internal interface IVisitor<VTR>
    {
        VTR OnAdd<LT, RT>(IExpression<LT?, RT?, float?> expr) where LT : struct where RT : struct;
    }

    internal interface IExpression<T, T1, T2>
    {
    }

    class Visitor : IVisitor<object>
    {
        public object OnAdd<LT, RT>(IExpression<LT?, RT?, float?> expr) where LT : struct where RT : struct
        {
            throw new NotImplementedException();
        }
    }

    internal class MyClass<LT, RT> : IExpression<LT?, RT?, float?> where LT:struct where RT:struct 
    {
        VTR Process<VTR>(IVisitor<VTR> visitor)
        {
            return visitor.OnAdd(this);//OnAdd - cannot be inferred from usage
        }
    }