[AST] Re-establish assertion about inheritance relationship in getSuperclassForDecl()

When we fail to find a particular superclass type of a type in ill-formed
code, make sure that we were right to look here at all. This re-establishes
a check I had recently weakened.

Author: DougGregor

lib/AST/Type.cpp

@@ -3193,6 +3193,15 @@ Type TypeBase::getSuperclassForDecl(const ClassDecl *baseClass,
     t = t->getSuperclass(useArchetypes);
   }
 
+#ifndef NDEBUG
+  auto *currentClass = getConcreteTypeForSuperclassTraversing(this)
+      ->getClassOrBoundGenericClass();
+  while (currentClass && currentClass != baseClass)
+    currentClass = currentClass->getSuperclassDecl();
+  assert(currentClass == baseClass &&
+         "no inheritance relationship between given classes");
+#endif
+
   return ErrorType::get(this);
 }
 

lib/Sema/TypeCheckType.cpp

@@ -300,7 +300,9 @@ Type TypeChecker::resolveTypeInContext(
       if (!sig)
         return ErrorType::get(Context);
       auto superclassType = sig->getSuperclassBound(selfType);
-      assert(superclassType);
+      if (!superclassType)
+        return ErrorType::get(Context);
+
       selfType = superclassType;
     }
   }