Exclude override checks for pairs from a Java class parent

Author: EugeneFlesselle

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

@@ -224,6 +224,9 @@ object RefChecks {
      *     See neg/i12828.scala for an example where this matters.
      *   - They overriding/overridden appear in linearization order.
      *     See neg/i5094.scala for an example where this matters.
+     *   - They overriding/overridden appear in linearization order,
+     *     or the parent is a Java class (because linearization does not apply to java classes).
+     *     See neg/i5094.scala and pos/i18654.scala for examples where this matters.
      *   - The overridden symbol is not `abstract override`. For such symbols
      *     we need a more extensive test since the virtual super chain depends
      *     on the precise linearization order, which might be different for the
@@ -232,7 +235,7 @@ object RefChecks {
     override def canBeHandledByParent(sym1: Symbol, sym2: Symbol, parent: Symbol): Boolean =
       isOverridingPair(sym1, sym2, parent.thisType)
         .showing(i"already handled ${sym1.showLocated}: ${sym1.asSeenFrom(parent.thisType).signature}, ${sym2.showLocated}: ${sym2.asSeenFrom(parent.thisType).signature} = $result", refcheck)
-      && inLinearizationOrder(sym1, sym2, parent)
+      && (inLinearizationOrder(sym1, sym2, parent) || parent.is(JavaDefined))
       && !sym2.is(AbsOverride)
 
     /** Checks the subtype relationship tp1 <:< tp2.