[Bug c++/109235] New: nodiscard attribute ignored with deduction guide

[Bug c++/109235] New: nodiscard attribute ignored with deduction guide

[tejo@bang-olufsen.dk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109235

            Bug ID: 109235
           Summary: nodiscard attribute ignored with deduction guide
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tejo@bang-olufsen.dk
  Target Milestone: ---

I have tried implementing std::experimental::scope_exit and since this is a
RAII object, I marked it with [[nodiscard]] to ensure all instances are
assigned to a variable.

It appears, however, that GCC ignores the nodiscard attribute when a deduction
guide is present. See this Godbolt link for an example:

https://godbolt.org/z/v9e8dqW1o

Notice how Clang correctly flags the problem.

[Bug c++/109235] nodiscard attribute ignored with deduction guide

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109235

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 54721
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54721&action=edit
Full testcase with -std=c++20

Please attach the testcase next time or put it inline and not just a link to
godbolt .

[Bug c++/109235] nodiscard attribute ignored on temporary created

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109235

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-03-21
            Summary|nodiscard attribute ignored |nodiscard attribute ignored
                   |with deduction guide        |on temporary created

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This has nothing to do with deduction guides either.
Reduced testcase:
```
struct [[nodiscard]] scope_exit {
        scope_exit(int);
};


int main()
{
    scope_exit{0};
}

```

[Bug c++/109235] nodiscard attribute on class not copied to the constructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109235

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|nodiscard attribute ignored |nodiscard attribute on
                   |on temporary created        |class not copied to the
                   |                            |constructors

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
If we move the nodiscard to the constructor, GCC errors out correctly.

[Bug c++/109235] nodiscard attribute on class not copied to the constructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109235

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #3)
> If we move the nodiscard to the constructor, GCC errors out correctly.

That is true even on the original testcase.

[Bug c++/109235] nodiscard attribute on class not copied to the constructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109235

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|NEW                         |RESOLVED

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 85973.

*** This bug has been marked as a duplicate of bug 85973 ***

[Bug c++/109235] nodiscard attribute on class not copied to the constructors

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109235

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Teis Johansen from comment #0)
> I have tried implementing std::experimental::scope_exit

N.B. this is already present in the upcoming GCC 13:

https://gcc.gnu.org/gcc-13/changes.html#libstdcxx