[PATCH] D101192: Add support for #elifdef and #elifndef

[Aaron Ballman via cfe-commits]

On Thu, Oct 21, 2021 at 2:40 PM Richard Smith - zygoloid via
Phabricator <reviews at reviews.llvm.org> wrote:
>
> rsmith added a comment.
>
> > In this version of the patch, I am supporting the feature in all C and C++ modes. This seems like more useful functionality than diagnosing an unknown preprocessor directive in older language modes, and is a conforming extension in those modes anyway as use of an unknown directive is undefined behavior.
>
> Is this really true? I think:
>
>   #define X
>   #if 0
>   #elifdef X
>   #error
>   #endif
>
> ... is a valid translation unit in C++ and is strictly conforming in C11 and earlier; any //conditionally-supported-directive// (using the C++ terminology) within a //group// is ignored when skipping that //group//.
>
> That said... applying this to all language modes does seem like the right choice, even though it may be technically non-conforming.

Hrmmm. You're correct that processing within a skipped group is
special. C2x 6.10.1p9 says "...is skipped: directives are processed
only through the name that determines the directive in order to keep
track of the level of nested conditionals; the rest of the directives'
preprocessing tokens are ignored."

6.10p9 says "The execution of a non-directive preprocessing directive
results in undefined behavior."

I believe we don't *execute* the directive as part of "directives are
processed" because in order to execute the directive, we need to know
the remaining preprocessing tokens for it. I was originally thinking
that merely hitting the name was sufficient for us to enter into UB.
So I think you're correct that this is technically nonconforming.

That said, I still think it's the right decision because the
alternative is worse.

~Aaron