[Clang] Possible bug with labels at the end of compound statements in C

[OCHyams]

With this test case test.c:

void fun(void) {
label:
}

Before D133887 we got an error:

$ clang label.c -c -std=c17
label.c:3:1: error: expected statement
}
^
1 error generated.

With D133887 we just get a warning:

$ clang label.c -c -std=c17
label.c:3:1: warning: label at end of compound statement is a C2x extension [-Wc2x-extensions]
}
^
1 warning generated.

I'm not very familiar with the C standard but it seemed suspicious that this error has become a warning even though -std=c17 is specified in the example. Is this a bug?

Also see on compiler explorer with clang-15 vs trunk.

cc @cor3ntin and @Izaron from D133887.

[llvmbot]

@llvm/issue-subscribers-c

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[Izaron]

Hi! This isn't a bug, it's a C++2b (tentatively C++23) and C2x (tentatively C23) feature: https://github.com/cplusplus/papers/issues/1006

even though -std=c17 is specified in the example

Clang backports most of the features of more recent standards to all possible standards (with warnings) if it doesn't break important things.

If you want to forbid backporting features of the more recent standard, you can use -Werror=c2x-extensions flag

[AaronBallman]

This behavior is by design, closing as not a bug.

[OCHyams]

Ok, thanks for the info!

[AaronBallman]

Btw, the reference to the C paper for this feature is: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2508.pdf