[Bug c/101545] New: [[nodiscard]]: Incorrect warning when creating a function alias

[Bug c/101545] New: [[nodiscard]]: Incorrect warning when creating a function alias

[colomar.6.4.3 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101545

            Bug ID: 101545
           Summary: [[nodiscard]]: Incorrect warning when creating a
                    function alias
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: colomar.6.4.3 at gmail dot com
  Target Milestone: ---

The following code never discards that `42`, passing it to the caller of
foobar() (in a different translation unit), and forcing it to read the value.
The warnings about an ignored [[nodiscard]] are therefore incorrect.

I put the contents of the header in the same source file just for simplicity.


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

[[gnu::copy(foo)]] extern __typeof__(foo) bar;

[[nodiscard]] int foobar(void);


int foo(void)
{
        return 42;
}

int foobar(void)
{
        return bar();  /* This will return 42 */
}

[[gnu::alias("foo")]] [[gnu::copy(foo)]] extern __typeof__(foo) bar;


$ cc -Wall -Wextra -std=c2x -c nodiscard.c 
nodiscard.c:3:1: warning: 'nodiscard' attribute directive ignored
[-Wattributes]
    3 | [[gnu::copy(foo)]] extern __typeof__(foo) bar;
      | ^
nodiscard.c:18:1: warning: 'nodiscard' attribute directive ignored
[-Wattributes]
   18 | [[gnu::alias("foo")]] [[gnu::copy(foo)]] extern __typeof__(foo) bar;
      | ^

[Bug c/101545] [[nodiscard]]: Incorrect warning when creating a function alias

[colomar.6.4.3 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101545

--- Comment #1 from Alejandro Colomar <colomar.6.4.3 at gmail dot com> ---
The same code with [[gnu::warn_unused_result]] instead of [[nodiscard]] doesn't
trigger the warning.

[Bug c/101545] copy attribute is not copying the C++11/C23 attributes correctly.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101545

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-04-09
     Ever confirmed|0                           |1
            Summary|[[nodiscard]]: Incorrect    |copy attribute is not
                   |warning when creating a     |copying the C++11/C23
                   |function alias              |attributes correctly.
                 CC|                            |pinskia at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

It has nothing to do with aliases at all. All has to do with the copy attribute
not copying the C++11/C23 attributes correctly.

Even the following C++ code incorrectly warns.

Like:
```
[[maybe_unused]] int foo(void);

[[gnu::copy(foo)]] extern __typeof__(foo) bar;

```