1 files changed, 23 insertions, 0 deletions
diff --git a/src/class.c b/src/class.c
index d044cb68a..e5d60a693 100644
--- a/src/class.c
+++ b/src/class.c
@@ -174,6 +174,14 @@ MRB_API struct RClass*
 mrb_vm_define_module(mrb_state *mrb, mrb_value outer, mrb_sym id)
 {
   check_if_class_or_module(mrb, outer);
+  if (mrb_const_defined_at(mrb, outer, id)) {
+    mrb_value old = mrb_const_get(mrb, outer, id);
+
+    if (mrb_type(old) != MRB_TT_MODULE) {
+      mrb_raisef(mrb, E_TYPE_ERROR, "%S is not a module", mrb_inspect(mrb, old));
+    }
+    return mrb_class_ptr(old);
+  }
   return define_module(mrb, id, mrb_class_ptr(outer));
 }
 
@@ -255,6 +263,21 @@ mrb_vm_define_class(mrb_state *mrb, mrb_value outer, mrb_value super, mrb_sym id
     s = 0;
   }
   check_if_class_or_module(mrb, outer);
+  if (mrb_const_defined_at(mrb, outer, id)) {
+    mrb_value old = mrb_const_get(mrb, outer, id);
+
+    if (mrb_type(old) != MRB_TT_CLASS) {
+      mrb_raisef(mrb, E_TYPE_ERROR, "%S is not a class", mrb_inspect(mrb, old));
+    }
+    c = mrb_class_ptr(old);
+    if (s) {
+      /* check super class */
+      if (mrb_class_real(c->super) != s) {
+        mrb_raisef(mrb, E_TYPE_ERROR, "superclass mismatch for class %S", old);
+      }
+    }
+    return c;
+  }
   c = define_class(mrb, id, s, mrb_class_ptr(outer));
   mrb_class_inherited(mrb, mrb_class_real(c->super), c);