The problem
-----------

In the repository, the definition for ``AX_CHECK_COMPILE_FLAG`` in Python's ``configure.ac`` file is missing. If ``autoreconf`` is run, an invalid ``configure`` script is generated. The following is the behaviour of running ``autoreconf`` followed by ``configure``:

```
# In cpython repository top-level directory
$ autoreconf
$ mkdir build
$ cd build
$ ../configure  # <- using newly generated configure script
[... omitted ...]
checking for --enable-optimizations... no
../configure: line 6498: syntax error near unexpected token `-fno-semantic-interposition,'
../configure: line 6498: `      AX_CHECK_COMPILE_FLAG(-fno-semantic-interposition,'
```


The solution
------------

It appears a file was missing in the m4/ directory. The file matches this one from the Autoconf Archive:

https://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html

Simply adding the correct m4 file to m4/ should make ``autoreconf`` work.

> If ``autoreconf`` is run, ...

I never used autoreconf. If you don't run it, ./configure works fine.

I only use "autoconf" and "autoheader".

$ ./configure --enable-optimizations --enable-shared
(...)
$ grep semantic Makefile
CONFIGURE_CFLAGS_NODIST= -fno-semantic-interposition -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration -fvisibility=hidden
CONFIGURE_LDFLAGS_NODIST= -fno-semantic-interposition

From the configure.ac file:

> dnl ***********************************************
> dnl * Please run autoreconf to test your changes! *
> dnl ***********************************************

I take it to mean "if configure.ac is changed, run autoreconf first", and that's what I did. Could you let me know what is the intended way to modify configure.ac and have it take effect?

Ah, maybe the configure.ac comment should be updated to suggest running "autoconf" instead ;-)

The extra macros are provided by optional packages. On Fedora and Debian/Ubuntu the package is called autoconf-archive.

> The extra macros are provided by optional packages. On Fedora and Debian/Ubuntu the package is called autoconf-archive.

Thank you very much. This (and the patch) is clearing things up for me a lot. At first I thought I was supposed to copy the m4 files over from the autoconf archive into the m4 directory under the repo.

New changeset 5d6e8c1c1a5f667cdce99cb3c563ac922198678d by Christian Heimes in branch 'master':
bpo-43617: Check autoconf-archive package in configure.ac (GH-25016)
https://github.com/python/cpython/commit/5d6e8c1c1a5f667cdce99cb3c563ac922198678d

New changeset 064bc07f241dceec2fc577cbf5c31fa6d63fe320 by Christian Heimes in branch '3.9':
[3.9] bpo-43617: Check autoconf-archive package in configure.ac (GH-25016) (GH-25034)
https://github.com/python/cpython/commit/064bc07f241dceec2fc577cbf5c31fa6d63fe320

New changeset e516290976626cf8535b88a14b1b34e37f88a78a by Christian Heimes in branch '3.8':
[3.8] bpo-43617: Check autoconf-archive package in configure.ac (GH-25016) (GH-25035)
https://github.com/python/cpython/commit/e516290976626cf8535b88a14b1b34e37f88a78a

Thanks for the bug report! I ran into the issue a couple of weeks ago on one machine that had autoconf but not the archive package installed.

Hmm, I've seen this accomplished elsewhere using m4_pattern_forbid, which would make autoreconf fail with the following message:

```
configure.ac:17: error: possibly undefined macro: AX_C_FLOAT_WORDS_BIGENDIAN
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
configure.ac:5586: error: possibly undefined macro: AX_CHECK_OPENSSL
autoreconf: error: /usr/bin/autoconf failed with exit status: 1
```

Example patch attached.