Go back to checking all base classes.

I was hoping to avoid an explosion of tests, by doing it via parents
only.  But to do that we'd need to check even abstract classes and
traits, which we don't do now.  But even then, you could skip a parent
via a java parent I think?  So I'm just reverting back to how it was
before: all base classes of concrete classes.

Author: dwijnand

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

@@ -781,8 +781,8 @@ object RefChecks {
        *  can assume invariant refinement for classes in `constrainPatternType`.
        */
       def checkClassInheritanceInvariant() =
-        for (middle <- clazz.parentSyms)
-          for (baseCls <- middle.asClass.parentSyms)
+        for (middle <- clazz.info.baseClasses.tail)
+          for (baseCls <- middle.info.baseClasses.tail)
             if (baseCls.typeParams.exists(_.paramVarianceSign != 0))
               val middleStr = if middle.is(Case) then "case " else ""
               for (problem <- variantInheritanceProblems(baseCls, middle, "variant", middleStr))