The out-of-memory error should not be an instance of RuntimeError.
And arena-overflow error as well.  They should not be caught by
`rescue` by default.
MODIFY
src/error.c
src/error.c
@@ -472,7 +472,7 @@ mrb_no_method_error(mrb_state *mrb, mrb_sym id, mrb_value args, char const* fmt,
void
mrb_init_exception(mrb_state *mrb)
{
- struct RClass *exception, *runtime_error, *script_error, *stack_error;
+ struct RClass *exception, *script_error, *stack_error, *nomem_error;
mrb->eException_class = exception = mrb_define_class(mrb, "Exception", mrb->object_class); /* 15.2.22 */
MRB_SET_INSTANCE_TT(exception, MRB_TT_EXCEPTION);
@@ -486,13 +486,15 @@ mrb_init_exception(mrb_state *mrb)
mrb_define_method(mrb, exception, "set_backtrace", exc_set_backtrace, MRB_ARGS_REQ(1));
mrb->eStandardError_class = mrb_define_class(mrb, "StandardError", mrb->eException_class); /* 15.2.23 */
- runtime_error = mrb_define_class(mrb, "RuntimeError", mrb->eStandardError_class); /* 15.2.28 */
- mrb->nomem_err = mrb_obj_ptr(mrb_exc_new_str_lit(mrb, runtime_error, "Out of memory"));
-#ifdef MRB_GC_FIXED_ARENA
- mrb->arena_err = mrb_obj_ptr(mrb_exc_new_str_lit(mrb, runtime_error, "arena overflow error"));
-#endif
+ mrb_define_class(mrb, "RuntimeError", mrb->eStandardError_class); /* 15.2.28 */
script_error = mrb_define_class(mrb, "ScriptError", mrb->eException_class); /* 15.2.37 */
mrb_define_class(mrb, "SyntaxError", script_error); /* 15.2.38 */
stack_error = mrb_define_class(mrb, "SystemStackError", exception);
mrb->stack_err = mrb_obj_ptr(mrb_exc_new_str_lit(mrb, stack_error, "stack level too deep"));
+
+ nomem_error = mrb_define_class(mrb, "NoMemoryError", exception);
+ mrb->nomem_err = mrb_obj_ptr(mrb_exc_new_str_lit(mrb, nomem_error, "Out of memory"));
+#ifdef MRB_GC_FIXED_ARENA
+ mrb->arena_err = mrb_obj_ptr(mrb_exc_new_str_lit(mrb, nomem_error, "arena overflow error"));
+#endif
}

The out-of-memory error should not be an instance of RuntimeError.

And arena-overflow error as well.  They should not be caught by
`rescue` by default.