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

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

[cuzdav at gmail dot com]

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.

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

[johelegp at gmail dot com]

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

Johel Ernesto Guerrero Peña <johelegp at gmail dot com> changed:

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

--- Comment #1 from Johel Ernesto Guerrero Peña <johelegp at gmail dot com> ---
*** Bug 105743 has been marked as a duplicate of this bug. ***

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

[johelegp at gmail dot com]

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

--- Comment #2 from Johel Ernesto Guerrero Peña <johelegp at gmail dot com> ---
Simplified reproducer from Bug 105743:

See https://godbolt.org/z/xq16xac15.

```C++
void f(auto x) { x(0); }
void g() {
  static constexpr auto h = [](...) { };
  f([](auto x) { h(x); });
}
```

```
<source>: In function 'void g()':
<source>:3:25: warning: variable 'h' set but not used
[-Wunused-but-set-variable]
    3 |   static constexpr auto h = [](...) { };
      |                         ^
```

Arguably, `h(x)` could perform ADL on `x`. But perhaps it'd be better to stay
silent, like the other compilers.

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

[pinskia at gcc dot gnu.org]

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

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

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

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 88804. The problem is with templated lambdas (auto param is really a
templated lambda).

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