Hello all,
I want to build Python 3.10~b2 documentation using Sphinx 4.0.2 (released in May 2021) as RPM in Fedora with aim to ship it in Fedora 35.

The build fails with the following error:

Warning, treated as error:
/builddir/build/BUILD/Python-3.10.0b2/Doc/c-api/complex.rst:49:Error in declarator or parameters
Error in declarator or parameters
Invalid C declaration: Expected identifier, got keyword: complex [error at 39]
  Py_complex _Py_c_neg(Py_complex complex)
  ---------------------------------------^
make: *** [Makefile:51: build] Error 2


The Sphinx changelog mentions in Bug fixes in https://www.sphinx-doc.org/en/master/changes.html#id21 the likely cause:
C, properly reject function declarations when a keyword is used as parameter name.

Hmm. It's probably not the best name, and we can certainly change it.

But I'm a bit confused by the error message: `complex` isn't a keyword in either C or C++, so I'm not sure why Sphinx thinks it is.

As a test, gcc and clang both seem happy to treat this as valid C.  I think Sphinx is wrong to reject this.

mdickinson@mirzakhani Desktop % cat test.c
typedef struct {
    double real;
    double imag;
} Py_complex;

Py_complex _Py_c_neg(Py_complex complex);
mdickinson@mirzakhani Desktop % gcc -Wall -Wextra -std=c17 -c test.c
mdickinson@mirzakhani Desktop % clang -Wall -Wextra -std=c17 -c test.c
mdickinson@mirzakhani Desktop %

New changeset 7247f6f433846c6e37308a550e8e5eb6be379856 by Mark Dickinson in branch 'main':
bpo-44426: Use of 'complex' as a C variable name confuses Sphinx; change it to 'num'. (GH-26744)
https://github.com/python/cpython/commit/7247f6f433846c6e37308a550e8e5eb6be379856

New changeset c689e0a7e2a25621da82f22cc64d089eae05e753 by Miss Islington (bot) in branch '3.10':
bpo-44426: Use of 'complex' as a C variable name confuses Sphinx; change it to 'num'. (GH-26744) (GH-26760)
https://github.com/python/cpython/commit/c689e0a7e2a25621da82f22cc64d089eae05e753

New changeset 686c6f303a6e9e54b50401be0ae3dc6aa2fcf05a by Miss Islington (bot) in branch '3.9':
bpo-44426: Use of 'complex' as a C variable name confuses Sphinx; change it to 'num'. (GH-26744) (GH-26761)
https://github.com/python/cpython/commit/686c6f303a6e9e54b50401be0ae3dc6aa2fcf05a

> and we can certainly change it

Done. Closing here, but I've opened a Sphinx issue at https://github.com/sphinx-doc/sphinx/issues/9354

Thanks for the fix, this indeed worked. Documentation however still doesn't build successfully, this time with a traceback:

Warning, treated as error:
/builddir/build/BUILD/Python-3.10.0b2/Doc/c-api/object.rst:314:Error in declarator or parameters
Error in declarator or parameters
Invalid C declaration: Expected identifier, got keyword: default [error at 62]
  Py_ssize_t PyObject_LengthHint(PyObject *o, Py_ssize_t default)
  --------------------------------------------------------------^
make: *** [Makefile:51: build] Error 2

I'm afraid there will be more on the way as this item is resolved.

Thanks. That one's a genuine keyword. Looks like what we probably need to do is get a list of all these errors, so that they can all be fixed at once. Presumably running without the "-W" flag would make that easier.

Thanks for the suggestion, I reran the build without -W option. 

Fortunately, the output shows that the reported errors shall be the only ones.

Build logs from the test build for reference: https://download.copr.fedorainfracloud.org/results/ksurma/pygments-2.9.0/fedora-rawhide-x86_64/02257459-python3-docs/build.log.gz

Thanks; this looks like an easy fix, then. No time right now, but I'll aim to get to it at some point before the end of the weekend, if no-one beats me to it.

Actually, for 3.8.10 I had to add one more keyword which confused Sphinx: default.

Fixed in GH-26798 (not yet merged). With that change, the documentation builds cleanly for me with Python 3.9.5 / Sphinx 4.0.2 / blurb 1.0.8 / python-docs-theme 2021.5, using the command

$ python -m sphinx -b html -W . build/html

New changeset 291848195f85e23c01adb76d5a0ff9c6eb7f2614 by Mark Dickinson in branch 'main':
bpo-44426: Fix use of the C keyword 'default' as a variable name (GH-26798)
https://github.com/python/cpython/commit/291848195f85e23c01adb76d5a0ff9c6eb7f2614

New changeset 139c5778c26aaf25b51fcfabd88c99ba728beaea by Miss Islington (bot) in branch '3.10':
bpo-44426: Fix use of the C keyword 'default' as a variable name (GH-26798) (GH-26804)
https://github.com/python/cpython/commit/139c5778c26aaf25b51fcfabd88c99ba728beaea

New changeset 533bff4e9fe221a7c9cf12795fd2d002e87bfa6a by Miss Islington (bot) in branch '3.9':
bpo-44426: Fix use of the C keyword 'default' as a variable name (GH-26798) (GH-26805)
https://github.com/python/cpython/commit/533bff4e9fe221a7c9cf12795fd2d002e87bfa6a

I think this should be good now.

It looks good on my end, documentation builds just fine. Thank you!

Thanks! Closing.