What's a good way to enable access to private members of another instance of a class template in this case?

I'm working on a polygon class:

// A polygon is multiple (possibly closed) polycurves.
template <typename T, typename P = _point<T,2>>
struct _polygon {
    using point  = P;

    // default constructor
    _polygon() {
        offs_.push_back(0);
    }

    // build polygon from another type
    template <typename U>
    _polygon(const _polygon<U> &poly) {
        pnts_.reserve(poly.pnts_.size());
        offs_ = poly.offs_;
        for (const auto& pnt : poly.pnts_) {
            pnts_.push_back(point(pnt));
        }
    }

private:
    vector<point> pnts_; // list of points
    vector<int>   offs_; // offsets of start of polychains
};

The issue is in the converting constructor. I get an errror when accessing members of the other _polygon type:

polygon.h:376:28: error: ‘std::vector, std::allocator > > sk::_new_polygon::pnts_’ is private within this context pnts_.reserve(poly.pnts_.size());

Clearly an instance of a class template C isn't friends/doesn't have visibility into another instance C, which is a little surprising to me. How can I get access to the other polygon's internals to do the conversion in this case?