Omit unused parameters of lambda.

test.cpp

#include <functional>
#include <string>

void test(std::function<int(const std::string & a, const std::string & b, const std::string & c)> func)
{
    func({}, {}, {});
}

int main()
{
    test([](const std::string & a, const std::string & b, const std::string & c)
    {
        return 0;
    });
    test([](auto, auto, auto)
    {
        return 0;
    });
    test([](auto...)
    {
        return 0;
    });
    test([](...)
    {
        return 0;
    });
}