_decimal failed to build with system libmpdec 2.5

[felixonmars]

BPO	41302
Nosy	@doko42, @skrah, @felixonmars, @miss-islington
PRs	bpo-41302: Fix build with system libmpdec #21481 [3.9] bpo-41302: Fix build with system libmpdec (GH-21481) #21486 [3.8] bpo-41302: Support system libmpdec 2.5 for Python 3.8 #21488

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/skrah'
closed_at = <Date 2020-07-15.20:30:40.352>
created_at = <Date 2020-07-15.10:54:56.624>
labels = ['extension-modules', 'build', '3.8', '3.9', '3.10']
title = '_decimal failed to build with system libmpdec 2.5'
updated_at = <Date 2020-07-15.20:30:40.351>
user = 'https://github.com/felixonmars'

bugs.python.org fields:

activity = <Date 2020-07-15.20:30:40.351>
actor = 'skrah'
assignee = 'skrah'
closed = True
closed_date = <Date 2020-07-15.20:30:40.352>
closer = 'skrah'
components = ['Extension Modules']
creation = <Date 2020-07-15.10:54:56.624>
creator = 'felixonmars'
dependencies = []
files = []
hgrepos = []
issue_num = 41302
keywords = ['patch']
message_count = 9.0
messages = ['373676', '373677', '373679', '373682', '373691', '373692', '373721', '373722', '373723']
nosy_count = 4.0
nosy_names = ['doko', 'skrah', 'felixonmars', 'miss-islington']
pr_nums = ['21481', '21486', '21488']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue41302'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[felixonmars]

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).

[skrah]

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?