Issue #17752 has been updated by xtkoba (Tee KOBAYASHI).


I think it's just a matter of coding style to allow evaluating undefined identifiers, whether it is good or bad.

The example of `ext/nkf/nkf-utf8/nkf.h` seems intentional, just not bothering to write:
```c
#ifndef DEFAULT_NEWLINE
#define DEFAULT_NEWLINE 0
#endif
```

If this practice should be error-prone, we might have to change the style. I am neutral for now.

----------------------------------------
Feature #17752: Enable -Wundef for C extensions in repository
https://bugs.ruby-lang.org/issues/17752#change-91111

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
----------------------------------------
I would like to enable `-Wundef` for C extensions built/bundled with CRuby.

From https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
> -Wundef
>    Warn if an undefined identifier is evaluated in an #if directive. Such identifiers are replaced with zero.

I found this warning to be quite useful, notably when investigating why a given C extension did not include some code I expected, and then building those extensions on TruffleRuby.

There are a couple places not respecting this currently but they seem trivial to fix, I can do that.

For instance a confusing case is:
https://github.com/ruby/ruby/blob/9143d21b1bf2f16b1e847d569a588510726d8860/ext/nkf/nkf-utf8/nkf.h#L19
```
#if DEFAULT_NEWLINE == 0x0D0A
```
which without -Wundef would just exclude the code without any warning if DEFAULT_NEWLINE is not defined.

I'm not sure if we should/can enable it for C extensions in general (installed as gems), as if a C extensions uses -Werror and would have such a warning it would no longer build.

I can make a PR for this.
I'm not sure where to add -Wundef though, should it be in https://github.com/ruby/ruby/blob/9143d21b1bf2f16b1e847d569a588510726d8860/configure.ac#L620, or maybe in mkmf.rb?



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