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

["gccbugbjorn at fahller dot se" <gcc-bugzilla@gcc.gnu.org>]

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