-Wimplicit-fallthrough generating warnings

[nohlson]

Bug report

Bug description:

Due to the addition of -Wimplicit-fallthrough as a BASEFLAG new warnings are generated.

This should be reverted until tooling is created to track these new warnings per #112301

Warnings can be found in builds https://buildbot.python.org/all/#/builders/721/builds/1465/steps/3/logs/warnings__143_ from #121030

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-121040: Remove fallthrough warnings compiler option #121041
• gh-121040: Use __attribute__((fallthrough)) #121044

[sobolevn]

A potential fix could look something like:

#if __has_attribute(__fallthrough__)
#  define _Py_FALLTHROUGH __attribute__((__fallthrough__))
#else
#  define _Py_FALLTHROUGH do { } while (0)
#endif

[vstinner]

Previous issues:

• Add -Wimplicit-fallthrough=0 to Makefile ? #75106
• gcc 7 fallthrough warnings #75200

[vstinner]

A potential fix could look something like: (...)

I wrote PR gh-121044 to fix these compiler warnings; the PR adds a new _Py_FALLTHROUGH macro.

[corona10]

A potential fix could look something like:

It can not be the ultimate solution.

• Should check MSVC is accept this macro. (or do nothing if the compiler is MSVC)
• Even if we added the macro for our codebase, vendored libraries like libexpat should follow the new compiler rule.

[vstinner]

Should check MSVC is accept this macro. (or do nothing if the compiler is MSVC)

Does MSC implement a flag like -Wimplicit-fallthrough? If not, I don't think that we should care about MSC in _Py_FALLTHROUGH macro.

Even if we added the macro for our codebase, vendored libraries like libexpat should follow the new compiler rule.

If you want to use this compiler flag, you can use system libraries, instead of building embedded copies of libexpat and libmpdecimal.

[corona10]

If you want to use this compiler flag, you can use system libraries, instead of building embedded copies of libexpat and libmpdecimal.

IIUC, the original intention was to adopt that compiler flag as default, not optional.

[corona10]

Does MSC implement a flag like -Wimplicit-fallthrough? If not, I don't think that we should care about MSC in _Py_FALLTHROUGH macro.

But it emits the compiler warning so we should provide dummy macro for the MSVC
see: https://github.com/python/cpython/pull/121044/files#r1654967484

[vstinner]

But it emits the compiler warning so we should provide dummy macro for the MSVC
see: https://github.com/python/cpython/pull/121044/files#r1654967484

You misunderstood the warning. It was only about __has_attribute(). I fixed this issue.

[vstinner]

Warnings were fixed by 12af8ec. I close the issue.