If you don't have access to C++23 then you can wrap your lambda in another lambda that hides the extra argument, e.g. like this:
int main(int argc, char **argv) { auto fact = [&](int x) -> int { auto do_fact = [&](const auto &self, int x) -> int { return x ? x * self(self, x - 1) : 1; }; return do_fact(do_fact, x); }; return fact(argc);}clang with -O and gcc with -O2 will compile the above into a simple multiplication loop (clang tries to be extra clever at higher optimisation levels).