Make the expandion of context bounds for poly types slightly more elegant

[Cherry-picked a736592]

Author: KacperFKorban

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

@@ -527,8 +527,7 @@ object desugar {
         makeContextualFunction(paramTpts, paramNames, tree, paramsErased).withSpan(tree.span)
 
     if meth.hasAttachment(PolyFunctionApply) then
-      meth.removeAttachment(PolyFunctionApply)
-      // (kπ): deffer this until we can type the result?
+      // meth.removeAttachment(PolyFunctionApply)
       if ctx.mode.is(Mode.Type) then
         cpy.DefDef(meth)(tpt = meth.tpt.withAttachment(PolyFunctionApply, params))
       else
@@ -1250,29 +1249,35 @@ 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)
-        .withAttachment(PolyFunctionApply, List.empty)
-    )).withSpan(tree.span)
+  def makePolyFunctionType(tree: PolyFunction)(using Context): RefinedTypeTree = tree 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, List.empty)
+      )).withSpan(tree.span)
+    case PolyFunction(tparams: List[untpd.TypeDef] @unchecked, res) =>
+      RefinedTypeTree(ref(defn.PolyFunctionType), List(
+        DefDef(nme.apply, tparams :: Nil, res, EmptyTree)
+          .withFlags(Synthetic)
+          .withAttachment(PolyFunctionApply, List.empty)
+      )).withSpan(tree.span)
   end makePolyFunctionType
 
   /** Invent a name for an anonympus given of type or template `impl`. */

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

@@ -3595,14 +3595,17 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
   private def pushDownDeferredEvidenceParams(tpe: Type, params: List[untpd.ValDef], span: Span)(using Context): Type = tpe.dealias match {
     case tpe: MethodType =>
-      MethodType(tpe.paramNames)(paramNames => tpe.paramInfos, _ => pushDownDeferredEvidenceParams(tpe.resultType, params, span))
+      tpe.derivedLambdaType(tpe.paramNames, tpe.paramInfos, pushDownDeferredEvidenceParams(tpe.resultType, params, span))
     case tpe: PolyType =>
-      PolyType(tpe.paramNames)(paramNames => tpe.paramInfos, _ => pushDownDeferredEvidenceParams(tpe.resultType, params, span))
+      tpe.derivedLambdaType(tpe.paramNames, tpe.paramInfos, pushDownDeferredEvidenceParams(tpe.resultType, params, span))
     case tpe: RefinedType =>
-      // TODO(kπ): Doesn't seem right, but the PolyFunction ends up being a refinement
-      RefinedType(pushDownDeferredEvidenceParams(tpe.parent, params, span), tpe.refinedName, pushDownDeferredEvidenceParams(tpe.refinedInfo, params, span))
+      tpe.derivedRefinedType(
+        pushDownDeferredEvidenceParams(tpe.parent, params, span),
+        tpe.refinedName,
+        pushDownDeferredEvidenceParams(tpe.refinedInfo, params, span)
+      )
     case tpe @ AppliedType(tycon, args) if defn.isFunctionType(tpe) && args.size > 1 =>
-      AppliedType(tpe.tycon, args.init :+ pushDownDeferredEvidenceParams(args.last, params, span))
+      tpe.derivedAppliedType(tycon, args.init :+ pushDownDeferredEvidenceParams(args.last, params, span))
     case tpe =>
       val paramNames = params.map(_.name)
       val paramTpts = params.map(_.tpt)
@@ -3611,18 +3614,52 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       typed(ctxFunction).tpe
   }
 
-  private def addDownDeferredEvidenceParams(tree: Tree, pt: Type)(using Context): (Tree, Type) = {
+  private def extractTopMethodTermParams(tpe: Type)(using Context): (List[TermName], List[Type]) = tpe match {
+    case tpe: MethodType =>
+      tpe.paramNames -> tpe.paramInfos
+    case tpe: RefinedType if defn.isFunctionType(tpe.parent) =>
+      extractTopMethodTermParams(tpe.refinedInfo)
+    case _ =>
+      Nil -> Nil
+  }
+
+  private def removeTopMethodTermParams(tpe: Type)(using Context): Type = tpe match {
+    case tpe: MethodType =>
+      tpe.resultType
+    case tpe: RefinedType if defn.isFunctionType(tpe.parent) =>
+      tpe.derivedRefinedType(tpe.parent, tpe.refinedName, removeTopMethodTermParams(tpe.refinedInfo))
+    case tpe: AppliedType if defn.isFunctionType(tpe) =>
+      tpe.args.last
+    case _ =>
+      tpe
+  }
+
+  private def healToPolyFunctionType(tree: Tree)(using Context): Tree = tree match {
+    case defdef: DefDef if defdef.name == nme.apply && defdef.paramss.forall(_.forall(_.symbol.flags.is(TypeParam))) && defdef.paramss.size == 1 =>
+      val (names, types) = extractTopMethodTermParams(defdef.tpt.tpe)
+      val newTpe = removeTopMethodTermParams(defdef.tpt.tpe)
+      val newParams = names.lazyZip(types).map((name, tpe) => SyntheticValDef(name, TypeTree(tpe), flags = SyntheticTermParam))
+      val newDefDef = cpy.DefDef(defdef)(paramss = defdef.paramss ++ List(newParams), tpt = untpd.TypeTree(newTpe))
+      val nestedCtx = ctx.fresh.setNewTyperState()
+      typed(newDefDef)(using nestedCtx)
+    case _ => tree
+  }
+
+  private def addDeferredEvidenceParams(tree: Tree, pt: Type)(using Context): (Tree, Type) = {
     tree.getAttachment(desugar.PolyFunctionApply) match
       case Some(params) if params.nonEmpty =>
         tree.removeAttachment(desugar.PolyFunctionApply)
         val tpe = pushDownDeferredEvidenceParams(tree.tpe, params, tree.span)
         TypeTree(tpe).withSpan(tree.span) -> tpe
+      // case Some(params) if params.isEmpty =>
+      //   println(s"tree: $tree")
+      //   healToPolyFunctionType(tree) -> pt
       case _ => tree -> pt
   }
 
   /** Interpolate and simplify the type of the given tree. */
   protected def simplify(tree: Tree, pt: Type, locked: TypeVars)(using Context): Tree =
-    val (tree1, pt1) = addDownDeferredEvidenceParams(tree, pt)
+    val (tree1, pt1) = addDeferredEvidenceParams(tree, pt)
     if !tree1.denot.isOverloaded then // for overloaded trees: resolve overloading before simplifying
       if !tree1.tpe.widen.isInstanceOf[MethodOrPoly] // wait with simplifying until method is fully applied
          || tree1.isDef                              // ... unless tree is a definition

tests/pos/contextbounds-for-poly-functions.scala

@@ -32,7 +32,7 @@ type CmpWeak[X] = X => Boolean
 type Comparer2Weak = [X: Ord] => X => CmpWeak[X]
 val less4_0: [X: Ord] => X => X => Boolean =
   [X: Ord] => (x: X) => (y: X) => summon[Ord[X]].compare(x, y) < 0
-val less4: Comparer2Weak =
+val less4_1: Comparer2Weak =
   [X: Ord] => (x: X) => (y: X) => summon[Ord[X]].compare(x, y) < 0
 
 val less5 = [X: [X] =>> Ord[X]] => (x: X, y: X) => summon[Ord[X]].compare(x, y) < 0