Treat all type parameters and abstract types as sealed

This means that no type parameter can be instantiated with a type that
captures cap covariantly or invariantly in its type.

Two exceptions/special cases:

 - Type arguments for isInstanceOf and asInstanceOf are excluded, they can capture cap
   anywhere.
 - Refining variables in class types can still contain cap since they describe what
   comes from the constructor.

Test reclassifications:

 - i15922.scala was moved to pending. Not clear whether this should compile, and what changes would be necessary to get it there.
 - future-traverse.scala was moved to pending. Not clear how to make this compiler.
 - i15749a.scala was moved to neg. The issue description seems to indicate that the test should not compile, but I am not sure what the outcome should be.

Author: odersky

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

@@ -608,6 +608,13 @@ 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(directOwner: Symbol)(using Context) extends Var(directOwner):
+    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/CheckCaptures.scala

@@ -170,22 +170,7 @@ object CheckCaptures:
         t.dealiasKeepAnnots match
           case t: TypeRef =>
             if !seen.contains(t) then
-              capt.println(i"disallow $t, $tp, $what, ${t.isSealed}")
               seen += t
-              t.info match
-                case TypeBounds(_, hi) if !t.isSealed && !t.symbol.isParametricIn(carrier) =>
-                  if hi.isAny then
-                    val detailStr =
-                      if t eq tp then "variable"
-                      else i"refers to the type variable $t, which"
-                    report.error(
-                      em"""$what cannot $have $tp since
-                          |that type $detailStr is not sealed.
-                          |$addendum""",
-                      pos)
-                  else
-                    traverse(hi)
-                case _ =>
               traverseChildren(t)
           case AnnotatedType(_, ann) if ann.symbol == defn.UncheckedCapturesAnnot =>
             ()
@@ -557,7 +542,7 @@ class CheckCaptures extends Recheck, SymTransformer:
         val polyType = atPhase(thisPhase.prev):
           fn.tpe.widen.asInstanceOf[TypeLambda]
         for case (arg: TypeTree, formal, pname) <- args.lazyZip(polyType.paramRefs).lazyZip((polyType.paramNames)) do
-          if formal.isSealed then
+          if !tree.symbol.isTypeTestOrCast then
             def where = if fn.symbol.exists then i" in an argument of ${fn.symbol}" else ""
             disallowRootCapabilitiesIn(arg.knownType, NoSymbol,
               i"Sealed type variable $pname", "be instantiated to",

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

@@ -174,7 +174,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
                   val getterType =
                     mapInferred(refine = false)(tp.memberInfo(getter)).strippedDealias
                   RefinedType(core, getter.name,
-                      CapturingType(getterType, CaptureSet.Var(ctx.owner)))
+                      CapturingType(getterType, CaptureSet.RefiningVar(ctx.owner)))
                     .showing(i"add capture refinement $tp --> $result", capt)
                 else
                   core

tests/neg-custom-args/captures/capt1.check

@@ -33,8 +33,22 @@
 27 |    def m() = if x == null then y else y
    |
    | longer explanation available when compiling with `-explain`
+-- Error: tests/neg-custom-args/captures/capt1.scala:32:12 -------------------------------------------------------------
+32 |  val z2 = h[() -> Cap](() => x) // error // error
+   |           ^^^^^^^^^^^^
+   |           Sealed type variable X cannot be instantiated to () -> box C^ since
+   |           the part box C^ of that type captures the root capability `cap`.
+   |           This is often caused by a local capability in an argument of method h
+   |           leaking as part of its result.
 -- Error: tests/neg-custom-args/captures/capt1.scala:32:30 -------------------------------------------------------------
-32 |  val z2 = h[() -> Cap](() => x) // error
+32 |  val z2 = h[() -> Cap](() => x) // error // error
    |                              ^
    |                            (x : C^) cannot be referenced here; it is not included in the allowed capture set {}
    |                            of an enclosing function literal with expected type () -> box C^
+-- Error: tests/neg-custom-args/captures/capt1.scala:34:12 -------------------------------------------------------------
+34 |  val z3 = h[(() -> Cap) @retains(x)](() => x)(() => C())  // error
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |           Sealed type variable X cannot be instantiated to box () ->{x} Cap since
+   |           the part C^ of that type captures the root capability `cap`.
+   |           This is often caused by a local capability in an argument of method h
+   |           leaking as part of its result.

tests/neg-custom-args/captures/capt1.scala

@@ -29,8 +29,7 @@ def h4(x: Cap, y: Int): A =
 def foo() =
   val x: C @retains(caps.cap) = ???
   def h[X](a: X)(b: X) = a
-  val z2 = h[() -> Cap](() => x) // error
+  val z2 = h[() -> Cap](() => x) // error // error
     (() => C())
-  val z3 = h[(() -> Cap) @retains(x)](() => x)(() => C())  // ok
-  val z4 = h[(() -> Cap) @retains(x)](() => x)(() => C())  // what was inferred for z3
+  val z3 = h[(() -> Cap) @retains(x)](() => x)(() => C())  // error
 

tests/pos-custom-args/captures/i15749a.scala renamed to tests/neg-custom-args/captures/i15749a.scala

@@ -4,7 +4,7 @@ object u extends Unit
 
 type Top = Any^
 
-type Wrapper[T] = [X] -> (op: T ->{cap} X) -> X
+type Wrapper[+T] = [X] -> (op: T ->{cap} X) -> X
 
 def test =
 

tests/neg-custom-args/captures/i15772.check

@@ -22,11 +22,14 @@
    |                               Required: (C^ => Unit) -> Unit
    |
    | longer explanation available when compiling with `-explain`
--- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15772.scala:33:33 ---------------------------------------
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15772.scala:33:34 ---------------------------------------
 33 |  val boxed2 : Observe[C]^ = box2(c) // error
-   |                             ^^^^^^^
-   |                             Found:    (C{val arg: C^}^ => Unit) ->? Unit
-   |                             Required: Observe[C]^
+   |                                  ^
+   |                                  Found:    box C^
+   |                                  Required: box C{val arg: C^?}^?
+   |
+   |                                  Note that the universal capability `cap`
+   |                                  cannot be included in capture set ?
    |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15772.scala:44:2 ----------------------------------------

tests/neg-custom-args/captures/i16114.scala

@@ -13,19 +13,19 @@ def withCap[T](op: Cap^ => T): T = {
 def main(fs: Cap^): Unit = {
   def badOp(io: Cap^): Unit ->{} Unit = {
     val op1: Unit ->{io} Unit = (x: Unit) =>
-      expect[Cap^] {
+      expect[Cap^] {  // error
         io.use()
         fs  // error (limitation)
       }
 
     val op2: Unit ->{fs} Unit = (x: Unit) =>
-      expect[Cap^] {
+      expect[Cap^] { // error
         fs.use()
         io  // error (limitation)
       }
 
-    val op3: Unit ->{io} Unit = (x: Unit) =>  // ok
-      expect[Cap^] {
+    val op3: Unit ->{io} Unit = (x: Unit) =>
+      expect[Cap^] {  // error
         io.use()
         io
       }
@@ -34,7 +34,7 @@ def main(fs: Cap^): Unit = {
       expect[Cap^](io) // error
 
     val op: Unit -> Unit = (x: Unit) =>
-      expect[Cap^] {
+      expect[Cap^] { // error
         io.use()    // error
         io          // error
       }

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

@@ -1,8 +1,3 @@
--- Error: tests/neg-custom-args/captures/levels.scala:6:16 -------------------------------------------------------------
-6 |    private var v: T = init  // error
-  |                ^
-  |                Mutable variable v cannot have type T since
-  |                that type variable is not sealed.
 -- Error: tests/neg-custom-args/captures/levels.scala:17:13 ------------------------------------------------------------
 17 |  val _ = Ref[String => String]((x: String) => x) // error
    |          ^^^^^^^^^^^^^^^^^^^^^

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

@@ -3,7 +3,7 @@ class CC
 def test1(cap1: CC^) =
 
   class Ref[T](init: T):
-    private var v: T = init  // error
+    private var v: T = init
     def setV(x: T): Unit = v = x
     def getV: T = v
 

tests/neg-custom-args/captures/outer-var.check

@@ -35,13 +35,10 @@
    |        cannot be included in outer capture set {p} of variable y which is associated with method test
    |
    | longer explanation available when compiling with `-explain`
--- [E007] Type Mismatch Error: tests/neg-custom-args/captures/outer-var.scala:16:65 ------------------------------------
+-- Error: tests/neg-custom-args/captures/outer-var.scala:16:53 ---------------------------------------------------------
 16 |  var finalizeActions = collection.mutable.ListBuffer[() => Unit]() // error
-   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |                        Found:    scala.collection.mutable.ListBuffer[box () => Unit]
-   |                        Required: scala.collection.mutable.ListBuffer[box () ->? Unit]^?
-   |
-   |                        Note that the universal capability `cap`
-   |                        cannot be included in capture set ? of variable finalizeActions
-   |
-   | longer explanation available when compiling with `-explain`
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                        Sealed type variable A cannot be instantiated to box () => Unit since
+   |                        that type captures the root capability `cap`.
+   |                        This is often caused by a local capability in an argument of method apply
+   |                        leaking as part of its result.

tests/neg-custom-args/captures/sealed-classes.scala

@@ -11,4 +11,6 @@ def withCap[X](op: (Cap^) => X): X = {
 def leaking(c: Cap^): Id[Cap^{c}] = mkId(c)
 
 def test =
-  val bad = withCap(leaking)
+  val ll = (c: Cap^) => leaking(c)
+  val bad1 = withCap(ll)       // used to work, now error (?)
+  val bad2 = withCap(leaking)  // used to work, now error (?)

tests/pos-custom-args/captures/future-traverse.scala renamed to tests/pending/pos-custom-args/captures/future-traverse.scala

@@ -8,18 +8,18 @@ class ArrayBuffer[sealed A: ClassTag] extends Buffer[A]:
   def at(i: Int): A = ???
 
 class ArrayBufferBAD[A: ClassTag] extends Buffer[A]:
-  var elems: Array[A] = new Array[A](10) // error // error
+  var elems: Array[A] = new Array[A](10)
   def add(x: A): this.type = ???
   def at(i: Int): A = ???
 
 object ArrayBuffer:
-  def make[A: ClassTag](xs: A*) = new ArrayBuffer: // error
+  def make[A: ClassTag](xs: A*) = new ArrayBuffer:
     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
+  val x: Array[A] = new Array[A](10)
 
 
 

tests/pos-custom-args/captures/i15922.scala renamed to tests/pending/pos-custom-args/captures/i15922.scala

@@ -1,3 +1,4 @@
+import annotation.unchecked.uncheckedCaptures
 def test =
-  val tasks = new collection.mutable.ArrayBuffer[() => Unit]
+  val tasks = new collection.mutable.ArrayBuffer[(() => Unit) @uncheckedCaptures]
   val _: Unit = tasks.foreach(((task: () => Unit) => task()))

tests/neg-custom-args/captures/buffers.scala renamed to tests/pos-custom-args/captures/buffers.scala

@@ -52,7 +52,8 @@ class Pair[+A, +B](x: A, y: B):
 def test2(ct: CanThrow[Exception], fs: FileSystem) =
   def x: Int ->{ct} String = ???
   def y: Logger^{fs} = ???
-  def p = Pair(x, y)
+  def p = Pair[Int ->{ct} String, Logger^{fs}](x, y)
+  def p3 = Pair(x, y)
   def f = () => p.fst
 
 

tests/pos-custom-args/captures/foreach.scala

@@ -1,7 +1,7 @@
 import annotation.unchecked.uncheckedCaptures
 def test =
-  @uncheckedCaptures
-  var finalizeActions = collection.mutable.ListBuffer[() => Unit]()
+  val finalizeActionsInit = collection.mutable.ListBuffer[(() => Unit) @uncheckedCaptures]()
+  var finalizeActions = finalizeActionsInit
   val action = finalizeActions.remove(0)