Modified erasure so we have enough information ...

Modified erasure so we have enough information to determine whether we
need to use scala or java erasure semantics. This fixes the runtime
failure illustrated here:

  % scala29 -e 'java.util.Collections.max(null)'
  java.lang.NoSuchMethodError: java.util.Collections.max(Ljava/util/Collection;)Ljava/lang/Comparable;

Review by odersky.

Author: paulp

src/compiler/scala/tools/nsc/backend/jvm/GenJVM.scala

@@ -594,7 +594,7 @@ abstract class GenJVM extends SubComponent with GenJVMUtil with GenAndroid with
           if ((settings.check.value contains "genjvm")) {
             val normalizedTpe = atPhase(currentRun.erasurePhase)(erasure.prepareSigMap(memberTpe))
             val bytecodeTpe = owner.thisType.memberInfo(sym)
-            if (!sym.isType && !sym.isConstructor && !(erasure.erasure(normalizedTpe) =:= bytecodeTpe)) {
+            if (!sym.isType && !sym.isConstructor && !(erasure.erasure(sym, normalizedTpe) =:= bytecodeTpe)) {
               clasz.cunit.warning(sym.pos,
                   """|compiler bug: created generic signature for %s in %s that does not conform to its erasure
                      |signature: %s

src/compiler/scala/tools/nsc/transform/AddInterfaces.scala

@@ -26,15 +26,13 @@ abstract class AddInterfaces extends InfoTransform {
    */
   override def phaseNewFlags: Long = lateDEFERRED | lateINTERFACE
 
-  /** Type reference after erasure; to be defined in subclass
-   *  <code>Erasure</code>.
+  /** Type reference after erasure; defined in Erasure.
    */
   def erasedTypeRef(sym: Symbol): Type
 
-  /** Erasure type-map; to be defined in subclass
-   *  <code>Erasure</code>.
+  /** Erasure calculation; defined in Erasure.
    */
-  def erasure: TypeMap
+  def erasure(sym: Symbol, tpe: Type): Type
 
   /** A lazily constructed map that associates every non-interface trait with
    *  its implementation class.
@@ -160,39 +158,34 @@ abstract class AddInterfaces extends InfoTransform {
     }
 
     override def complete(sym: Symbol) {
+      /** If `tp` refers to a non-interface trait, return a
+       *  reference to its implementation class. Otherwise return `tp`.
+       */
+      def mixinToImplClass(tp: Type): Type = erasure(sym,
+        tp match { //@MATN: no normalize needed (comes after erasure)
+          case TypeRef(pre, sym, args) if sym.needsImplClass =>
+            typeRef(pre, implClass(sym), args)
+          case _ =>
+            tp
+        }
+      )
       def implType(tp: Type): Type = tp match {
         case ClassInfoType(parents, decls, _) =>
           assert(phase == implClassPhase)
           ClassInfoType(
-            ObjectClass.tpe :: (parents.tail map mixinToImplClass filter (_.typeSymbol != ObjectClass))
-              ::: List(iface.tpe),
+            ObjectClass.tpe +: (parents.tail map mixinToImplClass filter (_.typeSymbol != ObjectClass)) :+ iface.tpe,
             implDecls(sym, decls),
-            sym)
-        case PolyType(tparams, restpe) =>
+            sym
+          )
+        case PolyType(_, restpe) =>
           implType(restpe)
       }
-      sym.setInfo(implType(atPhase(currentRun.erasurePhase)(iface.info)))
+      sym setInfo implType(atPhase(currentRun.erasurePhase)(iface.info))
     }
 
     override def load(clazz: Symbol) { complete(clazz) }
   }
 
-  /** If type <code>tp</code> refers to a non-interface trait, return a
-   *  reference to its implementation class. Otherwise return <code>tp</code>
-   *  itself.
-   *
-   *  @param tp ...
-   *  @return   ...
-   */
-  private def mixinToImplClass(tp: Type): Type =
-    erasure(
-      tp match { //@MATN: no normalize needed (comes after erasure)
-        case TypeRef(pre, sym, args) if (sym.needsImplClass) =>
-          typeRef(pre, implClass(sym), args)
-        case _ =>
-          tp
-      })
-
   def transformMixinInfo(tp: Type): Type = tp match {
     case ClassInfoType(parents, decls, clazz) =>
       if (clazz.needsImplClass) {

src/compiler/scala/tools/nsc/transform/Erasure.scala

@@ -88,7 +88,7 @@ abstract class Erasure extends AddInterfaces
       val eparams    = typeParamsToExistentials(ClassClass, ClassClass.typeParams)
       val upperBound = (
         if (isPhantomClass(sym)) AnyClass.tpe
-        else if (sym.isLocalClass) erasure.intersectionDominator(tp.parents) // erasure(tp)
+        else if (sym.isLocalClass) intersectionDominator(tp.parents)
         else tp.widen
       )
 
@@ -134,8 +134,9 @@ abstract class Erasure extends AddInterfaces
    *   - For a typeref P.C[Ts] where C refers to an alias type, the erasure of C's alias.
    *   - For a typeref P.C[Ts] where C refers to an abstract type, the
    *     erasure of C's upper bound.
-   *   - For a non-empty type intersection (possibly with refinement),
-   *     the erasure of its first parent.
+   *   - For a non-empty type intersection (possibly with refinement)
+   *      - in scala, the erasure of the intersection dominator
+   *      - in java, the erasure of its first parent                  <--- @PP: not yet in spec.
    *   - For an empty type intersection, java.lang.Object.
    *   - For a method type (Fs)scala.Unit, (|Fs|)scala#Unit.
    *   - For any other method type (Fs)Y, (|Fs|)|T|.
@@ -146,29 +147,84 @@ abstract class Erasure extends AddInterfaces
    *     parents |Ps|, but with duplicate references of Object removed.
    *   - for all other types, the type itself (with any sub-components erased)
    */
-  object erasure extends TypeMap {
-    // Compute the dominant part of the intersection type with given `parents` according to new spec.
-    def intersectionDominator(parents: List[Type]): Type =
+  def erasure(sym: Symbol, tp: Type): Type = {
+    if (sym != NoSymbol && sym.enclClass.isJavaDefined) {
+      val res = javaErasure(tp)
+      if (verifyJavaErasure && sym.isMethod) {
+        val old = scalaErasure(tp)
+        if (!(res =:= old))
+          log("Identified divergence between java/scala erasure:\n  scala: " + old + "\n   java: " + res)
+      }
+      res
+    }
+    else scalaErasure(tp)
+  }
+
+  /** Scala's more precise erasure than java's is problematic as follows:
+   *
+   *  - Symbols are read from classfiles and populated with types
+   *  - The textual signature read from the bytecode is forgotten
+   *  - Bytecode generation must know the precise signature of a method
+   *  - the signature is derived from the erasure of the method type
+   *  - If that derivation does not adhere to the rules by which the original
+   *    signature was created, a NoSuchMethod error will result.
+   *
+   *  For this reason and others (such as distinguishing constructors from other methods)
+   *  erasure is now (Symbol, Type) => Type rather than Type => Type.
+   */
+  object scalaErasure extends ErasureMap {
+    /** In scala, calculate a useful parent.
+     *  An intersection such as `Object with Trait` erases to Trait.
+     */
+    def mergeParents(parents: List[Type]): Type =
+      intersectionDominator(parents)
+  }
+  object javaErasure extends ErasureMap {
+    /** In java, always take the first parent.
+     *  An intersection such as `Object with Trait` erases to Object.
+     */
+    def mergeParents(parents: List[Type]): Type =
       if (parents.isEmpty) ObjectClass.tpe
-      else {
-        val psyms = parents map (_.typeSymbol)
-        if (psyms contains ArrayClass) {
-          // treat arrays specially
-          arrayType(
-            intersectionDominator(
-              parents filter (_.typeSymbol == ArrayClass) map (_.typeArgs.head)))
-        } else {
-          // implement new spec for erasure of refined types.
-          def isUnshadowed(psym: Symbol) =
-            !(psyms exists (qsym => (psym ne qsym) && (qsym isNonBottomSubClass psym)))
-          val cs = parents.iterator.filter { p => // isUnshadowed is a bit expensive, so try classes first
-            val psym = p.typeSymbol
-            psym.initialize
-            psym.isClass && !psym.isTrait && isUnshadowed(psym)
-          }
-          (if (cs.hasNext) cs else parents.iterator.filter(p => isUnshadowed(p.typeSymbol))).next()
+      else parents.head
+  }
+
+  /** The intersection dominator (SLS 3.7) of a list of types is computed as follows.
+   *
+   *  - If the list contains one or more occurrences of scala.Array with
+   *    type parameters El1, El2, ... then the dominator is scala.Array with
+   *    type parameter of intersectionDominator(List(El1, El2, ...)).           <--- @PP: not yet in spec.
+   *  - Otherwise, the list is reduced to a subsequence containing only types
+   *    which are not subtypes of other listed types (the span.)
+   *  - If the span is empty, the dominator is Object.
+   *  - If the span contains a class Tc which is not a trait and which is
+   *    not Object, the dominator is Tc.                                        <--- @PP: "which is not Object" not in spec.
+   *  - Otherwise, the dominator is the first element of the span.
+   */
+  def intersectionDominator(parents: List[Type]): Type = {
+    if (parents.isEmpty) ObjectClass.tpe
+    else {
+      val psyms = parents map (_.typeSymbol)
+      if (psyms contains ArrayClass) {
+        // treat arrays specially
+        arrayType(
+          intersectionDominator(
+            parents filter (_.typeSymbol == ArrayClass) map (_.typeArgs.head)))
+      } else {
+        // implement new spec for erasure of refined types.
+        def isUnshadowed(psym: Symbol) =
+          !(psyms exists (qsym => (psym ne qsym) && (qsym isNonBottomSubClass psym)))
+        val cs = parents.iterator.filter { p => // isUnshadowed is a bit expensive, so try classes first
+          val psym = p.typeSymbol
+          psym.initialize
+          psym.isClass && !psym.isTrait && isUnshadowed(psym)
         }
+        (if (cs.hasNext) cs else parents.iterator.filter(p => isUnshadowed(p.typeSymbol))).next()
       }
+    }
+  }
+
+  abstract class ErasureMap extends TypeMap {
+    def mergeParents(parents: List[Type]): Type
 
     def apply(tp: Type): Type = {
       tp match {
@@ -183,7 +239,7 @@ abstract class Erasure extends AddInterfaces
             else typeRef(apply(pre), sym, args map this)
           else if (sym == AnyClass || sym == AnyValClass || sym == SingletonClass || sym == NotNullClass) erasedTypeRef(ObjectClass)
           else if (sym == UnitClass) erasedTypeRef(BoxedUnitClass)
-          else if (sym.isRefinementClass) apply(intersectionDominator(tp.parents))
+          else if (sym.isRefinementClass) apply(mergeParents(tp.parents))
           else if (sym.isClass) typeRef(apply(rebindInnerClass(pre, sym)), sym, List())  // #2585
           else apply(sym.info) // alias type or abstract type
         case PolyType(tparams, restpe) =>
@@ -196,12 +252,13 @@ abstract class Erasure extends AddInterfaces
             if (restpe.typeSymbol == UnitClass)
               erasedTypeRef(UnitClass)
             else if (settings.YdepMethTpes.value)
-              // this replaces each typeref that refers to an argument by the type `p.tpe` of the actual argument p (p in params)
+              // this replaces each typeref that refers to an argument
+              // by the type `p.tpe` of the actual argument p (p in params)
               apply(mt.resultType(params map (_.tpe)))
             else
               apply(restpe))
         case RefinedType(parents, decls) =>
-          apply(intersectionDominator(parents))
+          apply(mergeParents(parents))
         case AnnotatedType(_, atp, _) =>
           apply(atp)
         case ClassInfoType(parents, decls, clazz) =>
@@ -229,8 +286,8 @@ abstract class Erasure extends AddInterfaces
             else if (!sym.owner.isPackageClass) traverse(pre)
           case PolyType(_, _) | ExistentialType(_, _) =>
             result = true
-          case RefinedType(parents, decls) =>
-            if (!parents.isEmpty) traverse(parents.head)
+          case RefinedType(parents, _) =>
+            parents foreach traverse
           case ClassInfoType(parents, _, _) =>
             parents foreach traverse
           case AnnotatedType(_, atp, _) =>
@@ -242,6 +299,7 @@ abstract class Erasure extends AddInterfaces
     }
   }
 
+  private def verifyJavaErasure = settings.Xverify.value || settings.debug.value
   private def needsJavaSig(tp: Type) = !settings.Ynogenericsig.value && NeedsSigCollector.collect(tp)
 
   // only refer to type params that will actually make it into the sig, this excludes:
@@ -418,7 +476,7 @@ abstract class Erasure extends AddInterfaces
               )
             }
           }
-          else jsig(erasure(tp), existentiallyBound, toplevel, primitiveOK)
+          else jsig(erasure(sym0, tp), existentiallyBound, toplevel, primitiveOK)
         case PolyType(tparams, restpe) =>
           assert(tparams.nonEmpty)
           def boundSig(bounds: List[Type]) = {
@@ -447,7 +505,7 @@ abstract class Erasure extends AddInterfaces
           println("something's wrong: "+sym0+":"+sym0.tpe+" has a bounded wildcard type")
           jsig(bounds.hi, existentiallyBound, toplevel, primitiveOK)
         case _ =>
-          val etp = erasure(tp)
+          val etp = erasure(sym0, tp)
           if (etp eq tp) throw new UnknownSig
           else jsig(etp)
       }
@@ -465,7 +523,7 @@ abstract class Erasure extends AddInterfaces
 
   /** Type reference after erasure */
   def erasedTypeRef(sym: Symbol): Type =
-    typeRef(erasure(sym.owner.tpe), sym, List())
+    typeRef(erasure(sym, sym.owner.tpe), sym, List())
 
   /** Remove duplicate references to class Object in a list of parent classes */
   private def removeDoubleObject(tps: List[Type]): List[Type] = tps match {
@@ -487,25 +545,25 @@ abstract class Erasure extends AddInterfaces
     if (sym == Object_asInstanceOf)
       sym.info
     else if (sym == Object_isInstanceOf || sym == ArrayClass)
-      PolyType(sym.info.typeParams, erasure(sym.info.resultType))
+      PolyType(sym.info.typeParams, erasure(sym, sym.info.resultType))
     else if (sym.isAbstractType)
       TypeBounds(WildcardType, WildcardType)
     else if (sym.isTerm && sym.owner == ArrayClass) {
       if (sym.isClassConstructor)
         tp match {
-          case MethodType(params, TypeRef(pre, sym, args)) =>
-            MethodType(cloneSymbols(params) map (p => p.setInfo(erasure(p.tpe))),
-                       typeRef(erasure(pre), sym, args))
+          case MethodType(params, TypeRef(pre, sym1, args)) =>
+            MethodType(cloneSymbols(params) map (p => p.setInfo(erasure(sym, p.tpe))),
+                       typeRef(erasure(sym, pre), sym1, args))
         }
       else if (sym.name == nme.apply)
         tp
       else if (sym.name == nme.update)
         (tp: @unchecked) match {
           case MethodType(List(index, tvar), restpe) =>
-            MethodType(List(index.cloneSymbol.setInfo(erasure(index.tpe)), tvar),
+            MethodType(List(index.cloneSymbol.setInfo(erasure(sym, index.tpe)), tvar),
                        erasedTypeRef(UnitClass))
         }
-      else erasure(tp)
+      else erasure(sym, tp)
     } else if (
       sym.owner != NoSymbol &&
       sym.owner.owner == ArrayClass &&
@@ -522,7 +580,7 @@ abstract class Erasure extends AddInterfaces
         else
           erasure(tp)
 */
-      transformMixinInfo(erasure(tp))
+      transformMixinInfo(erasure(sym, tp))
     }
   }
 
@@ -903,7 +961,7 @@ abstract class Erasure extends AddInterfaces
         val other = opc.overridden
         //Console.println("bridge? " + member + ":" + member.tpe + member.locationString + " to " + other + ":" + other.tpe + other.locationString)//DEBUG
         if (atPhase(currentRun.explicitouterPhase)(!member.isDeferred)) {
-          val otpe = erasure(other.tpe)
+          val otpe = erasure(owner, other.tpe)
           val bridgeNeeded = atPhase(phase.next) (
             !(other.tpe =:= member.tpe) &&
             !(deconstMap(other.tpe) =:= deconstMap(member.tpe)) &&
@@ -940,7 +998,7 @@ abstract class Erasure extends AddInterfaces
                              ((fun, vparams) => Apply(fun, vparams map Ident)))
                       });
                   if (settings.debug.value)
-                    log("generating bridge from " + other + "(" + Flags.flagsToString(bridge.flags)  + ")" + ":" + otpe + other.locationString + " to " + member + ":" + erasure(member.tpe) + member.locationString + " =\n " + bridgeDef);
+                    log("generating bridge from " + other + "(" + Flags.flagsToString(bridge.flags)  + ")" + ":" + otpe + other.locationString + " to " + member + ":" + erasure(owner, member.tpe) + member.locationString + " =\n " + bridgeDef);
                   bridgeDef
                 }
               } :: bridges
@@ -1004,16 +1062,20 @@ abstract class Erasure extends AddInterfaces
           val level = unboundedGenericArrayLevel(arg.tpe)
           def isArrayTest(arg: Tree) =
             gen.mkRuntimeCall("isArray", List(arg, Literal(Constant(level))))
+
           global.typer.typedPos(tree.pos) {
             if (level == 1) isArrayTest(qual)
-            else
-              gen.evalOnce(qual, currentOwner, unit) { qual1 =>
-                gen.mkAnd(
-                  Apply(TypeApply(Select(qual1(), fun.symbol),
-                                  List(TypeTree(erasure(arg.tpe)))),
-                        List()),
-                  isArrayTest(qual1()))
-              }
+            else gen.evalOnce(qual, currentOwner, unit) { qual1 =>
+              gen.mkAnd(
+                gen.mkMethodCall(
+                  qual1(),
+                  fun.symbol,
+                  List(erasure(fun.symbol, arg.tpe)),
+                  Nil
+                ),
+                isArrayTest(qual1())
+              )
+            }
           }
         case TypeApply(fun, args) if (fun.symbol.owner != AnyClass &&
                                       fun.symbol != Object_asInstanceOf &&
@@ -1030,7 +1092,7 @@ abstract class Erasure extends AddInterfaces
             // need to do the cast in adaptMember
             treeCopy.Apply(
               tree,
-              SelectFromArray(qual, name, erasure(qual.tpe)).copyAttrs(fn),
+              SelectFromArray(qual, name, erasure(tree.symbol, qual.tpe)).copyAttrs(fn),
               args)
 
         case Apply(fn @ Select(qual, _), Nil) if interceptedMethods(fn.symbol) =>
@@ -1150,7 +1212,7 @@ abstract class Erasure extends AddInterfaces
 
         case Literal(ct) if ct.tag == ClassTag
                          && ct.typeValue.typeSymbol != definitions.UnitClass =>
-          treeCopy.Literal(tree, Constant(erasure(ct.typeValue)))
+          treeCopy.Literal(tree, Constant(erasure(NoSymbol, ct.typeValue)))
 
         case _ =>
           tree
@@ -1168,10 +1230,10 @@ abstract class Erasure extends AddInterfaces
           val tree1 = preErase(tree)
           tree1 match {
             case EmptyTree | TypeTree() =>
-              tree1 setType erasure(tree1.tpe)
+              tree1 setType erasure(NoSymbol, tree1.tpe)
             case DefDef(_, _, _, _, tpt, _) =>
               val result = super.transform(tree1) setType null
-              tpt.tpe = erasure(tree1.symbol.tpe).resultType
+              tpt.tpe = erasure(tree1.symbol, tree1.symbol.tpe).resultType
               result
             case _ =>
               super.transform(tree1) setType null