Collection Expressions constructable from multiple types

[TonyValenti]

How do I make an object collection-expression constructable from multiple types without needing to implement IEnumerable<T>?

This is what I want to do:

    internal class Program
    {
        static void Main(string[] args)
        {

            MyCollection V1 = [
                "Test",
            ];

            MyCollection V2 = [
                typeof(string),
            ];

        }
    }


    public class MyCollectionBuilder {
        public static MyCollection Create(ReadOnlySpan<Type> items) {
            return new MyCollection();
        }

        public static MyCollection Create(ReadOnlySpan<string> items) {
            return new MyCollection();
        }
    }

    [CollectionBuilder(typeof(MyCollectionBuilder), nameof(MyCollectionBuilder.Create))]
    public class MyCollection {

    }

[HaloFour]

The target type of the collection expression must be foreach-able, in that the compiler recognizes how to iterate over that collection. Easiest way is to implement IEnumerable<T> for one specific type, but being compliant with the foreach pattern is sufficient. This doesn't permit a single collection type to support more than one arbitrary element type, not without generics anyway.

[jnm2]

Existing discussion here: #9008