Safe equality for core classes:

Name, Symbol, Denotation, Type.

This uncovered two nonsensical comparisons, one in CollectEntryPoints,
the other in DottyBackendInterface.

Author: odersky

src/dotty/tools/backend/jvm/DottyBackendInterface.scala

@@ -861,7 +861,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile)(implicit ctx: Context
               "If possible, please file a bug on issues.scala-lang.org.")
 
           tp match {
-            case ThisType(ArrayClass)               => ObjectReference.asInstanceOf[ct.bTypes.ClassBType] // was introduced in 9b17332f11 to fix SI-999, but this code is not reached in its test, or any other test
+            case tp: ThisType if tp.cls == ArrayClass => ObjectReference.asInstanceOf[ct.bTypes.ClassBType] // was introduced in 9b17332f11 to fix SI-999, but this code is not reached in its test, or any other test
             case ThisType(sym)                      => storage.getClassBTypeAndRegisterInnerClass(sym.asInstanceOf[ct.int.Symbol])
            // case t: SingletonType                 => primitiveOrClassToBType(t.classSymbol)
             case t: SingletonType                  => t.underlying.toTypeKind(ct)(storage)

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

@@ -97,7 +97,7 @@ object Denotations {
    *
    *  @param symbol  The referencing symbol, or NoSymbol is none exists
    */
-  abstract class Denotation(val symbol: Symbol) extends util.DotClass with printing.Showable {
+  abstract class Denotation(val symbol: Symbol) extends util.DotClass with printing.Showable with EqClassOf[Denotation] {
 
     /** The type info of the denotation, exists only for non-overloaded denotations */
     def info(implicit ctx: Context): Type

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

@@ -35,7 +35,7 @@ object Names {
    *  3. Names are intended to be encoded strings. @see dotc.util.NameTransformer.
    *     The encoding will be applied when converting a string to a name.
    */
-  abstract class Name extends DotClass
+  abstract class Name extends DotClass with EqClassOf[Name]
     with PreName
     with collection.immutable.Seq[Char]
     with IndexedSeqOptimized[Char, Name] {

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

@@ -509,7 +509,7 @@ object SymDenotations {
      */
     final def isSelfSym(implicit ctx: Context) = owner.infoOrCompleter match {
       case ClassInfo(_, _, _, _, selfInfo) =>
-        selfInfo == symbol ||
+        (selfInfo eq symbol) ||
           selfInfo.isInstanceOf[Type] && name == nme.WILDCARD
       case _ => false
     }

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

@@ -365,7 +365,7 @@ object Symbols {
    *  @param coord  The coordinates of the symbol (a position or an index)
    *  @param id     A unique identifier of the symbol (unique per ContextBase)
    */
-  class Symbol private[Symbols] (val coord: Coord, val id: Int) extends DotClass with printing.Showable {
+  class Symbol private[Symbols] (val coord: Coord, val id: Int) extends DotClass with printing.Showable with EqClassOf[Symbol] {
 
     type ThisName <: Name
 

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

@@ -70,7 +70,7 @@ object Types {
    *                       +- ErrorType
    *                       +- WildcardType
    */
-  abstract class Type extends DotClass with Hashable with printing.Showable {
+  abstract class Type extends DotClass with Hashable with printing.Showable with EqClassOf[Type] {
 
 // ----- Tests -----------------------------------------------------
 

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

@@ -75,7 +75,7 @@ class CollectEntryPoints extends MiniPhaseTransform {
       val javaPlatform = ctx.platform.asInstanceOf[JavaPlatform]
       if (javaPlatform.hasJavaMainMethod(companion))
         failNoForwarder("companion contains its own main method")
-      else if (companion != NoSymbol && companion.info.member(nme.main) != NoSymbol)
+      else if (companion.exists && companion.info.member(nme.main).exists)
       // this is only because forwarders aren't smart enough yet
         failNoForwarder("companion contains its own main method (implementation restriction: no main is allowed, regardless of signature)")
       else if (companion.flags is Flags.Trait)