        for case (arg: TypeTree, pinfo, pname) <- args.lazyZip(polyType.paramInfos).lazyZip((polyType.paramNames)) do
-          if pinfo.bounds.hi.hasAnnotation(defn.Caps_SealedAnnot) then
+        for case (arg: TypeTree, formal, pname) <- args.lazyZip(polyType.paramRefs).lazyZip((polyType.paramNames)) do
+          if formal.isSealed then
             def where = if fn.symbol.exists then i" in an argument of ${fn.symbol}" else ""
             disallowRootCapabilitiesIn(arg.knownType, fn.symbol,
               i"Sealed type variable $pname", "be instantiated to",
@@ -1315,6 +1313,23 @@ class CheckCaptures extends Recheck, SymTransformer:
             traverseChildren(tp)
       check.traverse(info)
 
+    def checkArraysAreSealedIn(tp: Type, pos: SrcPos)(using Context): Unit =
+      val check = new TypeTraverser:
+        def traverse(t: Type): Unit =
+          t match
+            case AppliedType(tycon, arg :: Nil) if tycon.typeSymbol == defn.ArrayClass =>
+              if !(pos.span.isSynthetic && ctx.reporter.errorsReported) then
+                CheckCaptures.disallowRootCapabilitiesIn(arg, NoSymbol,
+                  "Array", "have element type",
+                  "Since arrays are mutable, they have to be treated like variables,\nso their element type must be sealed.",
+                  pos)
+              traverseChildren(t)
+            case defn.RefinedFunctionOf(rinfo: MethodType) =>
+              traverse(rinfo)
+            case _ =>
+              traverseChildren(t)
+      check.traverse(tp)
+
     /** Perform the following kinds of checks
      *   - Check all explicitly written capturing types for well-formedness using `checkWellFormedPost`.
      *   - Check that arguments of TypeApplys and AppliedTypes conform to their bounds.
@@ -1340,6 +1355,8 @@ class CheckCaptures extends Recheck, SymTransformer:
               case _ =>
           case _: ValOrDefDef | _: TypeDef =>
             checkNoLocalRootIn(tree.symbol, tree.symbol.info, tree.symbol.srcPos)
+          case tree: TypeTree =>
+            checkArraysAreSealedIn(tree.tpe, tree.srcPos)
           case _ =>
         end check
       end checker

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

@@ -596,9 +596,9 @@ abstract class Recheck extends Phase, SymTransformer:
 
   /** Show tree with rechecked types instead of the types stored in the `.tpe` field */
   override def show(tree: untpd.Tree)(using Context): String =
-    atPhase(thisPhase) {
-      super.show(addRecheckedTypes.transform(tree.asInstanceOf[tpd.Tree]))
-    }
+    atPhase(thisPhase):
+      withMode(Mode.Printing):
+        super.show(addRecheckedTypes.transform(tree.asInstanceOf[tpd.Tree]))
 end Recheck
 
 /** A class that can be used to test basic rechecking without any customaization */

tests/neg-custom-args/captures/buffers.check

@@ -0,0 +1,26 @@
+-- Error: tests/neg-custom-args/captures/buffers.scala:11:6 ------------------------------------------------------------
+11 |  var elems: Array[A] = new Array[A](10) // error // error
+   |      ^
+   |      mutable variable elems cannot have type Array[A] since
+   |      that type refers to the type variable A, which is not sealed.
+-- Error: tests/neg-custom-args/captures/buffers.scala:16:38 -----------------------------------------------------------
+16 |  def make[A: ClassTag](xs: A*) = new ArrayBuffer: // error
+   |                                      ^^^^^^^^^^^
+   |                            Sealed type variable A cannot be instantiated to box A^? since
+   |                            that type refers to the type variable A, which is not sealed.
+   |                            This is often caused by a local capability in an argument of constructor ArrayBuffer
+   |                            leaking as part of its result.
+-- Error: tests/neg-custom-args/captures/buffers.scala:11:13 -----------------------------------------------------------
+11 |  var elems: Array[A] = new Array[A](10) // error // error
+   |             ^^^^^^^^
+   |             Array cannot have element type A since
+   |             that type refers to the type variable A, which is not sealed.
+   |             Since arrays are mutable, they have to be treated like variables,
+   |             so their element type must be sealed.
+-- Error: tests/neg-custom-args/captures/buffers.scala:22:9 ------------------------------------------------------------
+22 |  val x: Array[A] = new Array[A](10) // error
+   |         ^^^^^^^^
+   |         Array cannot have element type A since
+   |         that type refers to the type variable A, which is not sealed.
+   |         Since arrays are mutable, they have to be treated like variables,
+   |         so their element type must be sealed.

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

@@ -0,0 +1,30 @@
+import reflect.ClassTag
+
+class Buffer[A]
+
+class ArrayBuffer[sealed A: ClassTag] extends Buffer[A]:
+  var elems: Array[A] = new Array[A](10)
+  def add(x: A): this.type = ???
+  def at(i: Int): A = ???
+
+class ArrayBufferBAD[A: ClassTag] extends Buffer[A]:
+  var elems: Array[A] = new Array[A](10) // error // error
+  def add(x: A): this.type = ???
+  def at(i: Int): A = ???
+
+object ArrayBuffer:
+  def make[A: ClassTag](xs: A*) = new ArrayBuffer: // error
+    elems = xs.toArray
+  def apply[sealed A: ClassTag](xs: A*) = new ArrayBuffer:
+    elems = xs.toArray  // ok
+
+class EncapsArray[A: ClassTag]:
+  val x: Array[A] = new Array[A](10) // error
+
+
+
+
+
+
+
+

tests/pos-special/stdlib/collection/IterableOnce.scala

@@ -165,7 +165,7 @@ final class IterableOnceExtensionMethods[A](private val it: IterableOnce[A]) ext
   def toBuffer[sealed B >: A]: mutable.Buffer[B] = mutable.ArrayBuffer.from(it)
 
   @deprecated("Use .iterator.toArray", "2.13.0")
-  def toArray[B >: A: ClassTag]: Array[B] = it match {
+  def toArray[sealed B >: A: ClassTag]: Array[B] = it match {
     case it: Iterable[B] => it.toArray[B]
     case _ => it.iterator.toArray[B]
   }
@@ -272,10 +272,11 @@ object IterableOnce {
     math.max(math.min(math.min(len, srcLen), destLen - start), 0)
 
   /** Calls `copyToArray` on the given collection, regardless of whether or not it is an `Iterable`. */
-  @inline private[collection] def copyElemsToArray[A, B >: A](elems: IterableOnce[A]^,
-                                                              xs: Array[B],
-                                                              start: Int = 0,
-                                                              len: Int = Int.MaxValue): Int =
+  @inline private[collection] def copyElemsToArray[A, sealed B >: A](
+      elems: IterableOnce[A]^,
+      xs: Array[B],
+      start: Int = 0,
+      len: Int = Int.MaxValue): Int =
     elems match {
       case src: Iterable[A] => src.copyToArray[B](xs, start, len)
       case src              => src.iterator.copyToArray[B](xs, start, len)
@@ -889,7 +890,7 @@ trait IterableOnceOps[+A, +CC[_], +C] extends Any { this: IterableOnce[A]^ =>
    *  @note    Reuse: $consumesIterator
    */
   @deprecatedOverriding("This should always forward to the 3-arg version of this method", since = "2.13.4")
-  def copyToArray[B >: A](xs: Array[B]): Int = copyToArray(xs, 0, Int.MaxValue)
+  def copyToArray[sealed B >: A](xs: Array[B]): Int = copyToArray(xs, 0, Int.MaxValue)
 
   /** Copy elements to an array, returning the number of elements written.
    *
@@ -906,7 +907,7 @@ trait IterableOnceOps[+A, +CC[_], +C] extends Any { this: IterableOnce[A]^ =>
    *  @note    Reuse: $consumesIterator
    */
   @deprecatedOverriding("This should always forward to the 3-arg version of this method", since = "2.13.4")
-  def copyToArray[B >: A](xs: Array[B], start: Int): Int = copyToArray(xs, start, Int.MaxValue)
+  def copyToArray[sealed B >: A](xs: Array[B], start: Int): Int = copyToArray(xs, start, Int.MaxValue)
 
   /** Copy elements to an array, returning the number of elements written.
    *
@@ -923,7 +924,7 @@ trait IterableOnceOps[+A, +CC[_], +C] extends Any { this: IterableOnce[A]^ =>
    *
    *  @note    Reuse: $consumesIterator
    */
-  def copyToArray[B >: A](xs: Array[B], start: Int, len: Int): Int = {
+  def copyToArray[sealed B >: A](xs: Array[B], start: Int, len: Int): Int = {
     val it = iterator
     var i = start
     val end = start + math.min(len, xs.length - start)
@@ -1318,7 +1319,7 @@ trait IterableOnceOps[+A, +CC[_], +C] extends Any { this: IterableOnce[A]^ =>
     *
     * Implementation note: DO NOT call [[Array.from]] from this method.
     */
-  def toArray[B >: A: ClassTag]: Array[B] =
+  def toArray[sealed B >: A: ClassTag]: Array[B] =
     if (knownSize >= 0) {
       val destination = new Array[B](knownSize)
       copyToArray(destination, 0)

tests/pos-special/stdlib/collection/Iterator.scala

@@ -259,7 +259,7 @@ trait Iterator[+A] extends IterableOnce[A] with IterableOnceOps[A, Iterator, Ite
       }
       // segment must have data, and must be complete unless they allow partial
       val ok = index > 0 && (partial || index == size)
-      if (ok) buffer = builder.result().asInstanceOf[Array[B]]
+      if (ok) buffer = builder.result().asInstanceOf[Array[B @uncheckedCaptures]]
       else prev = null
       ok
     }

tests/pos-special/stdlib/collection/SeqView.scala

@@ -16,6 +16,7 @@ package collection
 import scala.annotation.nowarn
 import language.experimental.captureChecking
 import caps.unsafe.unsafeAssumePure
+import scala.annotation.unchecked.uncheckedCaptures
 
 /** !!! Scala 2 difference: Need intermediate trait SeqViewOps to collect the
  *  necessary functionality over which SeqViews are defined, and at the same
@@ -195,7 +196,7 @@ object SeqView {
           //     contains items of another type, we'd get a CCE anyway)
           //   - the cast doesn't actually do anything in the runtime because the
           //     type of A is not known and Array[_] is Array[AnyRef]
-          immutable.ArraySeq.unsafeWrapArray(arr.asInstanceOf[Array[A]])
+          immutable.ArraySeq.unsafeWrapArray(arr.asInstanceOf[Array[A @uncheckedCaptures]])
         }
       }
       evaluated = true

tests/pos-special/stdlib/collection/StringOps.scala

@@ -964,7 +964,7 @@ final class StringOps(private val s: String) extends AnyVal {
     else if (s.equalsIgnoreCase("false")) false
     else throw new IllegalArgumentException("For input string: \""+s+"\"")
 
-  def toArray[B >: Char](implicit tag: ClassTag[B]): Array[B] =
+  def toArray[sealed B >: Char](implicit tag: ClassTag[B]): Array[B] =
     if (tag == ClassTag.Char) s.toCharArray.asInstanceOf[Array[B]]
     else new WrappedString(s).toArray[B]
 

tests/pos-special/stdlib/collection/mutable/ArrayBuffer.scala

@@ -241,7 +241,7 @@ class ArrayBuffer[sealed A] private (initialElements: Array[AnyRef], initialSize
   @nowarn("""cat=deprecation&origin=scala\.collection\.Iterable\.stringPrefix""")
   override protected[this] def stringPrefix = "ArrayBuffer"
 
-  override def copyToArray[B >: A](xs: Array[B], start: Int, len: Int): Int = {
+  override def copyToArray[sealed B >: A](xs: Array[B], start: Int, len: Int): Int = {
     val copied = IterableOnce.elemsToCopyToArray(length, xs.length, start, len)
     if(copied > 0) {
       Array.copy(array, 0, xs, start, copied)
@@ -258,7 +258,7 @@ class ArrayBuffer[sealed A] private (initialElements: Array[AnyRef], initialSize
   override def sortInPlace[B >: A]()(implicit ord: Ordering[B]): this.type = {
     if (length > 1) {
       mutationCount += 1
-      scala.util.Sorting.stableSort(array.asInstanceOf[Array[B]], 0, length)
+      scala.util.Sorting.stableSort(array.asInstanceOf[Array[B @uncheckedCaptures]], 0, length)
     }
     this
   }

tests/pos-special/stdlib/collection/mutable/Buffer.scala

@@ -15,6 +15,7 @@ package mutable
 
 import scala.annotation.nowarn
 import language.experimental.captureChecking
+import scala.annotation.unchecked.uncheckedCaptures
 
 
 /** A `Buffer` is a growable and shrinkable `Seq`. */
@@ -185,7 +186,7 @@ trait IndexedBuffer[A] extends IndexedSeq[A]
     // There's scope for a better implementation which copies elements in place.
     var i = 0
     val s = size
-    val newElems = new Array[IterableOnce[A]^](s)
+    val newElems = new Array[(IterableOnce[A]^) @uncheckedCaptures](s)
     while (i < s) { newElems(i) = f(this(i)); i += 1 }
     clear()
     i = 0

tests/pos-special/stdlib/collection/mutable/StringBuilder.scala

@@ -110,7 +110,7 @@ final class StringBuilder(val underlying: java.lang.StringBuilder) extends Abstr
 
   override def toString: String = result()
 
-  override def toArray[B >: Char](implicit ct: scala.reflect.ClassTag[B]) =
+  override def toArray[sealed B >: Char](implicit ct: scala.reflect.ClassTag[B]) =
     ct.runtimeClass match {
       case java.lang.Character.TYPE => toCharArray.asInstanceOf[Array[B]]
       case _ => super.toArray