operator overloading for type-closed generic class

[aka-nse]

Maybe related to #1315.

In C#, operator overloading is supported as syntax sugar of static method calling.
For example, operator+ will be converted to the static method op_Addition.

BTW, the following code is valid:

using System;

public struct Container<T>
{
    public T Value { get; }
    
    public Container(T value) => Value = value;

    public static Container<T> operator+(Container<T> lhs, Container<T> rhs)
        => new Container<T>(FieldProvider<T>.Current.Add(lhs.Value, rhs.Value));
    /* is equivalent to :
    public static Container<T> op_Addition(Container<T> lhs, Container<T> rhs)
        => new Container<T>(FieldProvider<T>.Current.Add(lhs.Value, rhs.Value));
    */
}

public static class Program
{
    public static void Main()
    {
        var a = new Container<int>(1);
        var b = new Container<int>(2);
        var c = a + b;
        /* is equivalent to:
        var c = Container<int>.op_Addition(a, b);
        */
        Console.WriteLine(c.Value);
    }
}

but the following code is not valid:

using System;

public struct Container<T>
{
    public T Value { get; }
    
    public Container(T value) => Value = value;

    public static Container<int> operator+(Container<int> lhs, Container<int> rhs)
        => new Container<int>(FieldProvider<int>.Current.Add(lhs.Value, rhs.Value));
}

public static class Program
{
    public static void Main()
    {
        var a = new Container<int>(1);
        var b = new Container<int>(2);
        var c = a + b;
        Console.WriteLine(c.Value);
    }
}

although the following code is valid:

using System;

public struct Container<T>
{
    public T Value { get; }
    
    public Container(T value) => Value = value;

    public static Container<int> op_Addition(Container<int> lhs, Container<int> rhs)
        => new Container<int>(FieldProvider<int>.Current.Add(lhs.Value, rhs.Value));
}

public static class Program
{
    public static void Main()
    {
        var a = new Container<int>(1);
        var b = new Container<int>(2);
        var c = Container<int>.op_Addition(a, b);
        Console.WriteLine(c.Value);
    }
}

Like this, type-closed generic static method is available but such a operator is prohibited now.
In this proposal I suggest to allow operator overloading for type-closed generics such as 2nd code.

[iam3yal]

#164 should solve this and more.