Synthesize classTags in Typer.

Now diagnoses missing ClassTags of abstract types
as implicit failures.

Also: Simpler API of tpd.clsOf.

Author: odersky

src/dotty/DottyPredef.scala

@@ -1,37 +1,13 @@
 package dotty
 
-import scala.reflect.ClassTag
 import scala.reflect.runtime.universe.TypeTag
 import scala.Predef.???
 
-abstract class I1 {
-  implicit def classTag[T]: ClassTag[T] = ???
+/** unimplemented implicit for TypeTag */
+object DottyPredef {
   implicit def typeTag[T]: TypeTag[T] = ???
-  implicit val DoubleClassTag: ClassTag[Double]   = ClassTag.Double
-}
-abstract class I2 extends I1 {
-  implicit val FloatClassTag: ClassTag[Double]   = ClassTag.Double
-}
-abstract class I3 extends I2 {
-  implicit val LongClassTag: ClassTag[Long]       = ClassTag.Long
-}
-abstract class I4 extends I3 {
-  implicit val IntClassTag: ClassTag[Int]         = ClassTag.Int
-}
-abstract class I5 extends I4 {
-  implicit val ShortClassTag: ClassTag[Short]     = ClassTag.Short
-}
-abstract class I6 extends I5 {
-  implicit val ByteClassTag: ClassTag[Byte]       = ClassTag.Byte
-  implicit val CharClassTag: ClassTag[Char]       = ClassTag.Char
-  implicit val BooleanClassTag: ClassTag[Boolean] = ClassTag.Boolean
-  implicit val UnitClassTag: ClassTag[Unit]       = ClassTag.Unit
-  implicit val NullClassTag: ClassTag[Null]       = ClassTag.Null
-}
-
-/** implicits for ClassTag and TypeTag. Should be implemented with macros */
-object DottyPredef extends I6 {
 
-  /** ClassTags for final classes */
-  implicit val NothingClassTag: ClassTag[Nothing] = ClassTag.Nothing
+// not yet:
+// def classOf[T](implicit ctag: ClassTag[T]): Class[T] =
+//    ctag.runtimeClass
 }

src/dotty/tools/dotc/Compiler.scala

@@ -59,7 +59,6 @@ class Compiler {
            new SeqLiterals,
            new InterceptedMethods,
            new Getters,
-           new ClassTags,
            new ElimByName,
            new AugmentScala2Traits,
            new ResolveSuper),

src/dotty/tools/dotc/ast/tpd.scala

@@ -777,27 +777,6 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
       }
       else Assign(tree, rhs)
 
-    /** A tree in place of this tree that represents the class of type `tp`.
-     *  Contains special handling if the class is a primitive value class
-     *  and invokes a `default` method otherwise.
-     */
-    def clsOf(tp: Type, default: => Tree)(implicit ctx: Context): Tree = {
-      def TYPE(module: TermSymbol) =
-        ref(module).select(nme.TYPE_).ensureConforms(tree.tpe).withPos(tree.pos)
-      defn.scalaClassName(tp) match {
-        case tpnme.Boolean => TYPE(defn.BoxedBooleanModule)
-        case tpnme.Byte => TYPE(defn.BoxedByteModule)
-        case tpnme.Short => TYPE(defn.BoxedShortModule)
-        case tpnme.Char => TYPE(defn.BoxedCharModule)
-        case tpnme.Int => TYPE(defn.BoxedIntModule)
-        case tpnme.Long => TYPE(defn.BoxedLongModule)
-        case tpnme.Float => TYPE(defn.BoxedFloatModule)
-        case tpnme.Double => TYPE(defn.BoxedDoubleModule)
-        case tpnme.Unit => TYPE(defn.BoxedUnitModule)
-        case _ => default
-      }
-    }
-
     // --- Higher order traversal methods -------------------------------
 
     /** Apply `f` to each subtree of this tree */
@@ -842,6 +821,23 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
     }
   }
 
+  /** A tree that represents the class of the erasure of type `tp`. */
+  def clsOf(tp: Type)(implicit ctx: Context): Tree = {
+    def TYPE(module: TermSymbol) = ref(module).select(nme.TYPE_)
+    defn.scalaClassName(tp) match {
+      case tpnme.Boolean => TYPE(defn.BoxedBooleanModule)
+      case tpnme.Byte => TYPE(defn.BoxedByteModule)
+      case tpnme.Short => TYPE(defn.BoxedShortModule)
+      case tpnme.Char => TYPE(defn.BoxedCharModule)
+      case tpnme.Int => TYPE(defn.BoxedIntModule)
+      case tpnme.Long => TYPE(defn.BoxedLongModule)
+      case tpnme.Float => TYPE(defn.BoxedFloatModule)
+      case tpnme.Double => TYPE(defn.BoxedDoubleModule)
+      case tpnme.Unit => TYPE(defn.BoxedUnitModule)
+      case _ => Literal(Constant(TypeErasure.erasure(tp)))
+    }
+  }
+
   def applyOverloaded(receiver: Tree, method: TermName, args: List[Tree], targs: List[Type], expectedType: Type, isAnnotConstructor: Boolean = false)(implicit ctx: Context): Tree = {
     val typer = ctx.typer
     val proto = new FunProtoTyped(args, expectedType, typer)

src/dotty/tools/dotc/core/Definitions.scala

@@ -419,6 +419,9 @@ class Definitions {
   lazy val LanguageModuleRef          = ctx.requiredModule("dotty.language")
   def LanguageModuleClass(implicit ctx: Context) = LanguageModuleRef.symbol.moduleClass.asClass
   lazy val NonLocalReturnControlType: TypeRef   = ctx.requiredClassRef("scala.runtime.NonLocalReturnControl")
+  lazy val ClassTagType = ctx.requiredClassRef("scala.reflect.ClassTag")
+  def ClassTagClass(implicit ctx: Context) = ClassTagType.symbol.asClass
+  def ClassTagModule(implicit ctx: Context) = ClassTagClass.companionModule
 
   // Annotation base classes
   lazy val AnnotationType              = ctx.requiredClassRef("scala.annotation.Annotation")

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

@@ -290,6 +290,8 @@ class TreeUnpickler(reader: TastyReader, tastyName: TastyName.Table) {
           ConstantType(Constant(readName().toString))
         case NULLconst =>
           ConstantType(Constant(null))
+        case CLASSconst =>
+          ConstantType(Constant(readType()))
         case BYNAMEtype =>
           ExprType(readType())
       }

src/dotty/tools/dotc/transform/ClassOf.scala

@@ -31,7 +31,7 @@ class ClassOf extends MiniPhaseTransform {
   override def transformTypeApply(tree: TypeApply)(implicit ctx: Context, info: TransformerInfo): Tree =
     if (tree.symbol eq classOfMethod) {
       val targ = tree.args.head.tpe
-      tree.clsOf(targ, Literal(Constant(TypeErasure.erasure(targ))))
+      clsOf(targ).ensureConforms(tree.tpe).withPos(tree.pos)
     }
     else tree
 }

src/dotty/tools/dotc/transform/GetClass.scala

@@ -5,6 +5,7 @@ import ast.tpd
 import core.Contexts.Context
 import core.StdNames.nme
 import core.Phases.Phase
+import TypeUtils._
 import TreeTransforms.{MiniPhaseTransform, TransformerInfo}
 
 /** Rewrite `getClass` calls as follow:
@@ -24,7 +25,8 @@ class GetClass extends MiniPhaseTransform {
   override def transformApply(tree: Apply)(implicit ctx: Context, info: TransformerInfo): Tree = {
     import ast.Trees._
     tree match {
-      case Apply(Select(qual, nme.getClass_), Nil) => tree.clsOf(qual.tpe.widen, tree)
+      case Apply(Select(qual, nme.getClass_), Nil) if qual.tpe.widen.isPrimitiveValueType =>
+        clsOf(qual.tpe.widen).withPos(tree.pos)
       case _ => tree
     }
   }

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

@@ -1454,7 +1454,9 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
               case ambi: AmbiguousImplicits =>
                 implicitArgError(s"ambiguous implicits: ${ambi.explanation} of $where")
               case failure: SearchFailure =>
-                implicitArgError(d"no implicit argument of type $formal found for $where" + failure.postscript)
+                val arg = synthesizedClassTag(formal)
+                if (!arg.isEmpty) arg
+                else implicitArgError(d"no implicit argument of type $formal found for $where" + failure.postscript)
             }
           }
           if (errors.nonEmpty) {
@@ -1515,6 +1517,27 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         }
     }
 
+    /** If `formal` is of the form ClassTag[T], where `T` is a class type,
+     *  synthesize a class tag for `T`.
+     */
+    def synthesizedClassTag(formal: Type): Tree = {
+      if (formal.isRef(defn.ClassTagClass))
+        formal.argTypes match {
+          case arg :: Nil =>
+            arg.underlyingClassRef(refinementOK = false) match {
+              case tref: TypeRef =>
+                return ref(defn.ClassTagModule)
+                  .select(nme.apply)
+                  .appliedToType(arg)
+                  .appliedTo(clsOf(tref))
+                  .withPos(tree.pos.endPos)
+              case _ =>
+            }
+          case _ =>
+        }
+      EmptyTree
+    }
+
     /** Adapt an expression of constant type to a different constant type `tpe`. */
     def adaptConstant(tree: Tree, tpe: ConstantType): Tree = {
       def lit = Literal(tpe.value).withPos(tree.pos)