Inferring tracked

.github/workflows/ci.yaml

@@ -26,10 +26,10 @@ on:
     - cron: '0 3 * * *'  # Every day at 3 AM
   workflow_dispatch:
 
-# Cancels any in-progress runs within the same group identified by workflow name and GH reference (branch or tag) 
+# Cancels any in-progress runs within the same group identified by workflow name and GH reference (branch or tag)
 # For example it would:
 # - terminate previous PR CI execution after pushing more changes to the same PR branch
-# - terminate previous on-push CI run after merging new PR to main 
+# - terminate previous on-push CI run after merging new PR to main
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
@@ -831,7 +831,7 @@ jobs:
           path: .
       - name: Prepare MSI package
         shell: bash
-        run: | 
+        run: |
           msiInstaller="scala3-${{ env.RELEASE_TAG }}.msi"
           mv scala.msi "${msiInstaller}"
           sha256sum "${msiInstaller}" > "${msiInstaller}.sha256"
@@ -870,7 +870,7 @@ jobs:
       #       ${upload("tar.gz archive", s"$filename.tar.gz", "application/gzip", distribution)}
       #       ${upload("tar.gz archive SHA", s"$filename.tar.gz.sha256", "text/plain", distribution)}
       # """
-      # def uploadMSI() = 
+      # def uploadMSI() =
       #     val distribution = "Windows x86_64 MSI"
       #     s"""
       #       # $distribution

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1052,8 +1052,8 @@ object desugar {
         // we can reuse the constructor parameters; no derived params are needed.
         DefDef(
           className.toTermName, joinParams(constrTparams, defParamss), classTypeRef, creatorExpr
-        ) .withMods(companionMods | mods.flags.toTermFlags & (GivenOrImplicit | Inline) | finalFlag)
-          .withSpan(cdef.span) :: Nil
+        ).withMods(companionMods | mods.flags.toTermFlags & (GivenOrImplicit | Inline) | finalFlag)
+         .withSpan(cdef.span) :: Nil
       }
 
     val self1 = {

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -2752,6 +2752,9 @@ object SymDenotations {
     /** Sets all missing fields of given denotation */
     def complete(denot: SymDenotation)(using Context): Unit
 
+    /** Is this a completer for an explicit type tree */
+    def isNonInfering: Boolean = false
+
     def apply(sym: Symbol): LazyType = this
     def apply(module: TermSymbol, modcls: ClassSymbol): LazyType = this
 

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -278,6 +278,9 @@ class Namer { typer: Typer =>
                 if rhs.isEmpty || flags.is(Opaque) then flags |= Deferred
             if flags.is(Param) then tree.rhs else analyzeRHS(tree.rhs)
 
+        def isNonInferingTree(tree: ValOrDefDef): Boolean =
+          !tree.tpt.isEmpty || tree.mods.isOneOf(TermParamOrAccessor)
+
         // to complete a constructor, move one context further out -- this
         // is the context enclosing the class. Note that the context in which a
         // constructor is recorded and the context in which it is completed are
@@ -291,6 +294,8 @@ class Namer { typer: Typer =>
 
         val completer = tree match
           case tree: TypeDef => TypeDefCompleter(tree)(cctx)
+          case tree: ValOrDefDef if Feature.enabled(Feature.modularity) && isNonInferingTree(tree) =>
+            NonInferingCompleter(tree)(cctx)
           case _ => Completer(tree)(cctx)
         val info = adjustIfModule(completer, tree)
         createOrRefine[Symbol](tree, name, flags, ctx.owner, _ => info,
@@ -1733,6 +1738,10 @@ class Namer { typer: Typer =>
     }
   }
 
+  class NonInferingCompleter(original: ValOrDefDef)(ictx: Context) extends Completer(original)(ictx) {
+    override def isNonInfering: Boolean = true
+  }
+
   /** Possible actions to perform when deciding on a forwarder for a member */
   private enum CanForward:
     case Yes
@@ -1782,7 +1791,7 @@ class Namer { typer: Typer =>
     sym.owner.typeParams.foreach(_.ensureCompleted())
     completeTrailingParamss(constr, sym, indexingCtor = true)
     if Feature.enabled(modularity) then
-      constr.termParamss.foreach(_.foreach(setTracked))
+      constr.termParamss.foreach(_.foreach(setTrackedConstrParam))
 
   /** The signature of a module valdef.
    *  This will compute the corresponding module class TypeRef immediately
@@ -1922,22 +1931,24 @@ class Namer { typer: Typer =>
     def wrapRefinedMethType(restpe: Type): Type =
       wrapMethType(addParamRefinements(restpe, paramSymss))
 
+    def addTrackedIfNeeded(ddef: DefDef, owningSym: Symbol): Unit =
+      for params <- ddef.termParamss; param <- params do
+        val psym = symbolOfTree(param)
+        if needsTracked(psym, param, owningSym) then
+          psym.setFlag(Tracked)
+          setParamTrackedWithAccessors(psym, sym.maybeOwner.infoOrCompleter)
+
+    if Feature.enabled(modularity) then addTrackedIfNeeded(ddef, sym.maybeOwner)
+
     if isConstructor then
       // set result type tree to unit, but take the current class as result type of the symbol
       typedAheadType(ddef.tpt, defn.UnitType)
       val mt = wrapMethType(effectiveResultType(sym, paramSymss))
       if sym.isPrimaryConstructor then checkCaseClassParamDependencies(mt, sym.owner)
       mt
-    else if sym.isAllOf(Given | Method) && Feature.enabled(modularity) then
-      // set every context bound evidence parameter of a given companion method
-      // to be tracked, provided it has a type that has an abstract type member.
-      // Add refinements for all tracked parameters to the result type.
-      for params <- ddef.termParamss; param <- params do
-        val psym = symbolOfTree(param)
-        if needsTracked(psym, param) then psym.setFlag(Tracked)
-      valOrDefDefSig(ddef, sym, paramSymss, wrapRefinedMethType)
     else
-      valOrDefDefSig(ddef, sym, paramSymss, wrapMethType)
+      val paramFn = if Feature.enabled(Feature.modularity) && sym.isAllOf(Given | Method) then wrapRefinedMethType else wrapMethType
+      valOrDefDefSig(ddef, sym, paramSymss, paramFn)
   end defDefSig
 
   /** Complete the trailing parameters of a DefDef,
@@ -1986,36 +1997,97 @@ class Namer { typer: Typer =>
             cls.srcPos)
       case _ =>
 
-  /** Under x.modularity, we add `tracked` to context bound witnesses
-   *  that have abstract type members
+  private def setParamTrackedWithAccessors(psym: Symbol, ownerTpe: Type)(using Context): Unit =
+    for acc <- ownerTpe.decls.lookupAll(psym.name) if acc.is(ParamAccessor) do
+      acc.resetFlag(PrivateLocal)
+      psym.setFlag(Tracked)
+      acc.setFlag(Tracked)
+
+  /** `psym` needs tracked if it is referenced in any of the public signatures
+   *  of the defining class or when `psym` is a context bound witness with an
+   *  abstract type member
+   */
+  def needsTracked(psym: Symbol, param: ValDef, owningSym: Symbol)(using Context) =
+    lazy val abstractContextBound = isContextBoundWitnessWithAbstractMembers(psym, param, owningSym)
+    lazy val isRefInSignatures =
+      psym.maybeOwner.isPrimaryConstructor
+      && isReferencedInPublicSignatures(psym)
+    !psym.is(Tracked)
+    && psym.isTerm
+    && (
+      abstractContextBound
+      || isRefInSignatures
+    )
+
+  /** Under x.modularity, we add `tracked` to context bound witnesses and
+   *  explicit evidence parameters that have abstract type members
    */
-  def needsTracked(sym: Symbol, param: ValDef)(using Context) =
-    !sym.is(Tracked)
-    && param.hasAttachment(ContextBoundParam)
-    && sym.info.memberNames(abstractTypeNameFilter).nonEmpty
-
-  /** Under x.modularity, set every context bound evidence parameter of a class to be tracked,
-   *  provided it has a type that has an abstract type member. Reset private and local flags
-   *  so that the parameter becomes a `val`.
+  private def isContextBoundWitnessWithAbstractMembers(psym: Symbol, param: ValDef, owningSym: Symbol)(using Context): Boolean =
+    val accessorSyms = maybeParamAccessors(owningSym, psym)
+    (owningSym.isClass || owningSym.isAllOf(Given | Method))
+    && (param.hasAttachment(ContextBoundParam) || (psym.isOneOf(GivenOrImplicit) && !accessorSyms.forall(_.isOneOf(PrivateLocal))))
+    && psym.info.memberNames(abstractTypeNameFilter).nonEmpty
+
+  extension (sym: Symbol)
+    private def infoWithForceNonInferingCompleter(using Context): Type = sym.infoOrCompleter match
+      case tpe: LazyType if tpe.isNonInfering => sym.info
+      case tpe if sym.isType => sym.info
+      case info => info
+
+  /** Under x.modularity, we add `tracked` to term parameters whose types are
+   *  referenced in public signatures of the defining class
+   */
+  private def isReferencedInPublicSignatures(sym: Symbol)(using Context): Boolean =
+    val owner = sym.maybeOwner.maybeOwner
+    val accessorSyms = maybeParamAccessors(owner, sym)
+    def checkOwnerMemberSignatures(owner: Symbol): Boolean =
+      owner.infoOrCompleter match
+        case info: ClassInfo =>
+          info.decls.filter(_.isPublic)
+            .filter(_ != sym.maybeOwner)
+            .exists { decl =>
+              tpeContainsSymbolRef(decl.infoWithForceNonInferingCompleter, accessorSyms)
+            }
+        case _ => false
+    checkOwnerMemberSignatures(owner)
+
+  /** Check if any of syms are referenced in tpe */
+  private def tpeContainsSymbolRef(tpe: Type, syms: List[Symbol])(using Context): Boolean =
+    val acc = new ExistsAccumulator(
+      { tpe => tpe.termSymbol.exists && syms.contains(tpe.termSymbol) },
+      StopAt.Static,
+      forceLazy = false
+    ) {
+      override def apply(acc: Boolean, tpe: Type): Boolean = super.apply(acc, tpe.safeDealias)
+    }
+    acc(false, tpe)
+
+  private def maybeParamAccessors(owner: Symbol, sym: Symbol)(using Context): List[Symbol] = owner.infoOrCompleter match
+    case info: ClassInfo =>
+      info.decls.lookupAll(sym.name).filter(d => d.is(ParamAccessor)).toList
+    case _ => List(sym)
+
+  /** Under x.modularity, set every context bound evidence parameter or public
+   *  using parameter of a class to be tracked, provided it has a type that has
+   *  an abstract type member. Reset private and local flags so that the
+   *  parameter becomes a `val`.
    */
-  def setTracked(param: ValDef)(using Context): Unit =
+  def setTrackedConstrParam(param: ValDef)(using Context): Unit =
     val sym = symbolOfTree(param)
     sym.maybeOwner.maybeOwner.infoOrCompleter match
-      case info: ClassInfo if needsTracked(sym, param) =>
+      case info: ClassInfo
+        if !sym.is(Tracked) && isContextBoundWitnessWithAbstractMembers(sym, param, sym.maybeOwner.maybeOwner) =>
         typr.println(i"set tracked $param, $sym: ${sym.info} containing ${sym.info.memberNames(abstractTypeNameFilter).toList}")
-        for acc <- info.decls.lookupAll(sym.name) if acc.is(ParamAccessor) do
-          acc.resetFlag(PrivateLocal)
-          acc.setFlag(Tracked)
-          sym.setFlag(Tracked)
+        setParamTrackedWithAccessors(sym, info)
       case _ =>
 
   def inferredResultType(
-      mdef: ValOrDefDef,
-      sym: Symbol,
-      paramss: List[List[Symbol]],
-      paramFn: Type => Type,
-      fallbackProto: Type
-    )(using Context): Type =
+    mdef: ValOrDefDef,
+    sym: Symbol,
+    paramss: List[List[Symbol]],
+    paramFn: Type => Type,
+    fallbackProto: Type
+  )(using Context): Type =
 
     /** A type for this definition that might be inherited from elsewhere:
      *  If this is a setter parameter, the corresponding getter type.

docs/_docs/reference/experimental/modularity.md

@@ -116,6 +116,37 @@ ClsParam  ::=  {Annotation} [{Modifier | ‘tracked’} (‘val’ | ‘var’)]
 
 The (soft) `tracked` modifier is only allowed for `val` parameters of classes.
 
+**Tracked inference**
+
+In some cases `tracked` can be infered and doesn't have to be written
+explicitly. A common such case is when a class parameter is referenced in the
+signatures of the public members of the class. e.g.
+```scala 3
+class OrdSet(val ord: Ordering) {
+  type Set = List[ord.T]
+  def empty: Set = Nil
+
+  implicit class helper(s: Set) {
+    def add(x: ord.T): Set = x :: remove(x)
+    def remove(x: ord.T): Set = s.filter(e => ord.compare(x, e) != 0)
+    def member(x: ord.T): Boolean = s.exists(e => ord.compare(x, e) == 0)
+  }
+}
+```
+In the example above, `ord` is referenced in the signatures of the public
+members of `OrdSet`, so a `tracked` modifier will be inserted automatically.
+
+Another common case is when a context bound has an associated type (i.e. an abstract type member) e.g.
+```scala 3
+trait TC:
+  type Self
+  type T
+
+class Klass[A: {TC as tc}]
+```
+
+Here, `tc` is a context bound with an associated type `T`, so `tracked` will be inferred for `tc`.
+
 **Discussion**
 
 Since `tracked` is so useful, why not assume it by default? First, `tracked` makes sense only for `val` parameters. If a class parameter is not also a field declared using `val` then there's nothing to refine in the constructor result type. One could think of at least making all `val` parameters tracked by default, but that would be a backwards incompatible change. For instance, the following code would break:

tests/neg/infer-tracked-explicit-witness.scala

@@ -0,0 +1,18 @@
+import scala.language.experimental.modularity
+
+trait T:
+  type Self
+  type X
+  def foo: Self
+
+class D[C](using wd: C is T)
+class E(using we: Int is T)
+
+def Test =
+  given w: Int is T:
+    def foo: Int = 42
+    type X = Long
+  val d = D(using w)
+  summon[d.wd.X =:= Long] // error
+  val e = E(using w)
+  summon[e.we.X =:= Long] // error

tests/pos/infer-tracked-1.scala

@@ -0,0 +1,34 @@
+import scala.language.experimental.modularity
+import scala.language.future
+
+trait Ordering {
+  type T
+  def compare(t1:T, t2: T): Int
+}
+
+class SetFunctor(val ord: Ordering) {
+  type Set = List[ord.T]
+  def empty: Set = Nil
+
+  implicit class helper(s: Set) {
+    def add(x: ord.T): Set = x :: remove(x)
+    def remove(x: ord.T): Set = s.filter(e => ord.compare(x, e) != 0)
+    def member(x: ord.T): Boolean = s.exists(e => ord.compare(x, e) == 0)
+  }
+}
+
+object Test {
+  val orderInt = new Ordering {
+    type T = Int
+    def compare(t1: T, t2: T): Int = t1 - t2
+  }
+
+  val IntSet = new SetFunctor(orderInt)
+  import IntSet.*
+
+  def main(args: Array[String]) = {
+    val set = IntSet.empty.add(6).add(8).add(23)
+    assert(!set.member(7))
+    assert(set.member(8))
+  }
+}

tests/pos/infer-tracked-explicit-witness.scala

@@ -0,0 +1,18 @@
+import scala.language.experimental.modularity
+
+trait T:
+  type Self
+  type X
+  def foo: Self
+
+class D[C](using val wd: C is T)
+class E(using val we: Int is T)
+
+def Test =
+  given w: Int is T:
+    def foo: Int = 42
+    type X = Long
+  val d = D(using w)
+  summon[d.wd.X =:= Long]
+  val e = E(using w)
+  summon[e.we.X =:= Long]

tests/pos/infer-tracked-parent-refinements.scala

@@ -0,0 +1,8 @@
+import scala.language.experimental.modularity
+import scala.language.future
+
+trait WithValue { type Value = Int }
+
+case class Year(value: Int) extends WithValue {
+  val x: Value = 2
+}