Allow using static SomeClass to bring overloaded operators into scope.

[lostmsu]

This proposal is to enable adding operators to existing 3rd party types, or having different implementations of the same operators (for example, separate sets for GPU/CPU computation).

Proposal consists of two changes:

1. Enable defining operators for arbitrary types inside static classes. E.g.

static class ListMath {
  public static List operator+ (List a, List b)
    => new List(a.Count + b.Count).AddRange(a).AddRange(b);
}

2. Enable bringing them into scope via using static:

using static ListMath;

var a = new List{ 1 };
var b = new List{ 2 };
var c = a + b;

3. (optional, but highly desired) Allow operators in 1 to be generic:

public static List<T> operator+<T> (List<T> a, List<T> b) => new List(a.Count + b.Count).AddRange(a).AddRange(b);

Resolution of these operators should follow resolution rules of extension methods. E.g. if either operand has matching native operator defined, use that, otherwise use this extension operator.

[Unknown6656]

related: dotnet/roslyn#11159, dotnet/roslyn#6136, dotnet/roslyn#5165, dotnet/roslyn#4945, dotnet/roslyn#5624 (afaik some of the referenced proposals would allow extension operators, which should be what you request)

[lostmsu]

@Unknown6656 that was moved to #192

[Unknown6656]

Ah thanks, I was unable to find it with a superficial search.