New scheme to reject root captures

Reject root captures by considering unbox operations. This allows us
to ignore root captures buried under type applications.

Author: odersky

Diff for: compiler/src/dotty/tools/dotc/cc/CaptureOps.scala

@@ -3,7 +3,7 @@ package dotc
 package cc
 
 import core.*
-import Types.*, Symbols.*, Contexts.*, Annotations.*
+import Types.*, Symbols.*, Contexts.*, Annotations.*, Flags.*
 import ast.{tpd, untpd}
 import Decorators.*, NameOps.*
 import config.Printers.capt
@@ -85,3 +85,27 @@ extension (tp: Type)
         isImpure = true).appliedTo(args)
     case _ =>
       tp
+
+extension (sym: Symbol)
+
+  /** Does this symbol allow results carrying the universal capability?
+   *  Currently this is true only for function type applies (since their
+   *  results are unboxed) and `erasedValue` since this function is magic in
+   *  that is allows to conjure global capabilies from nothing (aside: can we find a
+   *  more controlled way to achieve this?).
+   *  But it could be generalized to other functions that so that they can take capability
+   *  classes as arguments.
+   */
+  def allowsRootCapture(using Context): Boolean =
+    sym == defn.Compiletime_erasedValue
+    || defn.isFunctionClass(sym.maybeOwner)
+
+  def unboxesResult(using Context): Boolean =
+    def containsEnclTypeParam(tp: Type): Boolean = tp.strippedDealias match
+      case tp @ TypeRef(pre: ThisType, _) => tp.symbol.is(Param)
+      case tp: TypeParamRef => true
+      case tp: AndOrType => containsEnclTypeParam(tp.tp1) || containsEnclTypeParam(tp.tp2)
+      case tp: RefinedType => containsEnclTypeParam(tp.parent) || containsEnclTypeParam(tp.refinedInfo)
+      case _ => false
+    containsEnclTypeParam(sym.info.finalResultType)
+    && !sym.allowsRootCapture

Diff for: compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -172,6 +172,10 @@ sealed abstract class CaptureSet extends Showable:
   def - (ref: CaptureRef)(using Context): CaptureSet =
     this -- ref.singletonCaptureSet
 
+  def disallowRootCapability(handler: () => Unit)(using Context): this.type =
+    if isUniversal then handler()
+    this
+
   def filter(p: CaptureRef => Boolean)(using Context): CaptureSet =
     if this.isConst then
       val elems1 = elems.filter(p)
@@ -276,6 +280,7 @@ object CaptureSet:
     var deps: Deps = emptySet
     def isConst = isSolved
     def isAlwaysEmpty = false
+    var addRootHandler: () => Unit = () => ()
 
     private def recordElemsState()(using VarState): Boolean =
       varState.getElems(this) match
@@ -296,6 +301,7 @@ object CaptureSet:
     def addNewElems(newElems: Refs, origin: CaptureSet)(using Context, VarState): CompareResult =
       if !isConst && recordElemsState() then
         elems ++= newElems
+        if isUniversal then addRootHandler()
         // assert(id != 2 || elems.size != 2, this)
         (CompareResult.OK /: deps) { (r, dep) =>
           r.andAlso(dep.tryInclude(newElems, this))
@@ -312,6 +318,10 @@ object CaptureSet:
       else
         CompareResult.fail(this)
 
+    override def disallowRootCapability(handler: () => Unit)(using Context): this.type =
+      addRootHandler = handler
+      super.disallowRootCapability(handler)
+
     private var computingApprox = false
 
     final def upperApprox(origin: CaptureSet)(using Context): CaptureSet =

Diff for: compiler/src/dotty/tools/dotc/transform/Recheck.scala

@@ -329,6 +329,7 @@ abstract class Recheck extends Phase, IdentityDenotTransformer:
         case tree: Alternative => recheckAlternative(tree, pt)
         case tree: PackageDef => recheckPackageDef(tree)
         case tree: Thicket => defn.NothingType
+        case tree: Import => defn.NothingType
 
       tree match
         case tree: NameTree => recheckNamed(tree, pt)

Diff for: compiler/src/dotty/tools/dotc/typer/CheckCaptures.scala

@@ -76,16 +76,6 @@ object CheckCaptures:
       if remaining.accountsFor(firstRef) then
         report.warning(em"redundant capture: $remaining already accounts for $firstRef", ann.srcPos)
 
-  /** Does this function allow type arguments carrying the universal capability?
-   *  Currently this is true only for `erasedValue` since this function is magic in
-   *  that is allows to conjure global capabilies from nothing (aside: can we find a
-   *  more controlled way to achieve this?).
-   *  But it could be generalized to other functions that so that they can take capability
-   *  classes as arguments.
-   */
-  private def allowUniversalArguments(fn: Tree)(using Context): Boolean =
-    fn.symbol == defn.Compiletime_erasedValue
-
 class CheckCaptures extends Recheck:
   thisPhase =>
 
@@ -309,6 +299,26 @@ class CheckCaptures extends Recheck:
         includeBoxedCaptures(res, tree.srcPos)
       res
 
+    override def recheckFinish(tpe: Type, tree: Tree, pt: Type)(using Context): Type =
+      val typeToCheck = tree match
+        case _: Ident | _: Select | _: Apply | _: TypeApply if tree.symbol.unboxesResult =>
+          tpe
+        case _: Try =>
+          tpe
+        case ValDef(_, tpt, _) if tree.symbol.is(Mutable) =>
+          tree.symbol.info
+        case _ =>
+          NoType
+      if typeToCheck.exists then
+        typeToCheck.widenDealias match
+          case wtp @ CapturingType(parent, refs, _) =>
+            refs.disallowRootCapability { () =>
+              val kind = if tree.isInstanceOf[ValDef] then "mutable variable" else "expression"
+              report.error(em"the $kind's type $wtp is not allowed to capture the root capability `*`", tree.srcPos)
+            }
+          case _ =>
+      super.recheckFinish(tpe, tree, pt)
+
     override def checkUnit(unit: CompilationUnit)(using Context): Unit =
       Setup(preRecheckPhase, thisPhase, recheckDef)
         .traverse(ctx.compilationUnit.tpdTree)
@@ -319,45 +329,6 @@ class CheckCaptures extends Recheck:
           show(unit.tpdTree) // this dows not print tree, but makes its variables visible for dependency printing
       }
 
-    def checkNotGlobal(tree: Tree, tp: Type, isVar: Boolean, allArgs: Tree*)(using Context): Unit =
-      for ref <- tp.captureSet.elems do
-        val isGlobal = ref match
-          case ref: TermRef => ref.isRootCapability
-          case _ => false
-        if isGlobal then
-          val what = if ref.isRootCapability then "universal" else "global"
-          val notAllowed = i" is not allowed to capture the $what capability $ref"
-          def msg =
-            if allArgs.isEmpty then
-              i"${if isVar then "type of mutable variable" else "result type"} ${tree.knownType}$notAllowed"
-            else tree match
-              case tree: InferredTypeTree =>
-                i"""inferred type argument ${tree.knownType}$notAllowed
-                    |
-                    |The inferred arguments are: [${allArgs.map(_.knownType)}%, %]"""
-              case _ => s"type argument$notAllowed"
-          report.error(msg, tree.srcPos)
-
-    def checkNotGlobal(tree: Tree, allArgs: Tree*)(using Context): Unit =
-      tree match
-        case LambdaTypeTree(_, restpt) =>
-          checkNotGlobal(restpt, allArgs*)
-        case _ =>
-          checkNotGlobal(tree, tree.knownType, isVar = false, allArgs*)
-
-    def checkNotGlobalDeep(tree: Tree)(using Context): Unit =
-      val checker = new TypeTraverser:
-        def traverse(tp: Type): Unit = tp match
-          case tp: TypeRef =>
-            tp.info match
-              case TypeBounds(_, hi) => traverse(hi)
-              case _ =>
-          case tp: TermRef =>
-          case _ =>
-            checkNotGlobal(tree, tp, isVar = true)
-            traverseChildren(tp)
-      checker.traverse(tree.knownType)
-
     object PostCheck extends TreeTraverser:
       def traverse(tree: Tree)(using Context) = trace{i"post check $tree"} {
         tree match
@@ -370,10 +341,6 @@ class CheckCaptures extends Recheck:
                 checkWellformedPost(annot.tree)
               case _ =>
             }
-          case tree1 @ TypeApply(fn, args) if !allowUniversalArguments(fn) =>
-            for arg <- args do
-              //println(i"checking $arg in $tree: ${tree.knownType.captureSet}")
-              checkNotGlobal(arg, args*)
           case t: ValOrDefDef if t.tpt.isInstanceOf[InferredTypeTree] =>
             val sym = t.symbol
             val isLocal =
@@ -396,10 +363,6 @@ class CheckCaptures extends Recheck:
                         |The type needs to be declared explicitly.""", t.srcPos)
                 case _ =>
               inferred.foreachPart(checkPure, StopAt.Static)
-          case t: ValDef if t.symbol.is(Mutable) =>
-            checkNotGlobalDeep(t.tpt)
-          case t: Try =>
-            checkNotGlobal(t)
           case _ =>
         traverseChildren(tree)
       }

Diff for: tests/neg-custom-args/captures/capt-test.scala

@@ -19,8 +19,8 @@ def handle[E <: Exception, R <: Top](op: (CanThrow[E]) => R)(handler: E => R): R
   catch case ex: E => handler(ex)
 
 def test: Unit =
-  val b = handle[Exception, () => Nothing] {  // error
+  val b = handle[Exception, () => Nothing] {
     (x: CanThrow[Exception]) => () => raise(new Exception)(using x)
-  } {
+  } { // error
     (ex: Exception) => ???
   }

Diff for: tests/neg-custom-args/captures/real-try.check

@@ -1,8 +1,20 @@
--- Error: tests/neg-custom-args/captures/real-try.scala:10:2 -----------------------------------------------------------
-10 |  try  // error
+-- Error: tests/neg-custom-args/captures/real-try.scala:12:2 -----------------------------------------------------------
+12 |  try  // error
    |  ^
-   |  result type {*} () -> Unit is not allowed to capture the universal capability *.type
-11 |    () => foo(1)
-12 |  catch
-13 |    case _: Ex1 => ???
-14 |    case _: Ex2 => ???
+   |  the expression's type {*} () -> Unit is not allowed to capture the root capability `*`
+13 |    () => foo(1)
+14 |  catch
+15 |    case _: Ex1 => ???
+16 |    case _: Ex2 => ???
+-- Error: tests/neg-custom-args/captures/real-try.scala:18:2 -----------------------------------------------------------
+18 |  try  // error
+   |  ^
+   |  the expression's type {*} () -> ? Cell[Unit] is not allowed to capture the root capability `*`
+19 |    () => Cell(foo(1))
+20 |  catch
+21 |    case _: Ex1 => ???
+22 |    case _: Ex2 => ???
+-- Error: tests/neg-custom-args/captures/real-try.scala:30:4 -----------------------------------------------------------
+30 |  b.x // error
+   |  ^^^
+   |  the expression's type box {*} () -> Unit is not allowed to capture the root capability `*`

Diff for: tests/neg-custom-args/captures/real-try.scala

@@ -6,9 +6,25 @@ class Ex2 extends Exception("Ex2")
 def foo(i: Int): (CanThrow[Ex1], CanThrow[Ex2]) ?-> Unit =
   if i > 0 then throw new Ex1 else throw new Ex2
 
+class Cell[+T](val x: T)
+
 def test() =
   try  // error
     () => foo(1)
   catch
     case _: Ex1 => ???
     case _: Ex2 => ???
+
+  try  // error
+    () => Cell(foo(1))
+  catch
+    case _: Ex1 => ???
+    case _: Ex2 => ???
+
+  val b = try  // ok here, but error on use
+    Cell(() => foo(1))//: Cell[box {ev} () => Unit] <: Cell[box {*} () => Unit]
+  catch
+    case _: Ex1 => ???
+    case _: Ex2 => ???
+
+  b.x // error

Diff for: tests/neg-custom-args/captures/try.check

@@ -1,3 +1,11 @@
+-- Error: tests/neg-custom-args/captures/try.scala:24:3 ----------------------------------------------------------------
+22 |  val a = handle[Exception, CanThrow[Exception]] {
+23 |    (x: CanThrow[Exception]) => x
+24 |  }{ // error
+   |          ^
+   |          the expression's type {*} CT[Exception] is not allowed to capture the root capability `*`
+25 |    (ex: Exception) => ???
+26 |  }
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/try.scala:28:43 ------------------------------------------
 28 |  val b = handle[Exception, () -> Nothing] {  // error
    |                                           ^
@@ -7,19 +15,25 @@
 30 |  } {
 
 longer explanation available when compiling with `-explain`
--- Error: tests/neg-custom-args/captures/try.scala:22:28 ---------------------------------------------------------------
-22 |  val a = handle[Exception, CanThrow[Exception]] { // error
-   |                            ^^^^^^^^^^^^^^^^^^^
-   |                            type argument is not allowed to capture the universal capability (* : Any)
--- Error: tests/neg-custom-args/captures/try.scala:34:11 ---------------------------------------------------------------
-34 |  val xx = handle { // error
-   |           ^^^^^^
-   |           inferred type argument {*} () -> Int is not allowed to capture the universal capability (* : Any)
-   |
-   |           The inferred arguments are: [? Exception, {*} () -> Int]
--- Error: tests/neg-custom-args/captures/try.scala:46:13 ---------------------------------------------------------------
-46 |val global = handle {  // error
-   |             ^^^^^^
-   |             inferred type argument {*} () -> Int is not allowed to capture the universal capability (* : Any)
-   |
-   |             The inferred arguments are: [? Exception, {*} () -> Int]
+-- Error: tests/neg-custom-args/captures/try.scala:39:4 ----------------------------------------------------------------
+34 |  val xx = handle {
+35 |    (x: CanThrow[Exception]) =>
+36 |      () =>
+37 |        raise(new Exception)(using x)
+38 |        22
+39 |  } {  // error
+   |           ^
+   |           the expression's type {*} () -> Int is not allowed to capture the root capability `*`
+40 |    (ex: Exception) => () => 22
+41 |  }
+-- Error: tests/neg-custom-args/captures/try.scala:51:2 ----------------------------------------------------------------
+46 |val global = handle {
+47 |  (x: CanThrow[Exception]) =>
+48 |    () =>
+49 |      raise(new Exception)(using x)
+50 |      22
+51 |} {  // error
+   |             ^
+   |             the expression's type {*} () -> Int is not allowed to capture the root capability `*`
+52 |  (ex: Exception) => () => 22
+53 |}

Diff for: tests/neg-custom-args/captures/try.scala

@@ -19,9 +19,9 @@ def handle[E <: Exception, R <: Top](op: CanThrow[E] => R)(handler: E => R): R =
   catch case ex: E => handler(ex)
 
 def test =
-  val a = handle[Exception, CanThrow[Exception]] { // error
+  val a = handle[Exception, CanThrow[Exception]] {
     (x: CanThrow[Exception]) => x
-  }{
+  }{ // error
     (ex: Exception) => ???
   }
 
@@ -31,23 +31,23 @@ def test =
     (ex: Exception) => ???
   }
 
-  val xx = handle { // error
+  val xx = handle {
     (x: CanThrow[Exception]) =>
       () =>
         raise(new Exception)(using x)
         22
-  } {
+  } {  // error
     (ex: Exception) => () => 22
   }
   val yy = xx :: Nil
   yy // OK
 
 
-val global = handle {  // error
+val global = handle {
   (x: CanThrow[Exception]) =>
     () =>
       raise(new Exception)(using x)
       22
-} {
+} {  // error
   (ex: Exception) => () => 22
 }

Diff for: tests/neg-custom-args/captures/try3.scala

@@ -14,12 +14,12 @@ def raise[E <: Exception](ex: E)(using CanThrow[E]): Nothing =
 
 @main def Test: Int =
   def f(a: Boolean) =
-    handle { // error
+    handle {
       if !a then raise(IOException())
       (b: Boolean) =>
         if !b then raise(IOException())
         0
-    } {
+    } {  // error
       ex => (b: Boolean) => -1
     }
   val g = f(true)