-Wnull-pointer-arithmetic and -Wno-gnu

[nickdesaulniers]

The following code w/ ToT LLVM (clang-15) produces the following warnings:

void *x(void) {
    return (char*)0 + 42;
}

void *y(void) {
    return (void*)0 + 42;
}

<source>:2:21: warning: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Wnull-pointer-arithmetic]
    return (char*)0 + 42;
           ~~~~~~~~ ^
<source>:6:21: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
    return (void*)0 + 42;
           ~~~~~~~~ ^

for the Linux kernel, we're using -std=gnu11 -Wno-gnu, so we should not be observing ANY warnings that we're using GNU C extensions.

I suspect that these two checks should be split into two different named command line flags, such that the one pertaining to GNU C extensions is placed within the -Wgnu flag group, such that -Wno-gnu implicitly disables it.

cc @AaronBallman @andykaylor

(link #4440)

As reported in this thread.

[efriedma-quic]

See https://reviews.llvm.org/D37042 for why this warning exists in its current form.

[andykaylor]

Handling this is beyond my knowledge of the front end diagnostics infrastructure. I had even forgotten that I was the person who added this warning. Based on the discussion in the review, it sounds like marking the warning specifically as a GNU extension will cause it to be an error with -pedantic-errors, but not doing so is the reason it isn't suppressed by -Wno-gnu. Is that right?

@AaronBallman Can you help with this?

[AaronBallman]

I think the issue is that the GNU extension diagnostic was added to the -Wnull-pointer-arithmetic group, which is a part of -Wextra and not -Wgnu. I spotted some other diagnostics in the same boat, so I'll try to get this fixed up.

[nickdesaulniers]

https://reviews.llvm.org/D122224

[AaronBallman]

This should now be fixed in c3fe8dd.