[PATCH v2] cpp/c: Add -Wexpansion-to-defined

[Manuel López-Ibáñez]

On 10 August 2016 at 11:06, Paolo Bonzini <bonzini@gnu.org> wrote:
> While I disagree with their inclusion of the warning to -Wall, I think
> it is a good addition overall.  First, it is a logical extension of the
> existing warning for breaking defined across a macro and its caller.
> Second, it is good to make these warnings for `defined' available with
> a command-line option other than -pedantic.  In fact this warning is
> not mandated by the standard and thus is a strange case of a non-pedwarn
> enabled by -pedantic.  As a side effect of using the command-line parsing
> machinery to attach the new warning to -pedantic, it would become an
> error for -pedantic-errors, which would be weird for a diagnostic that
> is not mandated by the standard.

Note that the definition of -pedantic-errors says: "in some cases
where there is undefined behavior at compile-time". Thus, this would
be allowed according to our current definitions. However, the
definition of -Wpedantic does not mention that, thus it could be a
pedwarn not controlled by -Wpedantic.

My only fear is that people not using -Wpedantic nor -pedantic-errors
expect that GNU extensions work. This is a GNU extension that defines
something that is undefined according to ISO. Enabling the warning
with -Wextra is just annoying those people who may not care about
other compilers.

Thus, my opinion is that the current definition of -Wpedantic is too
restrictive and it should contain the "in some cases where there is
undefined behavior at compile-time". And thus, this should be a
pedwarn enabled by -Wpedantic, not by -Wextra and an error with
-pedantic-errors. But you should wait for other opinions, specially
Joseph, before redoing it, even if you agree with me.

Cheers,

Manuel.