> -Werror-implicit-function-declaration isn't really an issue.  It is a standard warning 
> with very clear semantics.  If Ruby always builds with that warning promoted to an error 
> that's not likely to be an issue in practice.
> 
> The problem is when a new compiler (gcc, clang, or whatever) comes out with a *new* warning 
> that is enabled by default, or changes some existing warning in a way that causes it to 
> trigger in a way it did not before.  That will cause the build to break with -Werror.

You don't understand current situation correctly. We didn't and never use -Werror. The problem is,
we used -Werror=shorten-64-to-32 and other some specific -Werror= options and _Apple_ changed the
behavior of -Werror=shorten-64-to-32.

So, your comment completely make no sense.