[Bug c++/96400] New: False positive on Wunused-but-set-variable for static constexpr var used in lambda

[Bug c++/96400] New: False positive on Wunused-but-set-variable for static constexpr var used in lambda

[gccbugbjorn at fahller dot se]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96400

            Bug ID: 96400
           Summary: False positive on Wunused-but-set-variable for static
                    constexpr var used in lambda
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gccbugbjorn at fahller dot se
  Target Milestone: ---

Similar to Bug 96311, but not quite the same.

This code is flags "is_zero" as being unused but set:

template <typename Pred, typename Val>
bool test(Pred p, Val v)
{
    return p(v);
}

bool func(int* p)
{
    static constexpr auto is_zero = [](auto v) { return v == 0;};
    return test([](auto v){return is_zero(*v);}, p);
}

Output:
<source>: In function 'bool func(int*)':

<source>:9:27: warning: variable 'is_zero' set but not used
[-Wunused-but-set-variable]

    9 |     static constexpr auto is_zero = [](auto v) { return v == 0;};

      |                           ^~~~~~~


"is_zero" can correctly be used in the lambda without being captured, since
it's static. The warning only appears when using the extra indirection
"test()". Calling the anonymous lambda directly, without the indirection
"test()", does not display the warning.

Possibly "constexpr" is a red herring above.

Godbolt link:
https://godbolt.org/z/x4zb3s

[Bug c++/96400] False positive on Wunused-but-set-variable for static constexpr var used in lambda

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96400

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-07-27

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Here is the most reduced C++20 testcase which produce the warning:
void test(auto){}
void func()
{
    static auto is_zero = [](){ };
    test([](auto v){return is_zero(*v);});
}

Here is a C++17 testcase:
template<typename t> void test(t){}

void func()
{
    static auto is_zero = [](){ };
    test([](auto v){return is_zero(*v);});
}

[Bug c++/96400] False positive on Wunused-but-set-variable for static constexpr var used in lambda

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96400

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

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

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 105743 has been marked as a duplicate of this bug. ***

[Bug c++/96400] False positive on Wunused-but-set-variable for static constexpr var used in lambda

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96400

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

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

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Changing "[](auto v)" to "[](int *v)" removes the warning. So this is a dup of
bug 88804.

*** This bug has been marked as a duplicate of bug 88804 ***