Fix potential soundness hole when adding references to a mapped captu…

…re set (#18758)

Fix potential soundness hole when adding references to a set that is the
image of an
idempotent `tm` map `tm`. If the element `ref` did not come from the
source
of the set, we still assumed that `tm(ref) = ref`, so that we simply
added
the reference to the set and also back-propagated it to source. But that
is not
necessarily the case. (Although it is the case in our complete test
suite,
so I am not sure this case can actually arise in practice. Nevertheless,
it's better to not leave a potential soundness hole here.)

In the new implementation, we test whether `tm(ref) = ref`, and only
proceed
as before if that's the case. If not there are two sub-cases:

- `{ref} <:< tm(ref)` and the variance of the set is positive. In that
case we
can soundly add `tm(ref)` to the set while back-propagating `ref` as
before.
- Otherwise there's nothing obvious left to do except fail (which is
always
   sound).

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

@@ -19,11 +19,11 @@ private val Captures: Key[CaptureSet] = Key()
 
 object ccConfig:
 
-  /** Switch whether unpickled function types and byname types should be mapped to
-   *  impure types. With the new gradual typing using Fluid capture sets, this should
-   *  be no longer needed. Also, it has bad interactions with pickling tests.
+  /** If true, allow mappping capture set variables under captureChecking with maps that are neither
+   *  bijective nor idempotent. We currently do now know how to do this correctly in all
+   *  cases, though.
    */
-  private[cc] val adaptUnpickledFunctionTypes = false
+  inline val allowUnsoundMaps = false
 
   /** If true, use `sealed` as encapsulation mechanism instead of the
    *  previous global retriction that `cap` can't be boxed or unboxed.
@@ -48,7 +48,7 @@ def isCaptureCheckingOrSetup(using Context): Boolean =
  */
 def depFun(args: List[Type], resultType: Type, isContextual: Boolean, paramNames: List[TermName] = Nil)(using Context): Type =
   val make = MethodType.companion(isContextual = isContextual)
-  val mt = 
+  val mt =
     if paramNames.length == args.length then make(paramNames, args, resultType)
     else make(args, resultType)
   mt.toFunctionType(alwaysDependent = true)
@@ -106,22 +106,6 @@ extension (tree: Tree)
     case Apply(_, Typed(SeqLiteral(elems, _), _) :: Nil) => elems
     case _ => Nil
 
-  /** Under pureFunctions, add a @retainsByName(*)` annotation to the argument of
-   *  a by name parameter type, turning the latter into an impure by name parameter type.
-   */
-  def adaptByNameArgUnderPureFuns(using Context): Tree =
-    if ccConfig.adaptUnpickledFunctionTypes && Feature.pureFunsEnabledSomewhere then
-      val rbn = defn.RetainsByNameAnnot
-      Annotated(tree,
-        New(rbn.typeRef).select(rbn.primaryConstructor).appliedTo(
-          Typed(
-            SeqLiteral(ref(defn.captureRoot) :: Nil, TypeTree(defn.AnyType)),
-            TypeTree(defn.RepeatedParamType.appliedTo(defn.AnyType))
-          )
-        )
-      )
-    else tree
-
 extension (tp: Type)
 
   /** @pre `tp` is a CapturingType */
@@ -199,29 +183,6 @@ extension (tp: Type)
     case _ =>
       tp
 
-  /** Under pureFunctions, map regular function type to impure function type
-   */
-  def adaptFunctionTypeUnderPureFuns(using Context): Type = tp match
-    case AppliedType(fn, args)
-    if ccConfig.adaptUnpickledFunctionTypes && Feature.pureFunsEnabledSomewhere && defn.isFunctionClass(fn.typeSymbol) =>
-      val fname = fn.typeSymbol.name
-      defn.FunctionType(
-        fname.functionArity,
-        isContextual = fname.isContextFunction,
-        isImpure = true).appliedTo(args)
-    case _ =>
-      tp
-
-  /** Under pureFunctions, add a @retainsByName(*)` annotation to the argument of
-   *  a by name parameter type, turning the latter into an impure by name parameter type.
-   */
-  def adaptByNameArgUnderPureFuns(using Context): Type =
-    if ccConfig.adaptUnpickledFunctionTypes && Feature.pureFunsEnabledSomewhere then
-      AnnotatedType(tp,
-        CaptureAnnotation(CaptureSet.universal, boxed = false)(defn.RetainsByNameAnnot))
-    else
-      tp
-
   /** Is type known to be always pure by its class structure,
    *  so that adding a capture set to it would not make sense?
    */

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

@@ -16,7 +16,6 @@ import util.{SimpleIdentitySet, Property}
 import typer.ErrorReporting.Addenda
 import util.common.alwaysTrue
 import scala.collection.mutable
-import config.Config.ccAllowUnsoundMaps
 
 /** A class for capture sets. Capture sets can be constants or variables.
  *  Capture sets support inclusion constraints <:< where <:< is subcapturing.
@@ -667,7 +666,7 @@ object CaptureSet:
 
     private def mapIsIdempotent = tm.isInstanceOf[IdempotentCaptRefMap]
 
-    assert(ccAllowUnsoundMaps || mapIsIdempotent, tm.getClass)
+    assert(ccConfig.allowUnsoundMaps || mapIsIdempotent, tm.getClass)
 
     private def whereCreated(using Context): String =
       if stack == null then ""
@@ -683,7 +682,9 @@ object CaptureSet:
           // `r` is _one_ possible solution in `source` that would make an `r` appear in this set.
           // It's not necessarily the only possible solution, so the scheme is incomplete.
           source.tryInclude(elem, this)
-        else if !mapIsIdempotent && variance <= 0 && !origin.isConst && (origin ne initial) && (origin ne source) then
+        else if ccConfig.allowUnsoundMaps && !mapIsIdempotent
+            && variance <= 0 && !origin.isConst && (origin ne initial) && (origin ne source)
+        then
           // The map is neither a BiTypeMap nor an idempotent type map.
           // In that case there's no much we can do.
           // The scheme then does not propagate added elements back to source and rejects adding
@@ -697,8 +698,11 @@ object CaptureSet:
       def propagateIf(cond: Boolean): CompareResult =
         if cond then propagate else CompareResult.OK
 
-      if origin eq source then // elements have to be mapped
-        val mapped = extrapolateCaptureRef(elem, tm, variance)
+      val mapped = extrapolateCaptureRef(elem, tm, variance)
+      def isFixpoint =
+        mapped.isConst && mapped.elems.size == 1 && mapped.elems.contains(elem)
+
+      def addMapped =
         val added = mapped.elems.filter(!accountsFor(_))
         addNewElems(added)
           .andAlso:
@@ -707,6 +711,35 @@ object CaptureSet:
             else CompareResult.Fail(this :: Nil)
           .andAlso:
             propagateIf(!added.isEmpty)
+
+      def failNoFixpoint =
+        val reason =
+          if variance <= 0 then i"the set's variance is $variance"
+          else i"$elem gets mapped to $mapped, which is not a supercapture."
+        report.warning(em"""trying to add $elem from unrecognized source $origin of mapped set $this$whereCreated
+                            |The reference cannot be added since $reason""")
+        CompareResult.Fail(this :: Nil)
+
+      if origin eq source then // elements have to be mapped
+        addMapped
+          .andAlso:
+            if mapped.isConst then CompareResult.OK
+            else if mapped.asVar.recordDepsState() then { addAsDependentTo(mapped); CompareResult.OK }
+            else CompareResult.Fail(this :: Nil)
+      else if !isFixpoint then
+        // We did not yet observe the !isFixpoint condition in our tests, but it's a
+        // theoretical possibility. In that case, it would be inconsistent to both
+        // add `elem` to the set and back-propagate it. But if `{elem} <:< tm(elem)`
+        // and the variance of the set is positive, we can soundly add `tm(ref)` to
+        // the set while back-propagating `ref` as before. Otherwise there's nothing
+        // obvious left to do except fail (which is always sound).
+        if variance > 0
+            && elem.singletonCaptureSet.subCaptures(mapped, frozen = true).isOK then
+          // widen to fixpoint. mapped is known to be a fixpoint since tm is idempotent.
+          // The widening is sound, but loses completeness.
+          addMapped
+        else
+          failNoFixpoint
       else if accountsFor(elem) then
         CompareResult.OK
       else

compiler/src/dotty/tools/dotc/config/Config.scala

@@ -235,15 +235,4 @@ object Config {
    */
   inline val checkLevelsOnConstraints = false
   inline val checkLevelsOnInstantiation = true
-
-  /** If true, print capturing types in the form `{c} T`.
-   *  If false, print them in the form `T @retains(c)`.
-   */
-  inline val printCaptureSetsAsPrefix = true
-
-  /** If true, allow mappping capture set variables under captureChecking with maps that are neither
-   *  bijective nor idempotent. We currently do now know how to do this correctly in all
-   *  cases, though.
-   */
-  inline val ccAllowUnsoundMaps = false
 }

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -32,7 +32,6 @@ import ast.{Trees, tpd, untpd}
 import Trees._
 import Decorators._
 import transform.SymUtils._
-import cc.{adaptFunctionTypeUnderPureFuns, adaptByNameArgUnderPureFuns}
 import dotty.tools.dotc.quoted.QuotePatterns
 
 import dotty.tools.tasty.{TastyBuffer, TastyReader}
@@ -383,7 +382,7 @@ class TreeUnpickler(reader: TastyReader,
                 // Note that the lambda "rt => ..." is not equivalent to a wildcard closure!
                 // Eta expansion of the latter puts readType() out of the expression.
             case APPLIEDtype =>
-              postProcessFunction(readType().appliedTo(until(end)(readType())))
+              readType().appliedTo(until(end)(readType()))
             case TYPEBOUNDS =>
               val lo = readType()
               if nothingButMods(end) then
@@ -460,8 +459,7 @@ class TreeUnpickler(reader: TastyReader,
           val ref = readAddr()
           typeAtAddr.getOrElseUpdate(ref, forkAt(ref).readType())
         case BYNAMEtype =>
-          val arg = readType()
-          ExprType(if withPureFuns then arg else arg.adaptByNameArgUnderPureFuns)
+          ExprType(readType())
         case _ =>
           ConstantType(readConstant(tag))
       }
@@ -495,12 +493,6 @@ class TreeUnpickler(reader: TastyReader,
     def readTreeRef()(using Context): TermRef =
       readType().asInstanceOf[TermRef]
 
-    /** Under pureFunctions, map all function types to impure function types,
-     *  unless the unpickled class was also compiled with pureFunctions.
-     */
-    private def postProcessFunction(tp: Type)(using Context): Type =
-      if withPureFuns then tp else tp.adaptFunctionTypeUnderPureFuns
-
 // ------ Reading definitions -----------------------------------------------------
 
     private def nothingButMods(end: Addr): Boolean =
@@ -1240,8 +1232,7 @@ class TreeUnpickler(reader: TastyReader,
         case SINGLETONtpt =>
           SingletonTypeTree(readTree())
         case BYNAMEtpt =>
-          val arg = readTpt()
-          ByNameTypeTree(if withPureFuns then arg else arg.adaptByNameArgUnderPureFuns)
+          ByNameTypeTree(readTpt())
         case NAMEDARG =>
           NamedArg(readName(), readTree())
         case EXPLICITtpt =>
@@ -1453,7 +1444,7 @@ class TreeUnpickler(reader: TastyReader,
               val args = until(end)(readTpt())
               val tree = untpd.AppliedTypeTree(tycon, args)
               val ownType = ctx.typeAssigner.processAppliedType(tree, tycon.tpe.safeAppliedTo(args.tpes))
-              tree.withType(postProcessFunction(ownType))
+              tree.withType(ownType)
             case ANNOTATEDtpt =>
               Annotated(readTpt(), readTree())
             case LAMBDAtpt =>

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -33,7 +33,6 @@ import scala.collection.mutable
 import scala.collection.mutable.ListBuffer
 import scala.annotation.switch
 import reporting._
-import cc.{adaptFunctionTypeUnderPureFuns, adaptByNameArgUnderPureFuns}
 
 object Scala2Unpickler {
 
@@ -824,7 +823,7 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
         }
         val tycon = select(pre, sym)
         val args = until(end, () => readTypeRef())
-        if (sym == defn.ByNameParamClass2x) ExprType(args.head.adaptByNameArgUnderPureFuns)
+        if (sym == defn.ByNameParamClass2x) ExprType(args.head)
         else if (ctx.settings.scalajs.value && args.length == 2 &&
             sym.owner == JSDefinitions.jsdefn.ScalaJSJSPackageClass && sym == JSDefinitions.jsdefn.PseudoUnionClass) {
           // Treat Scala.js pseudo-unions as real unions, this requires a
@@ -833,7 +832,6 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
         }
         else if args.nonEmpty then
           tycon.safeAppliedTo(EtaExpandIfHK(sym.typeParams, args.map(translateTempPoly)))
-            .adaptFunctionTypeUnderPureFuns
         else if (sym.typeParams.nonEmpty) tycon.EtaExpand(sym.typeParams)
         else tycon
       case TYPEBOUNDStpe =>

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -640,8 +640,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
           try changePrec(GlobalPrec)(toText(arg) ~ "^" ~ toTextCaptureSet(captureSet))
           catch case ex: IllegalCaptureRef => toTextAnnot
         if annot.symbol.maybeOwner == defn.RetainsAnnot
-            && Feature.ccEnabled
-            && Config.printCaptureSetsAsPrefix && !printDebug
+            && Feature.ccEnabled && !printDebug
             && Phases.checkCapturesPhase.exists // might be missing on -Ytest-pickler
         then toTextRetainsAnnot
         else toTextAnnot