[Bug c++/96311] New: false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[Bug c++/96311] New: false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[tilkax at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

            Bug ID: 96311
           Summary: false positive for -Wunused-but-set-variable
                    (const/constexpr identifier used in generic lambda)
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tilkax at gmail dot com
  Target Milestone: ---

Using gcc version 11.0.0 20200723, this code:

void foo()
{
    constexpr int used = 0;
    [](auto unused)
    {
        return used;
    };
}

triggers an incorrect warning:

<source>:3:19: warning: variable 'used' set but not used
[-Wunused-but-set-variable]
    3 |     constexpr int used = 0;
      |                   ^~~~

It doesn't matter whether the lambda is called but it does need to be a generic
lambda to trigger the incorrect warning.

Also, both the lambda parameter as well as the lambda itself are unused here
but don't trigger any warnings :/

[Bug c++/96311] false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |marxin at gcc dot gnu.org
   Last reconfirmed|                            |2020-07-27
             Status|UNCONFIRMED                 |WAITING

--- Comment #1 from Martin Liška <marxin at gcc dot gnu.org> ---
Confirmed, but it seems the code is rejected with clang:

pr96311.c:6:10: error: variable 'used' cannot be implicitly captured in a
lambda with no capture-default specified
                return used;
                       ^
pr96311.c:3:16: note: 'used' declared here
        constexpr int used = 0;
                      ^
pr96311.c:4:2: note: lambda expression begins here
        [](auto unused)
        ^
pr96311.c:4:2: warning: expression result unused [-Wunused-value]
        [](auto unused)
        ^~~~~~~~~~~~~~~
2 warnings and 1 error generated.

Is it a valid test-case?

[Bug c++/96311] false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[tilkax at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

--- Comment #2 from Tillmann Karras <tilkax at gmail dot com> ---
(In reply to Martin Liška from comment #1)
> Confirmed, but it seems the code is rejected with clang:
> [...]
> Is it a valid test-case?

I think so.

There is a bug report for Clang: https://bugs.llvm.org/show_bug.cgi?id=25627
And MSVC used to have the same problem:
https://developercommunity.visualstudio.com/content/problem/367326/problems-with-capturing-constexpr-in-lambda.html

[Bug c++/96311] [8/9/10/11 Regression] false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW
            Summary|false positive for          |[8/9/10/11 Regression]
                   |-Wunused-but-set-variable   |false positive for
                   |(const/constexpr identifier |-Wunused-but-set-variable
                   |used in generic lambda)     |(const/constexpr identifier
                   |                            |used in generic lambda)
   Target Milestone|---                         |8.5
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Started with r8-3497-g281e6c1d8f1b4ca552d8ce2276ddecfcd6ffb15e

[Bug c++/96311] [8/9/10/11 Regression] false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2
                 CC|                            |jason at gcc dot gnu.org

[Bug c++/96311] [8/9/10/11 Regression] false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
If I compare
void
foo ()
{
  constexpr int used = 0;
  [](auto unused)
  {
    return used;
  };
}

void
bar ()
{
  constexpr int used = 0;
  [](int unused)
  {
    return used;
  };
}

then in bar mark_exp_read is called on used from check_return_expr ->
convert_for_initialization -> convert_from_assignment ->
perform_implicit_conversion_flags -> mark_rvalue_use -> mark_use ->
mark_exp_read, but for the generic lambda check_return_expr takes the goto
dependent; and doesn't mark anything.
It is only marked during instantiation of the lambda, such as when (0) as added
in between } and ; and in that case the warning is gone.

More complete testcase:
void
foo ()
{
  constexpr int used = 0;
  [](auto unused)
  {
    return used;
  };
}

void
bar ()
{
  constexpr int used = 0;
  [](int unused)
  {
    return used;
  };
}

void
baz ()
{
  constexpr int used = 0;
  [](auto unused)
  {
    return used;
  } (0);
}

void
qux ()
{
  constexpr int used = 0;
  [](auto unused)
  {
    return used + 1;
  };
}

void
corge ()
{
  constexpr int used = 0;
  [](auto unused)
  {
    return used + unused;
  };
}

This shows that if the variable is used in some non-dependent expression as in
qux (and not directly in return statement), it is marked already without
instantiation, but when it is only used directly in return stmt or used in a
dependent expression, it is marked only during instantiation of the lambda.

Though, when the lambda isn't really instantiated, the variable is not really
used, so not 100% sure the warning is unwanted.

[Bug c++/96311] [8/9/10/11 Regression] false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

康桓瑋 <hewillk at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hewillk at gmail dot com

--- Comment #5 from 康桓瑋 <hewillk at gmail dot com> ---
*** Bug 99361 has been marked as a duplicate of this bug. ***

[Bug c++/96311] [8/9/10/11 Regression] false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

Jason Merrill <jason at gcc dot gnu.org> changed:

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

[Bug c++/96311] [8/9/10/11 Regression] false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:b07dd9b0d0e501a0083da79e2bca17041c007ec8

commit r11-7995-gb07dd9b0d0e501a0083da79e2bca17041c007ec8
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Apr 5 16:22:51 2021 -0400

    c++: -Wunused, constant, and generic lambda [PR96311]

    We never called mark_use for a return value in a function with dependent
    return type.  In that situation we don't know if the use is as an rvalue or
    lvalue, but we can use mark_exp_read instead.

    gcc/cp/ChangeLog:

            PR c++/96311
            * typeck.c (check_return_expr): Call mark_exp_read in dependent
            case.

    gcc/testsuite/ChangeLog:

            PR c++/96311
            * g++.dg/cpp1y/lambda-generic-Wunused.C: New test.

[Bug c++/96311] [8/9/10 Regression] false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |11.0
            Summary|[8/9/10/11 Regression]      |[8/9/10 Regression] false
                   |false positive for          |positive for
                   |-Wunused-but-set-variable   |-Wunused-but-set-variable
                   |(const/constexpr identifier |(const/constexpr identifier
                   |used in generic lambda)     |used in generic lambda)

--- Comment #7 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed for 11 so far.

[Bug c++/96311] [9/10 Regression] false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|8.5                         |9.4

--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 8 branch is being closed.

[Bug c++/96311] [9/10 Regression] false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jason Merrill
<jason@gcc.gnu.org>:

https://gcc.gnu.org/g:8909ed58700694f5ba66499739edfea90a0fc813

commit r10-9846-g8909ed58700694f5ba66499739edfea90a0fc813
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Apr 5 16:22:51 2021 -0400

    c++: -Wunused, constant, and generic lambda [PR96311]

    We never called mark_use for a return value in a function with dependent
    return type.  In that situation we don't know if the use is as an rvalue or
    lvalue, but we can use mark_exp_read instead.

    gcc/cp/ChangeLog:

            PR c++/96311
            * typeck.c (check_return_expr): Call mark_exp_read in dependent
            case.

    gcc/testsuite/ChangeLog:

            PR c++/96311
            * g++.dg/cpp1y/lambda-generic-Wunused.C: New test.

[Bug c++/96311] [9/10 Regression] false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.4                         |9.5

--- Comment #10 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 9.4 is being released, retargeting bugs to GCC 9.5.

[Bug c++/96311] [9 Regression] false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

--- Comment #11 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-9 branch has been updated by Jason Merrill
<jason@gcc.gnu.org>:

https://gcc.gnu.org/g:aa3e2a211933efaeb6ecd0a69f196606b8a40b43

commit r9-10165-gaa3e2a211933efaeb6ecd0a69f196606b8a40b43
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Apr 5 16:22:51 2021 -0400

    c++: -Wunused, constant, and generic lambda [PR96311]

    We never called mark_use for a return value in a function with dependent
    return type.  In that situation we don't know if the use is as an rvalue or
    lvalue, but we can use mark_exp_read instead.

    gcc/cp/ChangeLog:

            PR c++/96311
            * typeck.c (check_return_expr): Call mark_exp_read in dependent
            case.

    gcc/testsuite/ChangeLog:

            PR c++/96311
            * g++.dg/cpp1y/lambda-generic-Wunused.C: New test.

[Bug c++/96311] [9 Regression] false positive for -Wunused-but-set-variable (const/constexpr identifier used in generic lambda)

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96311

Jason Merrill <jason at gcc dot gnu.org> changed:

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

--- Comment #12 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed for 9.5/10.4/11.