[Bug preprocessor/49928] Only workaround for "-Wundef" is "defined(Macro) && Macro", but it is undefined behavior?

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49928

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Johannes Schaub from comment #0)
> It appears that to inhibit a warning about using an undefined macro
> identifier, one has to employ the following work-around
> 
>     #define FOO BAR
>     #if FOO // warns about 'BAR'
> 
> Instead:
> 
>     #define FOO defined BAR && BAR 
>     #if FOO

I don't think a new GCC-specific builtin just to avoid an optional warning is a
good idea.

It seems to me that a better workaround that doesn't involve undefined
behaviour, and is much simpler to understand, would be to simply ensure that
BAR is always defined:

#define BAR 0
#define FOO BAR

If WebKit wants to be able to compile cleanly with -Wundef then they should
ensure all macros are defined, not create nasty hacks using undefined
behaviour.

So e.g.

// define everything to 0 initially
#define WTF_PLATFORM_CHROMIUM 0
#define WTF_PLATFORM_QT 0
#define WTF_PLATFORM_MAC 0

#if defined(BUILDING_CHROMIUM__)
# undef WTF_PLATFORM_CHROMIUM
# define WTF_PLATFORM_CHROMIUM 1
#elif ...


Alternatively, it would be nice to use:

#pragma GCC diagnostic ignored "-Wundef"

to suppress the warnings in code bases that are (by design) not compatible with
-Wundef ... unfortunately that doesn't work (PR 53431).