What's the problem with what I provided? You've introduced so many bugs!

• Your function now accepts any scalar as an argument.
  create_bar(123); is accepted.
  create_bar('abc'); is accepted.
  create_bar(['a', 'b']); is accepted.
  create_bar({key => 'val'}); is accepted.

• If the argument is a reference, it will unconditionally be treated as an object of type type_foo, even if it isn't even an object.

• warn gets executed when you pass undef.

• Do you even know what XSRETURN_UNDEF does? I don't, but I'm pretty sure it's not used properly.

You should be using type T_PTROBJ if you didn't accept undef, so you should base your code on T_PTROBJ.

Updated

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