Use jl.Class'es instead of ClassTag's in structural calls.

This simplifies the life of everyone: the expansion of structural
calls, the implementation of `scala.reflect.Selectable`, and the
reverse engineering performed by the Scala.js back-end.

As a side benefit, the new transformation supports bottom types in
the arguments. It was previously limited because we do not allow
the generation of `ClassTag`s for `Null` and `Nothing`: they are
unsound in arrays, which is what `ClassTag`s are designed for, but
on their own they have perfectly valid `Class`'es.

Author: sjrd

compiler/src/dotty/tools/backend/sjs/JSCodeGen.scala

@@ -3199,26 +3199,24 @@ class JSCodeGen()(using genCtx: Context) {
   }
 
   /** Gen JS code for an asInstanceOf cast (for reference types only) */
-  private def genAsInstanceOf(value: js.Tree, to: Type)(
-      implicit pos: Position): js.Tree = {
-
-    val sym = to.typeSymbol
+  private def genAsInstanceOf(value: js.Tree, to: Type)(implicit pos: Position): js.Tree =
+    genAsInstanceOf(value, toIRType(to))
 
-    if (sym == defn.ObjectClass || sym.isJSType) {
-      /* asInstanceOf[Object] always succeeds, and
-       * asInstanceOf to a raw JS type is completely erased.
-       */
-      value
-    } else if (sym == defn.NullClass) {
-      js.If(
-          js.BinaryOp(js.BinaryOp.===, value, js.Null()),
-          js.Null(),
-          genThrowClassCastException())(
-          jstpe.NullType)
-    } else if (sym == defn.NothingClass) {
-      js.Block(value, genThrowClassCastException())
-    } else {
-      js.AsInstanceOf(value, toIRType(to))
+  /** Gen JS code for an asInstanceOf cast (for reference types only) */
+  private def genAsInstanceOf(value: js.Tree, to: jstpe.Type)(implicit pos: Position): js.Tree = {
+    to match {
+      case jstpe.AnyType =>
+        value
+      case jstpe.NullType =>
+        js.If(
+            js.BinaryOp(js.BinaryOp.===, value, js.Null()),
+            js.Null(),
+            genThrowClassCastException())(
+            jstpe.NullType)
+      case jstpe.NothingType =>
+        js.Block(value, genThrowClassCastException())
+      case _ =>
+        js.AsInstanceOf(value, to)
     }
   }
 
@@ -3558,7 +3556,7 @@ class JSCodeGen()(using genCtx: Context) {
    *  {{{
    *  reflectiveSelectable(structural).selectDynamic("foo")
    *  reflectiveSelectable(structural).applyDynamic("bar",
-   *    ClassTag(classOf[Int]), ClassTag(classOf[String])
+   *    classOf[Int], classOf[String]
    *  )(
    *    6, "hello"
    *  )
@@ -3569,7 +3567,7 @@ class JSCodeGen()(using genCtx: Context) {
    *  {{{
    *  reflectiveSelectable(structural).selectDynamic("foo") // same as above
    *  reflectiveSelectable(structural).applyDynamic("bar",
-   *    wrapRefArray([ ClassTag(classOf[Int]), ClassTag(classOf[String]) : ClassTag ]
+   *    wrapRefArray([ classOf[Int], classOf[String] : jl.Class ]
    *  )(
    *    genericWrapArray([ Int.box(6), "hello" : Object ])
    *  )
@@ -3599,7 +3597,7 @@ class JSCodeGen()(using genCtx: Context) {
    *
    *  - the original receiver `structural`
    *  - the method name as a compile-time string `foo` or `bar`
-   *  - the `tp: Type`s that have been wrapped in `ClassTag(classOf[tp])`, as a
+   *  - the `tp: Type`s that have been wrapped in `classOf[tp]`, as a
    *    compile-time List[Type], from which we'll derive `jstpe.Type`s for the
    *    `asInstanceOf`s and `jstpe.TypeRef`s for the `MethodName.reflectiveProxy`
    *  - the actual arguments as a compile-time `List[Tree]`
@@ -3648,38 +3646,30 @@ class JSCodeGen()(using genCtx: Context) {
     } else {
       // Extract the param type refs and actual args from the 2nd and 3rd argument to applyDynamic
       args.tail match {
-        case WrapArray(classTagsArray: JavaSeqLiteral) :: WrapArray(actualArgsAnyArray: JavaSeqLiteral) :: Nil =>
-          // Extract jstpe.Type's and jstpe.TypeRef's from the ClassTag.apply(_) trees
-          val formalParamTypesAndTypeRefs = classTagsArray.elems.map {
-            // ClassTag.apply(classOf[tp]) -> tp
-            case Apply(fun, Literal(const) :: Nil)
-                if fun.symbol == defn.ClassTagModule_apply && const.tag == Constants.ClazzTag =>
+        case WrapArray(classOfsArray: JavaSeqLiteral) :: WrapArray(actualArgsAnyArray: JavaSeqLiteral) :: Nil =>
+          // Extract jstpe.Type's and jstpe.TypeRef's from the classOf[_] trees
+          val formalParamTypesAndTypeRefs = classOfsArray.elems.map {
+            // classOf[tp] -> tp
+            case Literal(const) if const.tag == Constants.ClazzTag =>
               toIRTypeAndTypeRef(const.typeValue)
-            // ClassTag.SpecialType -> erasure(SepecialType.typeRef) (e.g., ClassTag.Any -> Object)
-            case Apply(Select(classTagModule, name), Nil)
-                if classTagModule.symbol == defn.ClassTagModule &&
-                    defn.SpecialClassTagClasses.exists(_.name == name.toTypeName) =>
-              toIRTypeAndTypeRef(TypeErasure.erasure(
-                  defn.SpecialClassTagClasses.find(_.name == name.toTypeName).get.typeRef))
             // Anything else is invalid
-            case classTag =>
+            case otherTree =>
               report.error(
-                  "The ClassTags passed to Selectable.applyDynamic must be " +
-                  "literal ClassTag(classOf[T]) expressions " +
-                  "(typically compiler-generated). " +
+                  "The java.lang.Class[_] arguments passed to Selectable.applyDynamic must be " +
+                  "literal classOf[T] expressions (typically compiler-generated). " +
                   "Other uses are not supported in Scala.js.",
-                  classTag.sourcePos)
+                  otherTree.sourcePos)
               (jstpe.AnyType, jstpe.ClassRef(jsNames.ObjectClass))
           }
 
           // Gen the actual args, downcasting them to the formal param types
           val actualArgs = actualArgsAnyArray.elems.zip(formalParamTypesAndTypeRefs).map {
             (actualArgAny, formalParamTypeAndTypeRef) =>
               val genActualArgAny = genExpr(actualArgAny)
-              js.AsInstanceOf(genActualArgAny, formalParamTypeAndTypeRef._1)(genActualArgAny.pos)
+              genAsInstanceOf(genActualArgAny, formalParamTypeAndTypeRef._1)(genActualArgAny.pos)
           }
 
-          (formalParamTypesAndTypeRefs.map(_._2), actualArgs)
+          (formalParamTypesAndTypeRefs.map(pair => toParamOrResultTypeRef(pair._2)), actualArgs)
 
         case _ =>
           report.error(

compiler/src/dotty/tools/backend/sjs/JSEncoding.scala

@@ -231,13 +231,8 @@ object JSEncoding {
   }
 
   /** Computes the type ref for a type, to be used in a method signature. */
-  private def paramOrResultTypeRef(tpe: Type)(using Context): jstpe.TypeRef = {
-    toTypeRef(tpe) match {
-      case jstpe.ClassRef(ScalaRuntimeNullClassName)    => jstpe.NullRef
-      case jstpe.ClassRef(ScalaRuntimeNothingClassName) => jstpe.NothingRef
-      case otherTypeRef                                 => otherTypeRef
-    }
-  }
+  private def paramOrResultTypeRef(tpe: Type)(using Context): jstpe.TypeRef =
+    toParamOrResultTypeRef(toTypeRef(tpe))
 
   def encodeLocalSym(sym: Symbol)(
       implicit ctx: Context, pos: ir.Position, localNames: LocalNameGenerator): js.LocalIdent = {
@@ -284,6 +279,15 @@ object JSEncoding {
       ClassName(sym1.javaClassName)
   }
 
+  /** Converts a general TypeRef to a TypeRef to be used in a method signature. */
+  def toParamOrResultTypeRef(typeRef: jstpe.TypeRef): jstpe.TypeRef = {
+    typeRef match {
+      case jstpe.ClassRef(ScalaRuntimeNullClassName)    => jstpe.NullRef
+      case jstpe.ClassRef(ScalaRuntimeNothingClassName) => jstpe.NothingRef
+      case _                                            => typeRef
+    }
+  }
+
   def toIRTypeAndTypeRef(tp: Type)(using Context): (jstpe.Type, jstpe.TypeRef) = {
     val typeRefInternal = toTypeRefInternal(tp)
     (toIRTypeInternal(typeRefInternal), typeRefInternal._1)

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

@@ -11,6 +11,7 @@ import dotty.tools.dotc.core.Names.{Name, TermName}
 import dotty.tools.dotc.core.StdNames._
 import dotty.tools.dotc.core.Types._
 import dotty.tools.dotc.core.Decorators._
+import dotty.tools.dotc.core.TypeErasure
 import util.Spans._
 import core.Symbols._
 import core.Definitions
@@ -149,22 +150,22 @@ trait Dynamic {
    *  x1.applyDynamic("a")(a11, ..., a1n, ..., aN1, ..., aNn)
    *    .asInstanceOf[R]
    *  ```
-   *  If this call resolves to an `applyDynamic` method that takes a `ClassTag[?]*` as second
+   *  If this call resolves to an `applyDynamic` method that takes a `Class[?]*` as second
    *  parameter, we further rewrite this call to
    *  scala```
-   *  x1.applyDynamic("a", CT11, ..., CT1n, ..., CTN1, ... CTNn)
+   *  x1.applyDynamic("a", c11, ..., c1n, ..., cN1, ... cNn)
    *                  (a11, ..., a1n, ..., aN1, ..., aNn)
    *    .asInstanceOf[R]
    *  ```
-   *  where CT11, ..., CTNn are the class tags representing the erasure of T11, ..., TNn.
+   *  where c11, ..., cNn are the classOf constants representing the erasures of T11, ..., TNn.
    *
    *  It's an error if U is neither a value nor a method type, or a dependent method
    *  type.
    */
   def handleStructural(tree: Tree)(using Context): Tree = {
     val (fun @ Select(qual, name), targs, vargss) = decomposeCall(tree)
 
-    def structuralCall(selectorName: TermName, ctags: => List[Tree]) = {
+    def structuralCall(selectorName: TermName, classOfs: => List[Tree]) = {
       val selectable = adapt(qual, defn.SelectableClass.typeRef)
 
       // ($qual: Selectable).$selectorName("$name")
@@ -177,27 +178,27 @@ trait Dynamic {
         if (vargss.isEmpty) base
         else untpd.Apply(base, vargss.flatten.map(untpd.TypedSplice(_)))
 
-      // If function is an `applyDynamic` that takes a ClassTag* parameter,
-      // add `ctags`.
-      def addClassTags(tree: Tree): Tree = tree match
+      // If function is an `applyDynamic` that takes a Class* parameter,
+      // add `classOfs`.
+      def addClassOfs(tree: Tree): Tree = tree match
         case Apply(fn: Apply, args) =>
-          cpy.Apply(tree)(addClassTags(fn), args)
+          cpy.Apply(tree)(addClassOfs(fn), args)
         case Apply(fn @ Select(_, nme.applyDynamic), nameArg :: _ :: Nil) =>
           fn.tpe.widen match
             case mt: MethodType => mt.paramInfos match
-              case _ :: ctagsParam :: Nil
-              if ctagsParam.isRepeatedParam
-                 && ctagsParam.argInfos.head.isRef(defn.ClassTagClass) =>
-                  val ctagType = defn.ClassTagClass.typeRef.appliedTo(TypeBounds.empty)
+              case _ :: classOfsParam :: Nil
+              if classOfsParam.isRepeatedParam
+                 && classOfsParam.argInfos.head.isRef(defn.ClassClass) =>
+                  val jlClassType = defn.ClassClass.typeRef.appliedTo(TypeBounds.empty)
                   cpy.Apply(tree)(fn,
-                    nameArg :: seqToRepeated(SeqLiteral(ctags, TypeTree(ctagType))) :: Nil)
+                    nameArg :: seqToRepeated(SeqLiteral(classOfs, TypeTree(jlClassType))) :: Nil)
               case _ => tree
             case other => tree
         case _ => tree
-      addClassTags(typed(scall))
+      addClassOfs(typed(scall))
     }
 
-    def fail(name: Name, reason: String) =
+    def fail(reason: String): Tree =
       errorTree(tree, em"Structural access not allowed on method $name because it $reason")
 
     fun.tpe.widen match {
@@ -215,18 +216,21 @@ trait Dynamic {
         }
 
         if (isDependentMethod(tpe))
-          fail(name, i"has a method type with inter-parameter dependencies")
+          fail(i"has a method type with inter-parameter dependencies")
         else {
-          def ctags = tpe.paramInfoss.flatten.map(pt =>
-            implicitArgTree(defn.ClassTagClass.typeRef.appliedTo(pt.widenDealias :: Nil), fun.span.endPos))
-          structuralCall(nme.applyDynamic, ctags).cast(tpe.finalResultType)
+          def classOfs =
+            if tpe.paramInfoss.nestedExists(!TypeErasure.hasStableErasure(_)) then
+              fail(i"has a parameter type with an unstable erasure") :: Nil
+            else
+              TypeErasure.erasure(tpe).asInstanceOf[MethodType].paramInfos.map(clsOf(_))
+          structuralCall(nme.applyDynamic, classOfs).cast(tpe.finalResultType)
         }
 
       // (@allanrenucci) I think everything below is dead code
       case _: PolyType =>
-        fail(name, "is polymorphic")
+        fail("is polymorphic")
       case tpe =>
-        fail(name, i"has an unsupported type: $tpe")
+        fail(i"has an unsupported type: $tpe")
     }
   }
 }

docs/docs/reference/changed-features/structural-types-spec.md

@@ -35,13 +35,17 @@ def selectDynamic(name: String): T
 ```
 Often, the return type `T` is `Any`.
 
-The `applyDynamic` method is used for selections that are applied to arguments. It takes a method name and possibly `ClassTag`s representing its parameters types as well as the arguments to pass to the function.
+Unlike `scala.Dynamic`, there is no special meaning for an `updateDynamic` method.
+However, we reserve the right to give it meaning in the future.
+Consequently, it is recommended not to define any member called `updateDynamic` in `Selectable`s.
+
+The `applyDynamic` method is used for selections that are applied to arguments. It takes a method name and possibly `Class`es representing its parameters types as well as the arguments to pass to the function.
 Its signature should be of one of the two following forms:
 ```scala
 def applyDynamic(name: String)(args: Any*): T
-def applyDynamic(name: String, ctags: ClassTag[?]*)(args: Any*): T
+def applyDynamic(name: String, ctags: Class[?]*)(args: Any*): T
 ```
-Both versions are passed the actual arguments in the `args` parameter. The second version takes in addition a vararg argument of class tags that identify the method's parameter classes. Such an argument is needed
+Both versions are passed the actual arguments in the `args` parameter. The second version takes in addition a vararg argument of `java.lang.Class`es that identify the method's parameter classes. Such an argument is needed
 if `applyDynamic` is implemented using Java reflection, but it could be
 useful in other cases as well. `selectDynamic` and `applyDynamic` can also take additional context parameters in using clauses. These are resolved in the normal way at the callsite.
 
@@ -58,13 +62,13 @@ and `Rs` are structural refinement declarations, and given `v.a` of type `U`, we
   v.applyDynamic("a")(a11, ..., a1n, ..., aN1, ..., aNn)
     .asInstanceOf[R]
   ```
-  If this call resolves to an `applyDynamic` method of the second form that takes a `ClassTag[?]*` argument, we further rewrite this call to
+  If this call resolves to an `applyDynamic` method of the second form that takes a `Class[?]*` argument, we further rewrite this call to
   ```scala
-  v.applyDynamic("a", CT11, ..., CT1n, ..., CTN1, ... CTNn)(
+  v.applyDynamic("a", c11, ..., c1n, ..., cN1, ... cNn)(
     a11, ..., a1n, ..., aN1, ..., aNn)
     .asInstanceOf[R]
   ```
-   where each `CT_ij` is the class tag of the type of the formal parameter `Tij`
+   where each `c_ij` is the literal `java.lang.Class[?]` of the type of the formal parameter `Tij`, i.e., `classOf[Tij]`.
 
 - If `U` is neither a value nor a method type, or a dependent method
   type, an error is emitted.

docs/docs/reference/changed-features/structural-types.md

@@ -175,8 +175,8 @@ differences.
   `Selectable` is a trait which declares the access operations.
 
 - Two access operations, `selectDynamic` and `applyDynamic` are shared
-  between both approaches. In `Selectable`, `applyDynamic` also takes
-  `ClassTag` indicating the method's formal parameter types. `Dynamic`
-  comes with `updateDynamic`.
+  between both approaches. In `Selectable`, `applyDynamic` also may also take
+  `java.lang.Class` arguments indicating the method's formal parameter types.
+  `Dynamic` comes with `updateDynamic`.
 
 [More details](structural-types-spec.md)

library/src/scala/Selectable.scala

@@ -13,12 +13,13 @@ package scala
  *  `applyDynamic` is invoked for selections with arguments `v.m(...)`.
  *  If there's only one kind of selection, the method supporting the
  *  other may be omitted. The `applyDynamic` can also have a second parameter
- *  list of class tag arguments, i.e. it may alternatively have the signature
+ *  list of `java.lang.Class` arguments, i.e. it may alternatively have the
+ *  signature
  *
- *    def applyDynamic(name: String, paramClasses: ClassTag[_]*)(args: Any*): Any
+ *    def applyDynamic(name: String, paramClasses: Class[_]*)(args: Any*): Any
  *
- *  In this case the call will synthesize `ClassTag` arguments for all formal parameter
- *  types of the method in the structural type.
+ *  In this case the call will synthesize `Class` arguments for the erasure of
+ *  all formal parameter types of the method in the structural type.
  */
 trait Selectable extends Any
 

library/src/scala/reflect/Selectable.scala

@@ -27,10 +27,9 @@ trait Selectable extends scala.Selectable:
    *  @param paramTypes The class tags of the selected method's formal parameter types
    *  @param args       The arguments to pass to the selected method
    */
-  def applyDynamic(name: String, paramTypes: ClassTag[_]*)(args: Any*): Any =
+  def applyDynamic(name: String, paramTypes: Class[_]*)(args: Any*): Any =
     val rcls = selectedValue.getClass
-    val paramClasses = paramTypes.map(_.runtimeClass)
-    val mth = rcls.getMethod(name, paramClasses: _*)
+    val mth = rcls.getMethod(name, paramTypes: _*)
     ensureAccessible(mth)
     mth.invoke(selectedValue, args.asInstanceOf[Seq[AnyRef]]: _*)
 

tests/neg-scalajs/reflective-calls.scala

@@ -9,7 +9,7 @@ object Test {
     val receiver: Any = ???
     reflectiveSelectable(receiver).selectDynamic("foo") // OK
     reflectiveSelectable(receiver).applyDynamic("foo")() // OK
-    reflectiveSelectable(receiver).applyDynamic("foo", ClassTag(classOf[String]), ClassTag(classOf[List[_]]))("bar", Nil) // OK
+    reflectiveSelectable(receiver).applyDynamic("foo", classOf[String], classOf[List[_]])("bar", Nil) // OK
   }
 
   def badReceider(): Unit = {
@@ -25,21 +25,21 @@ object Test {
     reflectiveSelectable(receiver).applyDynamic(methodName)() // error
   }
 
-  def nonLiteralClassTag(): Unit = {
+  def nonLiteralClassOf(): Unit = {
     val receiver: Any = ???
-    val myClassTag: ClassTag[String] = ClassTag(classOf[String])
-    reflectiveSelectable(receiver).applyDynamic("foo", myClassTag, ClassTag(classOf[List[_]]))("bar", Nil) // error
+    val myClassOf: Class[String] = classOf[String]
+    reflectiveSelectable(receiver).applyDynamic("foo", myClassOf, classOf[List[_]])("bar", Nil) // error
   }
 
-  def classTagVarArgs(): Unit = {
+  def classOfVarArgs(): Unit = {
     val receiver: Any = ???
-    val classTags: List[ClassTag[_]] = List(ClassTag(classOf[String]), ClassTag(classOf[List[_]]))
-    reflectiveSelectable(receiver).applyDynamic("foo", classTags: _*)("bar", Nil) // error
+    val classOfs: List[Class[_]] = List(classOf[String], classOf[List[_]])
+    reflectiveSelectable(receiver).applyDynamic("foo", classOfs: _*)("bar", Nil) // error
   }
 
   def argsVarArgs(): Unit = {
     val receiver: Any = ???
     val args: List[Any] = List("bar", Nil)
-    reflectiveSelectable(receiver).applyDynamic("foo", ClassTag(classOf[String]), ClassTag(classOf[List[_]]))(args: _*) // error
+    reflectiveSelectable(receiver).applyDynamic("foo", classOf[String], classOf[List[_]])(args: _*) // error
   }
 }

tests/neg/structural.scala

@@ -1,6 +1,6 @@
 object Test3 {
   import scala.reflect.Selectable.reflectiveSelectable
-  def g(x: { type T ; def t: T ; def f(a: T): Boolean }) = x.f(x.t) // error: no ClassTag for x.T
+  def g(x: { type T ; def t: T ; def f(a: T): Boolean }) = x.f(x.t) // error: it has a parameter type with an unstable erasure
   g(new { type T = Int; def t = 4; def f(a:T) = true })
   g(new { type T = Any; def t = 4; def f(a:T) = true })
   val y: { type T = Int; def t = 4; def f(a:T) = true }  // error: illegal refinement // error: illegal refinement

tests/semanticdb/expect/Advanced.expect.scala

@@ -26,7 +26,7 @@ object Test/*<-advanced::Test.*/ {
   val s2/*<-advanced::Test.s2.*/ = s/*->advanced::Test.s.*/.s2/*->advanced::Structural#s2().*/
   val s2x/*<-advanced::Test.s2x.*/ = /*->scala::reflect::Selectable.reflectiveSelectable().*/s/*->advanced::Test.s.*/.s2/*->advanced::Structural#s2().*//*->scala::reflect::Selectable#selectDynamic().*/.x
   val s3/*<-advanced::Test.s3.*/ = s/*->advanced::Test.s.*/.s3/*->advanced::Structural#s3().*/
-  val s3x/*<-advanced::Test.s3x.*/ = /*->scala::reflect::Selectable.reflectiveSelectable().*/s/*->advanced::Test.s.*/.s3/*->advanced::Structural#s3().*//*->scala::reflect::Selectable#applyDynamic().*/.m/*->scala::reflect::ClassTag.apply().*/(???/*->scala::Predef.`???`().*/)
+  val s3x/*<-advanced::Test.s3x.*/ = /*->scala::reflect::Selectable.reflectiveSelectable().*/s/*->advanced::Test.s.*/.s3/*->advanced::Structural#s3().*//*->scala::reflect::Selectable#applyDynamic().*/.m(???/*->scala::Predef.`???`().*/)
 
   val e/*<-advanced::Test.e.*/ = new Wildcards/*->advanced::Wildcards#*/
   val e1/*<-advanced::Test.e1.*/ = e/*->advanced::Test.e.*/.e1/*->advanced::Wildcards#e1().*/