C++ circular dependency with header guards and forward declarations

I have a simple Vec3 classand a simple xMath class.

The xMath class is supposed to change the Vec3. There are just 2 files Vec3.hpp and xMath.hpp (and I am not allowed to create other files).

• both classes are in the same namespace
• there are header guards
• there are includes on both of them
• xMath is not recognized
• using forward declarations does not work, tried

Using Visual Studio 2019 Community Edition.

Getting errors:

• error C2027 use of undefined type 'ns::xMath'Vec3.hpp

Using the forward declarations, still getting errors:

• error C2653 Vec3 is not a class or a namespace name
• error C2027 use of undefined type 'ns::Vec3'

Here are the files.

//Vec3.hpp
#ifndef VEC3_H
#define VEC3_H

#include "xMath.hpp"

namespace ns
{
    //class xMath; //forward declaration does not work either..

    struct Vec3
    {
    public:

        float x;

        //constructor
        Vec3(float x) { this.x = x; }

        static float Change(Vec3* v)
        {
            v->x = xMath::Change(v);

            return t;
        }

    };
}

#endif

//xMath.hpp
#ifndef XMATH_H
#define XMATH_H

#include "Vec3.hpp"

namespace ns
{
    //class Vec3; //forward declaration does not work either..

    static class xMath
    {
    public:

        static float Change (Vec3* v)
        {
            return v->x;
        }
    };
}

#endif

Those requirements are very strange. Normally we'd put the implementation in its own file that includes both headers.

We can do something similar within the headers, though:

#ifndef XMATH_H
#define XMATH_H

// First the declarations:

namespace ns
{
    class Vec3;

    class xMath
    {
    public:
        static float Change (Vec3* v);
    };
}


// Now the implementation (which would normally be in xMath.cpp):

#include "Vec3.hpp"

float ns::xMath::Change (Vec3* v)
{
    return v->x;
}

#endif