Only track the most specific case and report an error directly, otherwise

Author: b-studios

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

@@ -791,37 +791,29 @@ object RefChecks {
         val seenParents = mutable.HashSet[Type]()
         val baseClasses: List[ClassSymbol] = clazz.info.baseClasses
 
-        // tracks types that we have seen for a particular base class in baseClasses
-        val seenTypes = MutableSymbolMap[List[Type]](baseClasses.size)
+        // tracks the most specific type that we have seen for a particular base class
+        val seenTypes = MutableSymbolMap[Type](baseClasses.size)
 
         // validate all base types of a class in reverse linear order.
         def register(tp: Type): Unit = {
           val baseClass = tp.classSymbol
           if (baseClasses contains baseClass) {
-            val alreadySeen = seenTypes.getOrElse(baseClass, Nil)
-            if (alreadySeen.forall(tp1 => !(tp1 <:< tp)))
-              seenTypes.update(baseClass, tp :: alreadySeen.filter(tp1 => !(tp <:< tp1)))
+            seenTypes.get(baseClass) match {
+              case Some(alreadySeen) if !(tp <:< alreadySeen) =>
+                val msg =
+                  em"""illegal inheritance;
+                      |
+                      |  $clazz inherits different type instances of $baseClass:
+                      |  $tp and $alreadySeen"""
+                report.error(msg, clazz.srcPos)
+              case _ => seenTypes.update(baseClass, tp)
+            }
           }
           val remaining = tp.parents filterNot seenParents
           seenParents ++= remaining
           remaining foreach register
         }
         register(tpe)
-
-        seenTypes.iterator.foreach {
-          case (cls, Nil) =>
-            assert(false) // this case should not be reachable
-          case (cls, _ :: Nil) =>
-            () // Ok
-          case (cls, tp1 :: tp2 :: _) =>
-            val msg =
-              em"""illegal inheritance;
-                  |
-                  |  $clazz inherits different type instances of $cls:
-                  |  $tp1 and $tp2"""
-
-            report.error(msg, clazz.srcPos)
-        }
       }
 
       checkParameterizedTraitsOK()