Survive ill-formed parent types

In the face of previous errors:

 - Constant#classBound cannot assume all class parents have good types
 - dominator cannot assume that all classes in an OrType inherit from the same parent

Author: odersky

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

@@ -153,7 +153,8 @@ object Constants {
      */
     def convertTo(pt: Type)(implicit ctx: Context): Constant = {
       def classBound(pt: Type): Type = pt.dealias.stripTypeVar match {
-        case tref: TypeRef if !tref.symbol.isClass => classBound(tref.info.bounds.lo)
+        case tref: TypeRef if !tref.symbol.isClass && tref.info.exists =>
+          classBound(tref.info.bounds.lo)
         case param: TypeParamRef =>
           ctx.typerState.constraint.entry(param) match {
             case TypeBounds(lo, hi) =>

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

@@ -140,7 +140,7 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
         val accu1 = if (accu exists (_ derivesFrom c)) accu else c :: accu
         if (cs == c.baseClasses) accu1 else dominators(rest, accu1)
       case Nil => // this case can happen because after erasure we do not have a top class anymore
-        assert(ctx.erasedTypes)
+        assert(ctx.erasedTypes || ctx.reporter.errorsReported)
         defn.ObjectClass :: Nil
     }
 

tests/neg/parser-stability-15.scala

@@ -0,0 +1,6 @@
+package x0
+class x0 {
+def x1 =
+x2 match          // error
+]                 // error
+case x3[] => x0   // error // error // error // error