Is [[nodiscard]] any different from [[gnu::warn_unused_result]]?

Question

I had some code that used the GCC extension [[gnu::warn_unused_result]] (a.k.a. __attribute__((__warn_unused_result__))). Now I attempted to use C2x's [[nodiscard]] and I got an incomprehensible error. I'm not sure if the usage of [[nodiscard]] is considerably different from the old GCC attribute, or if it's just a bug in GCC:

$ cat warn_unused_result.c 
[[gnu::warn_unused_result]]
int foo(void);

[[gnu::warn_unused_result]]
int bar(void);


int foo(void)
{
    return 1;
}

int bar(void)
{
    return foo();
}

$ cc -Wall -Wextra -Werror -std=c2x -c warn_unused_result.c

$ cat nodiscard.c 
[[nodiscard]]
int foo(void);

[[nodiscard]]
int bar(void);


int foo(void)
{
    return 1;
}

int bar(void)
{
    return foo();
}

$ cc -Wall -Wextra -Werror -std=c2x -c nodiscard.c          
nodiscard.c:2:1: error: 'nodiscard' attribute directive ignored [-Werror=attributes]
    2 | int foo(void);
      | ^~~
nodiscard.c:5:1: error: 'nodiscard' attribute directive ignored [-Werror=attributes]
    5 | int bar(void);
      | ^~~
cc1: all warnings being treated as errors

$ cc --version
cc (Debian 10.2.1-6) 10.2.1 20210110
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Obviously, I'm not ignoring the return value of any of the functions.

That 1 is never lost (the callers of bar() are in a different translation unit, which might be the problem triggering GCC's behavior?...)

Ahh, it seems that it is not yet implemented by GCC, and therefore ignored. However, it seems to at least consider it differently than a random attribute such as `[[foobar]]`, because the error is different `attribute directive ignored` vs `attribute ignored`; probably because it is understood to be C++. This is what happens when you attempt to use unreleased features :) — alx - recommends codidact, Jul 20 '21 at 22:25
Yeah, I get the same warnings when using [gcc v10.2](https://godbolt.org/z/Y5z9nWT5E), but they disappear when using [gcc v11.1](https://godbolt.org/z/8fYWbTzvj) — yano, Jul 20 '21 at 22:28
@yano Thanks! Upgrading to gcc from debian experimental (gcc 11.1.0) silenced the warnings :) — alx - recommends codidact, Jul 20 '21 at 22:33
However... Now I found an actual bug in `[[nodiscard]]`. Time to report it :) — alx - recommends codidact, Jul 20 '21 at 22:36

score 1 · Accepted Answer · answered Mar 24 '23 at 15:08

The difference is that C2X will likely suggest to implementations that the warning can be easily suppressed:

Evaluation of a nodiscard call as a void expression […] is discouraged unless explicitly cast to void.

By contrast, GCC's implementation of attribute warn_unused_result warns even in those cases:

Is [[nodiscard]] any different from [[gnu::warn_unused_result]]?

1 Answers1