Find Class/Module in nested singleton class; fix #3598

src/variable.c

@@ -953,14 +953,13 @@ mrb_vm_const_get(mrb_state *mrb, mrb_sym sym)
     if (c->iv && iv_get(mrb, c->iv, sym, &v)) {
       return v;
     }
-    if (c->tt == MRB_TT_SCLASS) {
+    for (c2 = c; c2 && c2->tt == MRB_TT_SCLASS;) {
       mrb_value klass;
-      klass = mrb_obj_iv_get(mrb, (struct RObject *)c,
+      klass = mrb_obj_iv_get(mrb, (struct RObject *)c2,
                              mrb_intern_lit(mrb, "__attached__"));
       c2 = mrb_class_ptr(klass);
-      if (c2->tt == MRB_TT_CLASS || c2->tt == MRB_TT_MODULE)
-        c = c2;
     }
+    if (c2->tt == MRB_TT_CLASS || c2->tt == MRB_TT_MODULE) c = c2;
     c2 = c;
     for (;;) {
       c2 = mrb_class_outer_module(mrb, c2);