Compilation error in dev branch when a struct is used as a vector/matrix index

We have a code that is currently working great with Eigen 3.3.7, and I wanted to use the new slicing features in the development branch that are planned for 3.4. But compilation of our code fails on the dev branch. I am not an expert in Eigen, so here is the minimum code to reproduce it ( I did it on https://godbolt.org/ with different compilers).

#include <Eigen/Core>
#include <iostream>
struct IndexType
{
	IndexType( int idx ) : mIndex( idx ) {}
	int mIndex;

        operator int () const { return mIndex; }
	int index() const { return mIndex; }
};

int main(){
    Eigen::Vector3d vec = Eigen::Vector3d::Zero();
    const IndexType index_1 = 1;   

    // working
    std::cout<< vec( index_1.index() ) <<std::endl;

    // working
    std::cout<< vec[ index_1 ] <<std::endl;

    // working for eigen 3.3.7, but not for current branch
    std::cout<< vec( index_1 ) <<std::endl;  

}

My guess is that for the line vec( index_1 ), Eigen considers the struct index_1 as a range ( feature added recently in the dev branch to facilitate slicing operations), overriding the conversion operator operator int ().