No declaration matches sparky::maths::vec2& sparky::maths::vec2::operator+=(const sparky::maths::vec2&)

#include <iostream>
#include <vector>
using namespace std;

// this should be inside the header file
namespace sparky
{
  namespace maths
  {
    class vec2
    {
      vector <int> v;

      vec2 & add( const vec2 & other)
      {
        for (size_t n=0; n<v.size() && n<other.v.size(); ++n)
        {
          v[n] += other.v[n];
        }
        return *this;
      }

    public:
      vec2 & operator += ( const vec2 & other );
      void push_back(const int i){ v.push_back(i); }

      friend ostream & operator << ( ostream & os, const vec2 & v2);

    };
  }
}

// this should be inside the source file
namespace sparky
{
  namespace maths
  {

    ostream & operator << (ostream & os, const vec2 & v2)
    {
      for (const auto &  x : v2.v)
        os << x << " ";
      return os;
    }
    vec2& vec2::operator+=(const vec2& other)
    {
      return add(other);
    }
  }
}


// testing the class
int main()
{
  sparky::maths::vec2 v,w;
  v.push_back(1);
  v.push_back(2);
  v.push_back(3);
  w.push_back(10);
  w.push_back(20);

  cout << "v " << v << ", w " << w  << "\n";
  v += w;
  cout << "v " << v << ", w " << w << "\n";
}