The specification says:

    The expression: a op= b
    is rewritten as: a.opOpAssign!("op")(b)

A similar statement is made about opIndexOpAssign. However, what actually happens is that the template parameter is set to "op=", as this program shows:

    struct S
    {
        void opIndexOpAssign(string op)(int x, int i)
        {
            pragma (msg, op);
        }

        void opOpAssign(string op)(int x)
        {
            pragma(msg, op);
        }
    }

    void main()
    {
        S s;
        s[1] += 2;
        s += 3;
    }

This prints the following on compilation:

  +=
  +=