Can't add pair(el1, el2) into static map

Question

I've got class Functions.
It contains of: static std::map<vector2d, double> functionValues;
Also I created my own vector2d clsss.
So, when I try to insert/emplace or add element through using of operator[] from member function of Function class into this map it fails.
So, the error code is C2676. I found no answear, so really appreciate any suggestions what can cause this problem. enter image description here Here is some code:

#pragma once

#include "vector2d.h"
#include <map>
extern unsigned int functionCallCounts;

class Function
{
private:
    static std::map<vector2d, double> functionValues;
    static int coef;
public:
    inline static void setCoef(int _coef) { coef = _coef; };
    static double calcFunction(vector2d _point);
};

#include "Function.h"
#include <cmath>

std::map<vector2d, double> Function::functionValues;
int Function::coef;

unsigned int functionCallCounts;

double Function::calcFunction(vector2d _point)
{
    for (auto e : functionValues)
    {
        if (_point == e.first)
        {
            return e.second;
        }
    }
    double functionValue = 2 * pow((_point.x - coef), 2) - _point.x * _point.y + 5 * pow(_point.y, 2);
    ++functionCallCounts;
    functionValues.insert(std::make_pair(_point, functionValue));
    return functionValue;
}

And vector2d

class vector2d
{
public:
    double x;
    double y;
    vector2d operator* (double coef);
    vector2d operator+ (vector2d v2);
    vector2d operator- (vector2d v2);
    vector2d operator* (vector2d v2);
    vector2d operator/ (vector2d v2);
    vector2d operator/ (double devideBy);
    double magnitude() const;

    double distance(const vector2d& vec2d) const;
    vector2d normalizedVec();
    void makeNormalized();

    vector2d();
    vector2d(double _x, double _y);

    bool operator== (const vector2d v2) const;
    friend vector2d operator*(const double coef, vector2d& v);
};