suppress warning "left operand of comma operator has no effect"

Question

In the following code, a array with repeated elements is created using a fold expression:

#include <array>
#include <utility>

template <std::size_t N>
class A {
    int m_x;

public:
    static constexpr std::size_t size = N;

    A(int x) : m_x{x} { }
    int f() const noexcept { return m_x; }
    int g() const noexcept { return m_x * m_x; }
};

template <typename U, std::size_t... I>
auto get_array_impl(const U& m, std::index_sequence<I...>) {
    return std::array<int, m.size + 1>{ m.g(), (I, m.f())... };
}

template <typename U, std::size_t... I>
auto get_array(const U& m) {
    return get_array_impl(m, std::make_index_sequence<m.size>{});
}


int main()
{
    A<3> x{8};
    auto a = get_array(x);
    
    return 0;
}

Check it live on Coliru.

Compiling the code gives a warning:

main.cpp:18:50: warning: left operand of comma operator has no effect [-Wunused-value]

   18 |     return std::array<int, m.size + 1>{ m.g(), (I, m.f())... };

While it's clear why the warning is created (I is practically discarded), it is also clear that this is not an issue, since the code does not really need the I-values, but rather uses the variadic expansion to repeat m.f().

How can I suppress the warning while still compiling with -Wall? Is there some variation of the syntax that gives the same results without raising the warning?

Answer 1

You can avoid compiler warnings by using statements that block specific warnings, such as

#pragma GCC diagnostic ignored "-Wunused-variable"