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cgccunused-variablesnodiscardc23

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

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?...)

Solution

  • 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: