[Bug c++/105571] New: Spurious "set but not used" on static constexpr local, used in lambda

["cuzdav at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105571

            Bug ID: 105571
           Summary: Spurious "set but not used" on static constexpr local,
                    used in lambda
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cuzdav at gmail dot com
  Target Milestone: ---

With -Wall, in every c++ language level on virtually every version of g++ as
far back as 4.9, g++ warns ("variable 'THIS_IS_USED' set but not used") on the
following complete code:


template <typename IterT, typename FuncT>
void for_each(IterT b, IterT e, FuncT f) {
    while (b != e) {
        f(*b++);
    }
}

volatile int sink; 
void xxx(int x) {
    sink = x;
}

void foo() {
    static constexpr auto THIS_IS_USED = 1;  
    int arr[10]{1,2,3,4,5,6,7,8,9,10};

    for_each(arr, arr+10, [](auto v) {
        xxx(THIS_IS_USED + v);
    });
}


<source>: In function 'void foo()':
<source>:15:27: warning: variable 'THIS_IS_USED' set but not used
[-Wunused-but-set-variable]
   15 |     static constexpr auto THIS_IS_USED = 1;
      |                           ^~~~~~~~~~~~
Compiler returned: 0

https://godbolt.org/z/b1s5Yqb3r


This appears to be related to the "auto" parameter in the lambda.  Changing it
to 'int' makes the problem go away.