Avoid rechecking already handled pairs only if in same order

Author: odersky

compiler/src/dotty/tools/dotc/transform/OverridingPairs.scala

@@ -34,7 +34,7 @@ object OverridingPairs {
      *  pair has already been treated in a parent class.
      *  This may be refined in subclasses. @see Bridges for a use case.
      */
-    protected def parents: Array[Symbol] = base.info.parents.toArray.map(_.typeSymbol)
+    protected def parents: Array[Symbol] = base.info.parents.toArray.map(_.classSymbol)
 
     /** Does `sym1` match `sym2` so that it qualifies as overriding when both symbols are
      *  seen as members of `self`? Types always match. Term symbols match if their membertypes
@@ -92,13 +92,22 @@ object OverridingPairs {
     private def isHandledByParent(sym1: Symbol, sym2: Symbol): Boolean =
       val commonParents = subParents(sym1.owner).intersect(subParents(sym2.owner))
       commonParents.nonEmpty
-      && commonParents.exists(i => canBeHandledByParent(sym1, sym2, parents(i).thisType))
+      && commonParents.exists(i => canBeHandledByParent(sym1, sym2, parents(i)))
 
     /** Can pair `sym1`/`sym2` be handled by parent `parentType` which is a common subtype
      *  of both symbol's owners? Assumed to be true by default, but overridden in RefChecks.
      */
-    protected def canBeHandledByParent(sym1: Symbol, sym2: Symbol, parentType: Type): Boolean =
-      true
+    protected def canBeHandledByParent(sym1: Symbol, sym2: Symbol, parent: Symbol): Boolean =
+      val owner1 = sym1.owner
+      val owner2 = sym2.owner
+      def precedesIn(bcs: List[ClassSymbol]): Boolean = (bcs: @unchecked) match
+        case bc :: bcs1 =>
+          if owner1 eq bc then true
+          else if owner2 eq bc then false
+          else precedesIn(bcs1)
+        case _ =>
+          false
+      precedesIn(parent.asClass.baseClasses)
 
     /** The scope entries that have already been visited as overridden
      *  (maybe excluded because of already handled by a parent).

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

@@ -514,9 +514,10 @@ object RefChecks {
 
       // We can exclude pairs safely from checking only of they also matched in
       // the parent class. See neg/i12828.scala for an example where this matters.
-      override def canBeHandledByParent(sym1: Symbol, sym2: Symbol, parentType: Type): Boolean =
-        considerMatching(sym1, sym2, parentType)
-         .showing(i"already handled ${sym1.showLocated}: ${sym1.asSeenFrom(parentType).signature}, ${sym2.showLocated}: ${sym2.asSeenFrom(parentType).signature} = $result", refcheck)
+      override def canBeHandledByParent(sym1: Symbol, sym2: Symbol, parent: Symbol): Boolean =
+        considerMatching(sym1, sym2, parent.thisType)
+         .showing(i"already handled ${sym1.showLocated}: ${sym1.asSeenFrom(parent.thisType).signature}, ${sym2.showLocated}: ${sym2.asSeenFrom(parent.thisType).signature} = $result", refcheck)
+        && super.canBeHandledByParent(sym1, sym2, parent)
     end opc
 
     while opc.hasNext do

tests/run/i5094.scala renamed to tests/neg/i5094.scala

@@ -9,7 +9,7 @@ trait SOIO extends IO {
 }
 trait SOSO extends SOIO with SO
 abstract class AS extends SO
-class L extends AS with SOSO
+class L extends AS with SOSO // error: cannot override final member
 object Test {
   def main(args: Array[String]): Unit = {
     new L