bug#63556: 29.0.90; Use of _Generic breaks Emacs build on GCC <4.9

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: eggert@cs.ucla.edu,  sbaugh@janestreet.com,  63556@debbugs.gnu.org
> Date: Thu, 18 May 2023 21:21:51 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Depends on how complex is that, and what problem does it solve.
> 
> The problem of using `long long' on systems with less than 64
> significant bits in their words.  I don't think that's a particularly
> good idea -- once such uses pile up, Emacs will gradually become slower
> and slower on such systems.

What does this have to do with _Generic?

> >>   #define foo(expression)                    \
> >>     (sizeof (expression) == sizeof (signed char)  \
> >>      ? code_for_signed_char                          \
> >>      : (sizeof (expression) == sizeof (short int) \
> >>    ? code_for_short_int                      \
> >>    : (sizeof (expression) == sizeof (int)    \
> >>       ? code_for_int                         \
> >>       ... and so on
> >
> > AFAIR, sizeof is a compile-time expression, the preprocessor cannot
> > evaluate it.
> 
> It doesn't have to be evaluated by the preprocessor (and nor is
> _Generic.)  It just needs to expand to an expression that does the right
> thing.

And what is "the right thing" in this case?

I guess I don't understand what this has to do with _Generic.