[Bug c++/95454] New: type-level nodiscard not applied to constructors

[Bug c++/95454] New: type-level nodiscard not applied to constructors

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95454

            Bug ID: 95454
           Summary: type-level nodiscard not applied to constructors
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: johelegp at gmail dot com
  Target Milestone: ---

The following does not warn for constructors. See https://godbolt.org/z/BTZwew.
```C++
struct [[nodiscard]] X {
    int x;
    X operator+() const { return *this; }
};
class [[nodiscard]] Y {
    int y;
public:
    Y operator+() const { return *this; }
};
void f()
{
    X{};
    Y{};
    X x;
    +x;
    Y y;
    +y;
}
```

[Bug c++/95454] type-level nodiscard not applied to constructors

[steve+gcc at tecwec dot eu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95454

Eric Estievenart <steve+gcc at tecwec dot eu> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |steve+gcc at tecwec dot eu

--- Comment #1 from Eric Estievenart <steve+gcc at tecwec dot eu> ---
Indeed, confirming with even simpler code ( https://godbolt.org/z/EPbsfxYcj ):

struct [[nodiscard]] Z {};

void f()
{
    Z{}; // should warn but does not
}

Tested all versions: 11.2, trunk, with -Wall -Wextra -....

This is really a need for modern code, even experienced developpers sometimes
forget the 'lock' in
Lock lock{&mutex};
and this has severe consequences ;-)

[Bug c++/95454] type-level nodiscard not applied to constructors

[kyrylo.bohdanenko at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95454

Kyrylo Bohdanenko <kyrylo.bohdanenko at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kyrylo.bohdanenko at gmail dot com

--- Comment #2 from Kyrylo Bohdanenko <kyrylo.bohdanenko at gmail dot com> ---
Still relevant as of the current trunk.

A workaround would be to declare the constructor(s) [[nodiscard]]:

struct Data {
    [[nodiscard]] Data() {}
};

int main() {
    Data{};
}

This actually issues a warning:

<source>: In function 'int main()':
<source>:6:11: warning: ignoring return value of 'Data::Data()', declared with
attribute 'nodiscard' [-Wunused-result]

Sandbox: https://godbolt.org/z/3Koj8rra7

[Bug c++/95454] type-level nodiscard not applied to constructors

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95454

--- Comment #3 from Johel Ernesto Guerrero Peña <johelegp at gmail dot com> ---
> A workaround would be to declare the constructor(s) [[nodiscard]]:

That's generally a [better
recommendation](https://github.com/mpusz/units/issues/136). Though I think
you'd have to `= default` SMFs to get the same effects as the  type-level
annotation.

[Bug c++/95454] type-level nodiscard not applied to constructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95454

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

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

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 85973.

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