Backport "Context Bounds for Polymorphic Functions" to 3.6 (#21972)

Backports #21643 to the 3.6.2.

PR submitted by the release tooling.

Author: WojciechMazur

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

@@ -52,6 +52,10 @@ object desugar {
    */
   val ContextBoundParam: Property.Key[Unit] = Property.StickyKey()
 
+  /** Marks a poly fcuntion apply method, so that we can handle adding evidence parameters to them in a special way
+   */
+  val PolyFunctionApply: Property.Key[Unit] = Property.StickyKey()
+
   /** What static check should be applied to a Match? */
   enum MatchCheck {
     case None, Exhaustive, IrrefutablePatDef, IrrefutableGenFrom
@@ -242,7 +246,7 @@ object desugar {
    *      def f$default$2[T](x: Int) = x + "m"
    */
   private def defDef(meth: DefDef, isPrimaryConstructor: Boolean = false)(using Context): Tree =
-    addDefaultGetters(elimContextBounds(meth, isPrimaryConstructor))
+    addDefaultGetters(elimContextBounds(meth, isPrimaryConstructor).asInstanceOf[DefDef])
 
   /** Drop context bounds in given TypeDef, replacing them with evidence ValDefs that
    *  get added to a buffer.
@@ -304,10 +308,8 @@ object desugar {
       tdef1
   end desugarContextBounds
 
-  private def elimContextBounds(meth: DefDef, isPrimaryConstructor: Boolean)(using Context): DefDef =
-    val DefDef(_, paramss, tpt, rhs) = meth
+  def elimContextBounds(meth: Tree, isPrimaryConstructor: Boolean = false)(using Context): Tree =
     val evidenceParamBuf = mutable.ListBuffer[ValDef]()
-
     var seenContextBounds: Int = 0
     def freshName(unused: Tree) =
       seenContextBounds += 1 // Start at 1 like FreshNameCreator.
@@ -317,7 +319,7 @@ object desugar {
       // parameters of the method since shadowing does not affect
       // implicit resolution in Scala 3.
 
-    val paramssNoContextBounds =
+    def paramssNoContextBounds(paramss: List[ParamClause]): List[ParamClause] =
       val iflag = paramss.lastOption.flatMap(_.headOption) match
         case Some(param) if param.mods.isOneOf(GivenOrImplicit) =>
           param.mods.flags & GivenOrImplicit
@@ -329,15 +331,32 @@ object desugar {
         tparam => desugarContextBounds(tparam, evidenceParamBuf, flags, freshName, paramss)
       }(identity)
 
-    rhs match
-      case MacroTree(call) =>
-        cpy.DefDef(meth)(rhs = call).withMods(meth.mods | Macro | Erased)
-      case _ =>
-        addEvidenceParams(
-          cpy.DefDef(meth)(
-            name = normalizeName(meth, tpt).asTermName,
-            paramss = paramssNoContextBounds),
-          evidenceParamBuf.toList)
+    meth match
+      case meth @ DefDef(_, paramss, tpt, rhs) =>
+        val newParamss = paramssNoContextBounds(paramss)
+        rhs match
+          case MacroTree(call) =>
+            cpy.DefDef(meth)(rhs = call).withMods(meth.mods | Macro | Erased)
+          case _ =>
+            addEvidenceParams(
+              cpy.DefDef(meth)(
+                name = normalizeName(meth, tpt).asTermName,
+                paramss = newParamss
+              ),
+              evidenceParamBuf.toList
+            )
+      case meth @ PolyFunction(tparams, fun) =>
+        val PolyFunction(tparams: List[untpd.TypeDef] @unchecked, fun) = meth: @unchecked
+        val Function(vparams: List[untpd.ValDef] @unchecked, rhs) = fun: @unchecked
+        val newParamss = paramssNoContextBounds(tparams :: vparams :: Nil)
+        val params = evidenceParamBuf.toList
+        if params.isEmpty then
+          meth
+        else
+          val boundNames = getBoundNames(params, newParamss)
+          val recur = fitEvidenceParams(params, nme.apply, boundNames)
+          val (paramsFst, paramsSnd) = recur(newParamss)
+          functionsOf((paramsFst ++ paramsSnd).filter(_.nonEmpty), rhs)
   end elimContextBounds
 
   def addDefaultGetters(meth: DefDef)(using Context): Tree =
@@ -465,6 +484,74 @@ object desugar {
       case _ =>
         (Nil, tree)
 
+  private def referencesName(vdef: ValDef, names: Set[TermName])(using Context): Boolean =
+    vdef.tpt.existsSubTree:
+      case Ident(name: TermName) => names.contains(name)
+      case _ => false
+
+  /** Fit evidence `params` into the `mparamss` parameter lists, making sure
+   * that all parameters referencing `params` are after them.
+   * - for methods the final parameter lists are := result._1 ++ result._2
+   * - for poly functions, each element of the pair contains at most one term
+   *   parameter list
+   *
+   * @param params the evidence parameters list that should fit into `mparamss`
+   * @param methName the name of the method that `mparamss` belongs to
+   * @param boundNames the names of the evidence parameters
+   * @param mparamss the original parameter lists of the method
+   * @return a pair of parameter lists containing all parameter lists in a
+   * reference-correct order; make sure that `params` is always at the
+   * intersection of the pair elements; this is relevant, for poly functions
+   * where `mparamss` is guaranteed to have exectly one term parameter list,
+   * then each pair element will have at most one term parameter list
+   */
+  private def fitEvidenceParams(
+    params: List[ValDef],
+    methName: Name,
+    boundNames: Set[TermName]
+  )(mparamss: List[ParamClause])(using Context): (List[ParamClause], List[ParamClause]) = mparamss match
+    case ValDefs(mparams) :: _ if mparams.exists(referencesName(_, boundNames)) =>
+      (params :: Nil) -> mparamss
+    case ValDefs(mparams @ (mparam :: _)) :: Nil if mparam.mods.isOneOf(GivenOrImplicit) =>
+      val normParams =
+        if params.head.mods.flags.is(Given) != mparam.mods.flags.is(Given) then
+          params.map: param =>
+            val normFlags = param.mods.flags &~ GivenOrImplicit | (mparam.mods.flags & (GivenOrImplicit))
+            param.withMods(param.mods.withFlags(normFlags))
+              .showing(i"adapted param $result ${result.mods.flags} for ${methName}", Printers.desugar)
+        else params
+      ((normParams ++ mparams) :: Nil) -> Nil
+    case mparams :: mparamss1 =>
+      val (fst, snd) = fitEvidenceParams(params, methName, boundNames)(mparamss1)
+      (mparams :: fst) -> snd
+    case Nil =>
+      Nil -> (params :: Nil)
+
+  /** Create a chain of possibly contextual functions from the parameter lists */
+  private def functionsOf(paramss: List[ParamClause], rhs: Tree)(using Context): Tree = paramss match
+    case Nil => rhs
+    case ValDefs(head @ (fst :: _)) :: rest if fst.mods.isOneOf(GivenOrImplicit) =>
+      val paramTpts = head.map(_.tpt)
+      val paramNames = head.map(_.name)
+      val paramsErased = head.map(_.mods.flags.is(Erased))
+      makeContextualFunction(paramTpts, paramNames, functionsOf(rest, rhs), paramsErased).withSpan(rhs.span)
+    case ValDefs(head) :: rest =>
+      Function(head, functionsOf(rest, rhs))
+    case TypeDefs(head) :: rest =>
+      PolyFunction(head, functionsOf(rest, rhs))
+    case _ =>
+      assert(false, i"unexpected paramss $paramss")
+      EmptyTree
+
+  private def getBoundNames(params: List[ValDef], paramss: List[ParamClause])(using Context): Set[TermName] =
+    var boundNames = params.map(_.name).toSet // all evidence parameter + context bound proxy names
+    for mparams <- paramss; mparam <- mparams do
+      mparam match
+        case tparam: TypeDef if tparam.mods.annotations.exists(WitnessNamesAnnot.unapply(_).isDefined) =>
+          boundNames += tparam.name.toTermName
+        case _ =>
+    boundNames
+
   /**  Add all evidence parameters in `params` as implicit parameters to `meth`.
    *   The position of the added parameters is determined as follows:
    *
@@ -479,36 +566,23 @@ object desugar {
   private def addEvidenceParams(meth: DefDef, params: List[ValDef])(using Context): DefDef =
     if params.isEmpty then return meth
 
-    var boundNames = params.map(_.name).toSet // all evidence parameter + context bound proxy names
-    for mparams <- meth.paramss; mparam <- mparams do
-      mparam match
-        case tparam: TypeDef if tparam.mods.annotations.exists(WitnessNamesAnnot.unapply(_).isDefined) =>
-          boundNames += tparam.name.toTermName
-        case _ =>
+    val boundNames = getBoundNames(params, meth.paramss)
 
-    def referencesBoundName(vdef: ValDef): Boolean =
-      vdef.tpt.existsSubTree:
-        case Ident(name: TermName) => boundNames.contains(name)
-        case _ => false
+    val fitParams = fitEvidenceParams(params, meth.name, boundNames)
 
-    def recur(mparamss: List[ParamClause]): List[ParamClause] = mparamss match
-      case ValDefs(mparams) :: _ if mparams.exists(referencesBoundName) =>
-        params :: mparamss
-      case ValDefs(mparams @ (mparam :: _)) :: Nil if mparam.mods.isOneOf(GivenOrImplicit) =>
-        val normParams =
-          if params.head.mods.flags.is(Given) != mparam.mods.flags.is(Given) then
-            params.map: param =>
-              val normFlags = param.mods.flags &~ GivenOrImplicit | (mparam.mods.flags & (GivenOrImplicit))
-              param.withMods(param.mods.withFlags(normFlags))
-                .showing(i"adapted param $result ${result.mods.flags} for ${meth.name}", Printers.desugar)
-          else params
-        (normParams ++ mparams) :: Nil
-      case mparams :: mparamss1 =>
-        mparams :: recur(mparamss1)
-      case Nil =>
-        params :: Nil
-
-    cpy.DefDef(meth)(paramss = recur(meth.paramss))
+    if meth.removeAttachment(PolyFunctionApply).isDefined then
+      // for PolyFunctions we are limited to a single term param list, so we
+      // reuse the fitEvidenceParams logic to compute the new parameter lists
+      // and then we add the other parameter lists as function types to the
+      // return type
+      val (paramsFst, paramsSnd) = fitParams(meth.paramss)
+      if ctx.mode.is(Mode.Type) then
+        cpy.DefDef(meth)(paramss = paramsFst, tpt = functionsOf(paramsSnd, meth.tpt))
+      else
+        cpy.DefDef(meth)(paramss = paramsFst, rhs = functionsOf(paramsSnd, meth.rhs))
+    else
+      val (paramsFst, paramsSnd) = fitParams(meth.paramss)
+      cpy.DefDef(meth)(paramss = paramsFst ++ paramsSnd)
   end addEvidenceParams
 
   /** The parameters generated from the contextual bounds of `meth`, as generated by `desugar.defDef` */
@@ -1224,27 +1298,29 @@ object desugar {
   /** Desugar [T_1, ..., T_M] => (P_1, ..., P_N) => R
    *  Into    scala.PolyFunction { def apply[T_1, ..., T_M](x$1: P_1, ..., x$N: P_N): R }
    */
-  def makePolyFunctionType(tree: PolyFunction)(using Context): RefinedTypeTree =
-    val PolyFunction(tparams: List[untpd.TypeDef] @unchecked, fun @ untpd.Function(vparamTypes, res)) = tree: @unchecked
-    val paramFlags = fun match
-      case fun: FunctionWithMods =>
-        // TODO: make use of this in the desugaring when pureFuns is enabled.
-        // val isImpure = funFlags.is(Impure)
-
-        // Function flags to be propagated to each parameter in the desugared method type.
-        val givenFlag = fun.mods.flags.toTermFlags & Given
-        fun.erasedParams.map(isErased => if isErased then givenFlag | Erased else givenFlag)
-      case _ =>
-        vparamTypes.map(_ => EmptyFlags)
-
-    val vparams = vparamTypes.lazyZip(paramFlags).zipWithIndex.map {
-      case ((p: ValDef, paramFlags), n) => p.withAddedFlags(paramFlags)
-      case ((p, paramFlags), n) => makeSyntheticParameter(n + 1, p).withAddedFlags(paramFlags)
-    }.toList
-
-    RefinedTypeTree(ref(defn.PolyFunctionType), List(
-       DefDef(nme.apply, tparams :: vparams :: Nil, res, EmptyTree).withFlags(Synthetic)
-    )).withSpan(tree.span)
+  def makePolyFunctionType(tree: PolyFunction)(using Context): RefinedTypeTree = (tree: @unchecked) match
+    case PolyFunction(tparams: List[untpd.TypeDef] @unchecked, fun @ untpd.Function(vparamTypes, res)) =>
+      val paramFlags = fun match
+        case fun: FunctionWithMods =>
+          // TODO: make use of this in the desugaring when pureFuns is enabled.
+          // val isImpure = funFlags.is(Impure)
+
+          // Function flags to be propagated to each parameter in the desugared method type.
+          val givenFlag = fun.mods.flags.toTermFlags & Given
+          fun.erasedParams.map(isErased => if isErased then givenFlag | Erased else givenFlag)
+        case _ =>
+          vparamTypes.map(_ => EmptyFlags)
+
+      val vparams = vparamTypes.lazyZip(paramFlags).zipWithIndex.map {
+        case ((p: ValDef, paramFlags), n) => p.withAddedFlags(paramFlags)
+        case ((p, paramFlags), n) => makeSyntheticParameter(n + 1, p).withAddedFlags(paramFlags)
+      }.toList
+
+      RefinedTypeTree(ref(defn.PolyFunctionType), List(
+        DefDef(nme.apply, tparams :: vparams :: Nil, res, EmptyTree)
+          .withFlags(Synthetic)
+          .withAttachment(PolyFunctionApply, ())
+      )).withSpan(tree.span)
   end makePolyFunctionType
 
   /** Invent a name for an anonympus given of type or template `impl`. */

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3460,7 +3460,7 @@ object Parsers {
      *
      *  TypTypeParamClause::=  ‘[’ TypTypeParam {‘,’ TypTypeParam} ‘]’
      *  TypTypeParam      ::=  {Annotation}
-     *                         (id | ‘_’) [HkTypeParamClause] TypeBounds
+     *                         (id | ‘_’) [HkTypeParamClause] TypeAndCtxBounds
      *
      *  HkTypeParamClause ::=  ‘[’ HkTypeParam {‘,’ HkTypeParam} ‘]’
      *  HkTypeParam       ::=  {Annotation} [‘+’ | ‘-’]
@@ -3491,7 +3491,9 @@ object Parsers {
             else ident().toTypeName
           val hkparams = typeParamClauseOpt(ParamOwner.Hk)
           val bounds =
-            if paramOwner.acceptsCtxBounds then typeAndCtxBounds(name) else typeBounds()
+            if paramOwner.acceptsCtxBounds then typeAndCtxBounds(name)
+            else if in.featureEnabled(Feature.modularity) && paramOwner == ParamOwner.Type then typeAndCtxBounds(name)
+            else typeBounds()
           TypeDef(name, lambdaAbstract(hkparams, bounds)).withMods(mods)
         }
       }

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

@@ -1917,7 +1917,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
   def typedPolyFunction(tree: untpd.PolyFunction, pt: Type)(using Context): Tree =
     val tree1 = desugar.normalizePolyFunction(tree)
     if (ctx.mode is Mode.Type) typed(desugar.makePolyFunctionType(tree1), pt)
-    else typedPolyFunctionValue(tree1, pt)
+    else typedPolyFunctionValue(desugar.elimContextBounds(tree1).asInstanceOf[untpd.PolyFunction], pt)
 
   def typedPolyFunctionValue(tree: untpd.PolyFunction, pt: Type)(using Context): Tree =
     val untpd.PolyFunction(tparams: List[untpd.TypeDef] @unchecked, fun) = tree: @unchecked
@@ -2471,7 +2471,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val TypeDef(_, impl: Template) = typed(refineClsDef): @unchecked
     val refinements1 = impl.body
     val seen = mutable.Set[Symbol]()
-    for (refinement <- refinements1) { // TODO: get clarity whether we want to enforce these conditions
+    for refinement <- refinements1 do // TODO: get clarity whether we want to enforce these conditions
       typr.println(s"adding refinement $refinement")
       checkRefinementNonCyclic(refinement, refineCls, seen)
       val rsym = refinement.symbol
@@ -2485,7 +2485,6 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       val member = refineCls.info.member(rsym.name)
       if (member.isOverloaded)
         report.error(OverloadInRefinement(rsym), refinement.srcPos)
-    }
     assignType(cpy.RefinedTypeTree(tree)(tpt1, refinements1), tpt1, refinements1, refineCls)
   }