In bpo-40874, mpdecimal.h in the vendored libmpdec has defines of UNUSED while the standalone released version of mpdecimal 2.5.0 doesn't. This breaks _decimal module build with system libmpdec due to UNUSED is undefined.

Errors are like:

cpython/Modules/_decimal/_decimal.c:277:36: error: expected ‘;’, ‘,’ or ‘)’ before ‘UNUSED’
  277 | dec_traphandler(mpd_context_t *ctx UNUSED) /* GCOV_NOT_REACHED */
      |                                    ^~~~~~

Reproducible in both 3.8 branch and master (didn't test 3.9, but should be affected too).

Thank you for the report, I'll add the define or remove UNUSED in 3.9
and 3.10.


3.8 is still supposed to use libmpdec-2.4.2, though it would be
harmless to use libmpdec-2.5.0.

Are you planning to use libmpdec-2.5.0 with 3.8?

Yes, I am currently defining it manually as a workaround for building 3.8.4 in Arch.

Also opened GH-21481 for this :)

New changeset 015efdbef7454a522e88cd79ba2b4cd77a5fb2a2 by Felix Yan in branch 'master':
bpo-41302: Fix build with system libmpdec (GH-21481)
https://github.com/python/cpython/commit/015efdbef7454a522e88cd79ba2b4cd77a5fb2a2

New changeset 8ca63f95f769c31f65a996fe065ff6dcfb490bbe by Miss Islington (bot) in branch '3.9':
bpo-41302: Fix build with system libmpdec (GH-21481)
https://github.com/python/cpython/commit/8ca63f95f769c31f65a996fe065ff6dcfb490bbe

Thanks for the patch!

The integrated libmpdec-2.4.2 in Python 3.8 still has a couple of
UNUSED, so the simple approach of moving the define would not work.
It would be easy to replace the few instances of UNUSED with void
casts like in 2.5.0.


Strictly speaking though, this would be a feature for 3.8 and not
a build fix.


On the other hand, if Debian also plans to use 2.5.0 with 3.8,
there would be a stronger case for this build feature.

I'm going to reclassify this as a build fix for 3.8. 3.8 promises:

#if !defined(MPD_VERSION_HEX) || MPD_VERSION_HEX < 0x02040100
  #error "libmpdec version >= 2.4.1 required"
#endif

So it seems reasonable to support at least two or three consecutive
system libmpdec versions, otherwise the >= is pointless.

New changeset 16eea45fbd3b7c3d1b222b7eb7a5d7ee427f70bd by Felix Yan in branch '3.8':
[3.8] bpo-41302: Support system libmpdec 2.5 for Python 3.8 (GH-21488)
https://github.com/python/cpython/commit/16eea45fbd3b7c3d1b222b7eb7a5d7ee427f70bd

Closing, thanks for all the patches!