Weird compiler warning about unused variable, is this a compiler bug?

Question

My question is about the following code:

#include <type_traits>
#include <utility>

template <int First, int Last, typename Functor>
constexpr void static_for(Functor&& f)
{
    if constexpr (First < Last)
    {
        f(std::integral_constant<int, First>{});
        static_for<First + 1, Last, Functor>(std::forward<Functor>(f));
    }
}

int main() {
    static_for<1, 3>([](int /*i*/){

    });
    return 0;
}

It produces the following compiler warning with MSVC (Visual Studio 2017 15.9.11, v141 toolset, /std:c++17):

warning C4100: 'f': unreferenced formal parameter

It's reproducible on Godbolt: https://godbolt.org/z/6gLDzu

Is this a compiler bug? I was going to report it to Microsoft, but then felt like asking for the community's opinion, maybe I'm missing something? The code does work and the functor is invoked the correct number of times, so it's not the case of the compiler mis-compiling the code and optimizing f out erroneously.

More like a QoI issue, [with workarounds](https://stackoverflow.com/a/49321420/817643). — StoryTeller - Unslander Monica, Apr 21 '19 at 19:19
In the case where First >= Last, the `if constexpr` causes the entire function body to vanish, and `f` is unused. — Raymond Chen, Apr 21 '19 at 19:20
@RaymondChen: but that does not happen anywhere during compilation of this code snippet, shouldn't the compiler see that before issuing the warning? Looks to me as if the warning is generated based on some heuristics instead of the actual AST, which I find odd. — Violet Giraffe, Apr 21 '19 at 19:39
Of course it happens. `First + 1` doesn't continue ad infinitum. — StoryTeller - Unslander Monica, Apr 21 '19 at 19:40
@StoryTeller: oh, great point there, my bad. Maybe I should re-write my implementation to avoid that last useless recursive call. You should make this an answer! — Violet Giraffe, Apr 21 '19 at 19:41
I wouldn't bother if I were you. It's clear and readable as is. `[[maybe_unused]]` fixes it with no cost. But YMMV. — StoryTeller - Unslander Monica, Apr 21 '19 at 19:42
As an aside, consider avoiding so many levels of instantiation by using fold-expressions and `std::index_sequence`. — Deduplicator, May 03 '19 at 15:34
@Deduplicator: what's your idea? Can you post an answer with a code sample? I'm not getting it. What would I unfold with a fold expression? — Violet Giraffe, Jun 15 '19 at 08:55
[This post](https://stackoverflow.com/a/55009125) has an efficient basic `static_for` from me. — Deduplicator, Jun 15 '19 at 09:07

score 3 · Accepted Answer · answered Apr 23 '19 at 13:27

3

In the last iteration of your static_for() First + 1 equals Last. This causes the body of the function to vanish and f is unused.

answered Apr 23 '19 at 13:27

pablo285

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Weird compiler warning about unused variable, is this a compiler bug?

1 Answers1