Feature or enhancement

The PyType_FromSpec family of functions provides a convenient and forward-compatible way mechanism for creating new types. It could in principle also be used to create callables providing a PEP-590 vector call interface, which has significant performance benefits for binding libraries (see the related discussion here).

One can already specify a member named __vectorcalloffset__ in PyType_FromSpec. This isn't fully working in the limited API, however. I encountered the following problems.

1. The Py_TPFLAGS_HAVE_VECTORCALL flag is not part of the limited API.
2. The PyVectorcall_NARGS() helper function is not part of the limited API.
3. One would normally set tp_call to the compatibility dispatch routine PyVectorcall_Call. It is, however, also not part of the public ABI.
4. Leaving tp_call unspecified is not an option. PyType_Ready() even throws an exception in type_ready_pre_checks() when tp_call is unspecified.

Pitch

I propose the following changes:

1. Adding Py_TPFLAGS_HAVE_VECTORCALL, PyVectorcall_NARGS(), and PyVectorcall_Call() to the limited API.
2. Redundant, but nice: Setting tp_call to PyVectorcall_Call when PyType_Ready encounters a type that doesn't have this field set.

Note that vector calls can be received and performed. This issue is just about the receiving end.

Previous discussion

See the discord thread https://discuss.python.org/t/ideas-for-forward-compatible-and-fast-extension-libraries-in-python-3-12/15993/12.