More precise and uniform modelling of "isCurrent"

For implemented getters and forwarded methods we need a notion of "isCurrent",
which means: would the getter or method before the implementation is added be
a member of the implementing class? Only in this case do we need to do anything.
The method formulation was previously weaker than the getter formulation, which
led to an error when compiling core (duplicate methods: andThen and size).

Author: odersky

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

@@ -157,14 +157,10 @@ class Mixin extends MiniPhaseTransform with SymTransformer { thisTransform =>
 
     def traitInits(mixin: ClassSymbol): List[Tree] =
       for (getter <- mixin.info.decls.filter(getr => getr.isGetter && !wasDeferred(getr)).toList) yield {
-        // isCurrent: getter is a member of implementing class
-        val isCurrent = getter.is(ExpandedName) || ctx.atPhase(thisTransform) { implicit ctx =>
-          cls.info.member(getter.name).suchThat(_.isGetter).symbol == getter
-        }
         val isScala2x = mixin.is(Scala2x)
         def default = Underscore(getter.info.resultType)
         def initial = transformFollowing(superRef(initializer(getter)).appliedToNone)
-        if (isCurrent)
+        if (isCurrent(getter) || getter.is(ExpandedName))
           // transformFollowing call is needed to make memoize & lazy vals run
           transformFollowing(
             DefDef(implementation(getter.asTerm), if (isScala2x) default else initial))

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

@@ -31,12 +31,13 @@ class MixinOps(cls: ClassSymbol, thisTransform: DenotTransformer)(implicit ctx:
     //sup.select(target)
   }
 
+  /** Is `sym` a member of implementing class `cls`? */
+  def isCurrent(sym: Symbol) = cls.info.member(sym.name).hasAltWith(_.symbol == sym)
+
   def needsForwarder(meth: Symbol): Boolean = {
     def needsDisambiguation = !meth.allOverriddenSymbols.forall(_ is Deferred)
-    def isOverridden = meth.overridingSymbol(cls).is(Method, butNot = Deferred)
     meth.is(Method, butNot = PrivateOrAccessorOrDeferred) &&
-    !isOverridden &&
-    !meth.isConstructor &&
+    isCurrent(meth) &&
     (needsDisambiguation || meth.owner.is(Scala2x))
   }