support class define in singleton class

src/class.c

@@ -227,6 +227,7 @@ mrb_vm_define_class(mrb_state *mrb, mrb_value outer, mrb_value super, mrb_sym id
   }
   switch (mrb_type(outer)) {
   case MRB_TT_CLASS:
+  case MRB_TT_SCLASS:
   case MRB_TT_MODULE:
     break;
   default:
@@ -1245,21 +1246,31 @@ mrb_class_path(mrb_state *mrb, struct RClass *c)
 {
   mrb_value path;
   const char *name;
-  mrb_int len;
   mrb_sym classpath = mrb_intern_lit(mrb, "__classpath__");
 
   path = mrb_obj_iv_get(mrb, (struct RObject*)c, classpath);
   if (mrb_nil_p(path)) {
     struct RClass *outer = mrb_class_outer_module(mrb, c);
     mrb_sym sym = mrb_class_sym(mrb, c, outer);
+    mrb_int len;
+
     if (sym == 0) {
       return mrb_nil_value();
     }
     else if (outer && outer != mrb->object_class) {
       mrb_value base = mrb_class_path(mrb, outer);
-      path = mrb_str_plus(mrb, base, mrb_str_new_lit(mrb, "::"));
+      path = mrb_str_buf_new(mrb, 0);
+      if (mrb_nil_p(base)) {
+        mrb_str_cat_lit(mrb, path, "#<Class:");
+        mrb_str_concat(mrb, path, mrb_ptr_to_str(mrb, outer));
+        mrb_str_cat_lit(mrb, path, ">");
+      }
+      else {
+        mrb_str_concat(mrb, path, base);
+      }
+      mrb_str_cat_lit(mrb, path, "::");
       name = mrb_sym2name_len(mrb, sym, &len);
-      mrb_str_concat(mrb, path, mrb_str_new(mrb, name, len));
+      mrb_str_cat(mrb, path, name, len);
     }
     else {
       name = mrb_sym2name_len(mrb, sym, &len);