[Bug c++/100666] New: warning: ignoring return value of 'constexpr _Tp&& std::forward(typename std::remove_reference<_Tp>::type&) [with _Tp = std::nullptr_t; typename std::remove_reference<_Tp>::type = std::nullptr_t]', declared with attribute 'nodiscard' [-Wunused-result]

[Bug c++/100666] New: warning: ignoring return value of 'constexpr _Tp&& std::forward(typename std::remove_reference<_Tp>::type&) [with _Tp = std::nullptr_t; typename std::remove_reference<_Tp>::type = std::nullptr_t]', declared with attribute 'nodiscard' [-Wunused-result]

[mh+gcc at glandium dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100666

            Bug ID: 100666
           Summary: warning: ignoring return value of 'constexpr _Tp&&
                    std::forward(typename
                    std::remove_reference<_Tp>::type&) [with _Tp =
                    std::nullptr_t; typename
                    std::remove_reference<_Tp>::type = std::nullptr_t]',
                    declared with attribute 'nodiscard' [-Wunused-result]
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mh+gcc at glandium dot org
  Target Milestone: ---

(Note I'm not entirely sure this is a GCC bug, but it doesn't happen with GCC <
11 and with clang)

Reduced testcase:
```
#include <utility>

extern void foo(...);

template <typename... Ts>
void bar(Ts&&... aArgs) {
    foo(std::forward<Ts>(aArgs)...);
}

void qux() {
    bar(nullptr);
}
```

This emits the following warning:
```
<source>: In instantiation of 'void bar(Ts&& ...) [with Ts =
{std::nullptr_t}]':
<source>:11:8:   required from here
<source>:7:25: warning: ignoring return value of 'constexpr _Tp&&
std::forward(typename std::remove_reference<_Tp>::type&) [with _Tp =
std::nullptr_t; typename std::remove_reference<_Tp>::type = std::nullptr_t]',
declared with attribute 'nodiscard' [-Wunused-result]
    7 |     foo(std::forward<Ts>(aArgs)...);
      |         ~~~~~~~~~~~~~~~~^~~~~~~
In file included from
/opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/bits/stl_pair.h:59,
                 from
/opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/utility:70,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/bits/move.h:77:5: note:
declared here
   77 |     forward(typename std::remove_reference<_Tp>::type& __t) noexcept
      |     ^~~~~~~
```
(https://godbolt.org/z/GW3xf71n3)

Incidentally, using the x86-64 gcc (static analysis) on compiler explorer fails
with:
```
during IPA pass: analyzer
<source>: In function 'void qux()':
<source>:11:9: internal compiler error: in make_region_for_type, at
analyzer/region-model.cc:5973
   11 |     bar(nullptr);
      |         ^~~~~~~
Please submit a full bug report,
with preprocessed source if appropriate.
See <https://gcc.gnu.org/bugs/> for instructions.
```

[Bug c++/100666] [9/10/11/12 Regression] warning: ignoring return value of 'constexpr _Tp&& std::forward(typename std::remove_reference<_Tp>::type&) [with _Tp = std::nullptr_t; ...]', declared with attribute 'nodiscard' [-Wunused-result]

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100666

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |7.5.0, 8.3.0, 9.2.0
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-05-19
      Known to fail|                            |10.1.0, 11.1.0, 12.0,
                   |                            |8.4.0, 9.3.0
             Status|UNCONFIRMED                 |NEW
                 CC|                            |jakub at gcc dot gnu.org
            Summary|warning: ignoring return    |[9/10/11/12 Regression]
                   |value of 'constexpr _Tp&&   |warning: ignoring return
                   |std::forward(typename       |value of 'constexpr _Tp&&
                   |std::remove_reference<_Tp>: |std::forward(typename
                   |:type&) [with _Tp =         |std::remove_reference<_Tp>:
                   |std::nullptr_t; typename    |:type&) [with _Tp =
                   |std::remove_reference<_Tp>: |std::nullptr_t; ...]',
                   |:type = std::nullptr_t]',   |declared with attribute
                   |declared with attribute     |'nodiscard'
                   |'nodiscard'                 |[-Wunused-result]
                   |[-Wunused-result]           |

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Mike Hommey from comment #0)
> (Note I'm not entirely sure this is a GCC bug, but it doesn't happen with
> GCC < 11 and with clang)

Yeah, this shouldn't warn.

Reduced:

[[nodiscard]] decltype(nullptr) null();

extern void foo(...);

void qux() {
    foo(null());
}


GCC 10 has the same warning, it's just that std::forward didn't have the
[[nodiscard]] attribute.

It only happens when a nodiscard function returns std::nullptr_t and it's
passed to an ellipsis. GCC 7 was OK, and 8.3 and 9.2

It started with r279680:

    re PR c++/92992 (Side-effects dropped when decltype(nullptr) typed
expression is passed to ellipsis)

            PR c++/92992
            * call.c (convert_arg_to_ellipsis): For decltype(nullptr) arguments
            that have side-effects use cp_build_compound_expr.

            * g++.dg/cpp0x/nullptr45.C: New test.

[Bug c++/100666] [9/10/11/12 Regression] warning: ignoring return value of 'constexpr _Tp&& std::forward(typename std::remove_reference<_Tp>::type&) [with _Tp = std::nullptr_t; ...]', declared with attribute 'nodiscard' [-Wunused-result]

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100666

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=92992
   Target Milestone|---                         |9.5

[Bug c++/100666] [9/10/11/12 Regression] warning: ignoring return value of 'constexpr _Tp&& std::forward(typename std::remove_reference<_Tp>::type&) [with _Tp = std::nullptr_t; ...]', declared with attribute 'nodiscard' [-Wunused-result]

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100666

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |jakub at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 50862
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50862&action=edit
gcc12-pr100666.patch

Untested fix.

[Bug c++/100666] [9/10/11/12 Regression] warning: ignoring return value of 'constexpr _Tp&& std::forward(typename std::remove_reference<_Tp>::type&) [with _Tp = std::nullptr_t; ...]', declared with attribute 'nodiscard' [-Wunused-result]

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100666

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:ad52d89808a947264397e920d7483090d4108f7b

commit r12-1043-gad52d89808a947264397e920d7483090d4108f7b
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue May 25 17:24:38 2021 +0200

    c++: Avoid -Wunused-value false positives on nullptr passed to ellipsis
[PR100666]

    When passing expressions with decltype(nullptr) type with side-effects to
    ellipsis, we pass (void *)0 instead, but for the side-effects evaluate them
    on the lhs of a COMPOUND_EXPR.  Unfortunately that means we warn about it
    if the expression is a call to nodiscard marked function, even when the
    result is really used, just needs to be transformed.

    Fixed by adding a warning_sentinel.

    2021-05-25  Jakub Jelinek  <jakub@redhat.com>

            PR c++/100666
            * call.c (convert_arg_to_ellipsis): For expressions with
NULLPTR_TYPE
            and side-effects, temporarily disable -Wunused-result warning when
            building COMPOUND_EXPR.

            * g++.dg/cpp1z/nodiscard8.C: New test.
            * g++.dg/cpp1z/nodiscard9.C: New test.

[Bug c++/100666] [9/10/11/12 Regression] warning: ignoring return value of 'constexpr _Tp&& std::forward(typename std::remove_reference<_Tp>::type&) [with _Tp = std::nullptr_t; ...]', declared with attribute 'nodiscard' [-Wunused-result]

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100666

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:718a78fcfb03ca3c552800b510ef3eb085bcbb76

commit r11-8492-g718a78fcfb03ca3c552800b510ef3eb085bcbb76
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue May 25 17:24:38 2021 +0200

    c++: Avoid -Wunused-value false positives on nullptr passed to ellipsis
[PR100666]

    When passing expressions with decltype(nullptr) type with side-effects to
    ellipsis, we pass (void *)0 instead, but for the side-effects evaluate them
    on the lhs of a COMPOUND_EXPR.  Unfortunately that means we warn about it
    if the expression is a call to nodiscard marked function, even when the
    result is really used, just needs to be transformed.

    Fixed by adding a warning_sentinel.

    2021-05-25  Jakub Jelinek  <jakub@redhat.com>

            PR c++/100666
            * call.c (convert_arg_to_ellipsis): For expressions with
NULLPTR_TYPE
            and side-effects, temporarily disable -Wunused-result warning when
            building COMPOUND_EXPR.

            * g++.dg/cpp1z/nodiscard8.C: New test.
            * g++.dg/cpp1z/nodiscard9.C: New test.

    (cherry picked from commit ad52d89808a947264397e920d7483090d4108f7b)

[Bug c++/100666] [9/10 Regression] warning: ignoring return value of 'constexpr _Tp&& std::forward(typename std::remove_reference<_Tp>::type&) [with _Tp = std::nullptr_t; ...]', declared with attribute 'nodiscard' [-Wunused-result]

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100666

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |11.1.1, 12.0
            Summary|[9/10/11/12 Regression]     |[9/10 Regression] warning:
                   |warning: ignoring return    |ignoring return value of
                   |value of 'constexpr _Tp&&   |'constexpr _Tp&&
                   |std::forward(typename       |std::forward(typename
                   |std::remove_reference<_Tp>: |std::remove_reference<_Tp>:
                   |:type&) [with _Tp =         |:type&) [with _Tp =
                   |std::nullptr_t; ...]',      |std::nullptr_t; ...]',
                   |declared with attribute     |declared with attribute
                   |'nodiscard'                 |'nodiscard'
                   |[-Wunused-result]           |[-Wunused-result]
      Known to fail|12.0                        |

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed on the trunk and for 11.2 so far.

[Bug c++/100666] [9/10 Regression] warning: ignoring return value of 'constexpr _Tp&& std::forward(typename std::remove_reference<_Tp>::type&) [with _Tp = std::nullptr_t; ...]', declared with attribute 'nodiscard' [-Wunused-result]

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100666

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:589a0f34b1d176b62b064c9815c341b6abf48a2b

commit r10-10614-g589a0f34b1d176b62b064c9815c341b6abf48a2b
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue May 25 17:24:38 2021 +0200

    c++: Avoid -Wunused-value false positives on nullptr passed to ellipsis
[PR100666]

    When passing expressions with decltype(nullptr) type with side-effects to
    ellipsis, we pass (void *)0 instead, but for the side-effects evaluate them
    on the lhs of a COMPOUND_EXPR.  Unfortunately that means we warn about it
    if the expression is a call to nodiscard marked function, even when the
    result is really used, just needs to be transformed.

    Fixed by adding a warning_sentinel.

    2021-05-25  Jakub Jelinek  <jakub@redhat.com>

            PR c++/100666
            * call.c (convert_arg_to_ellipsis): For expressions with
NULLPTR_TYPE
            and side-effects, temporarily disable -Wunused-result warning when
            building COMPOUND_EXPR.

            * g++.dg/cpp1z/nodiscard8.C: New test.
            * g++.dg/cpp1z/nodiscard9.C: New test.

    (cherry picked from commit ad52d89808a947264397e920d7483090d4108f7b)

[Bug c++/100666] [9/10 Regression] warning: ignoring return value of 'constexpr _Tp&& std::forward(typename std::remove_reference<_Tp>::type&) [with _Tp = std::nullptr_t; ...]', declared with attribute 'nodiscard' [-Wunused-result]

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100666

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-9 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:6a05023c8739f2848778074e1ea9311cf6825a0f

commit r9-10073-g6a05023c8739f2848778074e1ea9311cf6825a0f
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue May 25 17:24:38 2021 +0200

    c++: Avoid -Wunused-value false positives on nullptr passed to ellipsis
[PR100666]

    When passing expressions with decltype(nullptr) type with side-effects to
    ellipsis, we pass (void *)0 instead, but for the side-effects evaluate them
    on the lhs of a COMPOUND_EXPR.  Unfortunately that means we warn about it
    if the expression is a call to nodiscard marked function, even when the
    result is really used, just needs to be transformed.

    Fixed by adding a warning_sentinel.

    2021-05-25  Jakub Jelinek  <jakub@redhat.com>

            PR c++/100666
            * call.c (convert_arg_to_ellipsis): For expressions with
NULLPTR_TYPE
            and side-effects, temporarily disable -Wunused-result warning when
            building COMPOUND_EXPR.

            * g++.dg/cpp1z/nodiscard8.C: New test.
            * g++.dg/cpp1z/nodiscard9.C: New test.

    (cherry picked from commit ad52d89808a947264397e920d7483090d4108f7b)

[Bug c++/100666] [9/10 Regression] warning: ignoring return value of 'constexpr _Tp&& std::forward(typename std::remove_reference<_Tp>::type&) [with _Tp = std::nullptr_t; ...]', declared with attribute 'nodiscard' [-Wunused-result]

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100666

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed.