Python 3.8.0b3 has the fixed https://docs.python.org/3/c-api/tuple.html#c.PyStructSequence_NewType, but one of the documented features of PyStructSequence is the special https://docs.python.org/3/c-api/tuple.html#c.PyStructSequence_UnnamedField which is meant to be used for unnamed (and presumably also "hidden") fields.

However, this variable is not "exported" (via __declspec(dllexport) or the relevant Python C macro) and so my C extension cannot "import" it and use it.

My guess is that this passed testing because the only tests using it are internal modules linked into python38.dll, which are happy with the `extern` in the header:

Include\structseq.h:extern char* PyStructSequence_UnnamedField;

Editing one line in structseq.h seems to fix the issue.

I changed this

extern char* PyStructSequence_UnnamedField;

to

PyAPI_DATA(char*) PyStructSequence_UnnamedField;

rebuilt, and now my C extension can use PyStructSequence_UnnamedField.