I am attempting at access a class that is like a multidimensional array through the operator[].. only to discover that a comma separated set of parameters is reduced to the first parameter only.. This was a quite a surprise to me..

Does anyone know if this an actual bug or some kind of legalize gotcha in the c++ standard or compiler implementations?..

Its rather annoying as the compiler tends to reduce it without error or warning and bugs can slide by if the coder makes a mistake.

#include <memory>
#include <vector>
#include <stdexcept>
#include <iostream>

template <size_t D, size_t N>
struct A {
    //size_t operator[](int idx) {
    //    throw std::runtime_error("wtf!");
    //}

    size_t operator[](std::initializer_list<int> idx) {
        if (idx.size() != D) {
            throw std::runtime_error("lacking entries");
        }

        size_t offset = 0;
        for (size_t v : idx) {
            offset = offset*N + v;
        }
        return offset;
    }
};

int main() {
    A<3,10> a;    
    std::cout << a[1,2,3] << "n";    // BUG??
    std::cout << a[{1,2,3}] << "n";    
}

The godbolt is here: https://godbolt.org/z/WYeGn7Y4n

I expect a[1,2,3] to access the initializer list call and pass all parameters over however it accesses the single parameter one(or tries to)