https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124198
Bug ID: 124198
Summary: nested template-for generating warnings for
unused-but-set-variable
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: uchanahome8 at gmail dot com
Target Milestone: ---
Hi, I am using the gcc-trunk on compiler explorer:
https://godbolt.org/z/E8cWTs13d and was able to reproduce this on local build
of master branch as well.
This shouldn't have triggered the warning for unused-variable `J`.
Code:
```
#include <iostream>
#include <utility>
#include <array>
template <std::size_t start, std::size_t end>
consteval auto make_array() {
std::array<std::size_t, end - start> arr;
for (std::size_t i = start; i < end; ++i)
arr[i - start] = i;
return arr;
}
template <int... vals>
constexpr bool are_same() {
template for (constexpr auto I: make_array<0, sizeof...(vals)>()) {
template for (constexpr auto J: make_array<I + 1, sizeof...(vals)>()) {
if constexpr (vals...[I] == vals...[J])
return true;
}
}
return false;
}
int main() {
std::cout << are_same<1,2,3>();
return 0;
}
```
Flags: `-std=c++26 -Werror -Wunused-but-set-variable`.
GCC: `trunk`.
Error:
```
<source>: In instantiation of 'constexpr bool are_same() [with int ...vals =
{1, 2, 3}]':
<source>:25:33: required from here
25 | std::cout << are_same<1,2,3>();
| ~~~~~~~~~~~~~~~^~
<source>:16:38: error: variable 'J' set but not used
[-Werror=unused-but-set-variable=]
16 | template for (constexpr auto J: make_array<I + 1,
sizeof...(vals)>()) {
| ^
<source>:16:38: error: variable 'J' set but not used
[-Werror=unused-but-set-variable=]
<source>:16:38: error: variable 'J' set but not used
[-Werror=unused-but-set-variable=]
cc1plus: all warnings being treated as errors
Compiler returned: 1
```
