Align computeBaseClasses with checkedParentType

`checkedParentType` tests that the underlying class symbol of a type is a class.
So `computeBaseClasses` should assume exactly the same property.

Author: odersky

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -1467,8 +1467,10 @@ object SymDenotations {
         onBehalf.signalProvisional()
       val builder = new BaseDataBuilder
       for (p <- classParents) {
-        if (p.typeSymbol.isClass) builder.addAll(p.typeSymbol.asClass.baseClasses)
-        else assert(isRefinementClass || ctx.mode.is(Mode.Interactive), s"$this has non-class parent: $p")
+        p.underlyingClassRef(refinementOK = false).typeSymbol match {
+          case pcls: ClassSymbol => builder.addAll(pcls.baseClasses)
+          case _ => assert(isRefinementClass || ctx.mode.is(Mode.Interactive), s"$this has non-class parent: $p")
+        }
       }
       (classSymbol :: builder.baseClasses, builder.baseClassSet)
     }

tests/neg/parser-stability-12.scala

@@ -0,0 +1,3 @@
+trait x0[]
+     trait x1[x1 <:x0]
+     extends x1[