aio-libs / multidict / 36115378344

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Fail codegen_diff when a compared tree lacks a declaration (#1572)

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## What do these changes do?

`tools/codegen_diff.py` compiled both trees with `-w`. On GCC 13 and
older an implicit function declaration is only a warning in C11, so `-w`
hid it: a `--probe` calling `md_add_with_hash_steal_refs` compiled
against a `--ref` from before #1571 (where it was
`_md_add_with_hash_steal_refs`) built a call to an undeclared function,
and the tool exited 0 with a meaningless "N of M functions differ".

The builds now pass `-Werror=implicit-function-declaration` and no
longer pass `-w`. Simply adding the flag after `-w` is not enough: GCC
discards an inhibited warning before `-Werror=` gets to reclassify it,
which can be reproduced on GCC 14+ with `-fpermissive`. Compiler output
is captured and printed only when a build fails, so warnings stay quiet
otherwise. A failing build now exits non-zero with `before build (--ref
X) failed` or `after build (working tree) failed` instead of a
`CalledProcessError` traceback. `--self-test` gained two cases that
expect an undeclared call to fail, one of them with `-fpermissive` so
the check also covers the pre-14 behaviour on newer compilers.

## Are there changes in behavior for the user?

No, this is contributor tooling only.

## Is it a substantial burden for the maintainers to support this?

No.

## Related issue number

Follow-up to the review of #1571
(https://github.com/aio-libs/multidict/pull/1571#pullrequestreview-5308437866).

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist (`--self-test` cases)
- [x] Documentation reflects the changes
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt` N/A
- [x] Add a new news fragment into the `CHANGES/` folder
- [x] `make doc-spelling` passes and any new technical words are added
to `docs/spelling_wordlist.txt`

Drafted with Claude Code (Claude Opus 5.5); reviewed ... (continued)

778 of 1556 branches covered (50.0%)

Branch coverage included in aggregate %.

7968 of 8480 relevant lines covered (93.96%)

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