StaticAbstractMembers.cs

using System;

ReadOnlySpan<Vector2d> vectors = stackalloc Vector2d[] { new(1d, 1d), new(2d, 2d), };
Console.WriteLine(AddAll(vectors));

static T AddAll<T>(ReadOnlySpan<T> addables) where T: IAddable<T>
{
    var result = T.Zero;
    foreach (var a in addables) result += a;
    return result;
}

interface IAddable<T> where T : IAddable<T>
{
    static abstract T Zero { get; }
    static abstract T operator +(T t1, T t2);
}

record struct Vector2d(double X, double Y) : IAddable<Vector2d>
{
    public static Vector2d operator +(Vector2d first, Vector2d second)
        => new(first.X + second.X, first.Y + second.Y);
    public static Vector2d Zero => new(0d, 0d);
}