[Bug libstdc++/109941] New: [feat req] Add an option to mark STL types as nodiscard

[Bug libstdc++/109941] New: [feat req] Add an option to mark STL types as nodiscard

[roi.jacobson1 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109941

            Bug ID: 109941
           Summary: [feat req] Add an option to mark STL types as
                    nodiscard
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: roi.jacobson1 at gmail dot com
  Target Milestone: ---

I would find it useful if there was a macro or some other option to mark types
like std::error_code, std::errc, and std::expected as [[nodiscard]] so
developers will be forced to handle the errors.

[Bug libstdc++/109941] [feat req] Add an option to mark STL types as nodiscard

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109941

Jiang An <de34 at live dot cn> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |de34 at live dot cn

--- Comment #1 from Jiang An <de34 at live dot cn> ---
FYI MSVC STL currently applies [[nodiscard]] to std::lock_guard and
std::scoped_lock.

[Bug libstdc++/109941] [feat req] Add an option to mark STL types as nodiscard

[roi.jacobson1 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109941

--- Comment #2 from Roy Jacobson <roi.jacobson1 at gmail dot com> ---
Right, libc++ do that as well since 2019 -
https://reviews.llvm.org/D65900?id=213975.

Although with the RAII classes I think you need to [[nodiscard]] the
constructor which I think (?) is a compiler extension. And you're also much
less likely to have false positives with that so you can actually do that
unconditionally :)

[Bug libstdc++/109941] [feat req] Add an option to mark STL types as nodiscard

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109941

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Depends on|                            |85973

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That doesn't currently work with GCC, so there's not much point doing it.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85973
[Bug 85973] [[nodiscard]] on class shall emit a warning for unused anonymous
variable

[Bug libstdc++/109941] [feat req] Add an option to mark STL types as nodiscard

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109941

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement

[Bug libstdc++/109941] [feat req] Add an option to mark STL types as nodiscard

[roi.jacobson1 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109941

--- Comment #4 from Roy Jacobson <roi.jacobson1 at gmail dot com> ---
The linked bug is about nodiscard applied to the class together with ignoring
temporary objects. But GCC will when the type is the return value or when the
nodiscard is applied to the constructor:

class [[nodiscard]] err {};
err f();

void g() {
    f(); // Warning emitted
}

struct RAII {
    [[nodiscard]] RAII(int x);
};

void h() {
    RAII(0); // Warning emitted
}