[issue43615] [PATCH] Properly implement Py_UNREACHABLE macro using autoconf.

[STINNER Victor]

STINNER Victor <vstinner at python.org> added the comment:

> The headers Python.h and also the ones pulled in by it were actually from Python 3.8 release, which unconditionally defines the macro as a call to Py_FatalError.

Using __builtin_unreachable() is a recent change (bpo-38249), Python 3.9.0:

* commit eebaa9bfc593d5a46b293c1abd929fbfbfd28199

Fix for old GCC (bpo-41875):

* master: commit 24ba3b0df5e5f2f237d7b23b4017ba12f16320ae
* 3.9.1: commit cca896e13b230a934fdb709b7f10c5451ffc25ba

I prefer to not backport this feature, since, as you wrote, it's tricky to make sure that it's available on the C compiler.

I close the issue since Py_UNREACHABLE() works as expected on Python 3.9 and newer.

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stage: resolved -> 

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