Extension to run tasks concurrently and return an array when all have completed

Extensions.cs

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static partial class Extensions
{
    static readonly Task IncompleteTask = new TaskCompletionSource().Task;

    public static async Task<(T Item, Task<TResult> Task)[]>
        ToCompletedTaskArrayAsync<T, TResult>(
            this IEnumerable<T> source,
            Func<T, CancellationToken, Task<TResult>> function,
            int concurrency, // 0 = unbounded
            CancellationToken cancellationToken)
    {
        ArgumentNullException.ThrowIfNull(source, nameof(source));
        ArgumentNullException.ThrowIfNull(function, nameof(function));
        if (concurrency < 0) throw new ArgumentOutOfRangeException(nameof(concurrency), concurrency, null);

        using var concurrencySemaphore = concurrency > 0 ? new SemaphoreSlim(concurrency) : null;
        using var completionSemaphore = new SemaphoreSlim(0, int.MaxValue);
        var pendingTaskList = new List<Task<(int, (T, Task<TResult>))>?>(source is ICollection { Count: var count } ? count : 4);

        var i = 0;
        foreach (var item in source)
            pendingTaskList.Add(RunAsync(i++, item));

        var results = new (T, Task<TResult>)[pendingTaskList.Count];

        var si = 0;
        for (var pendingCount = pendingTaskList.Count; pendingCount > 0; pendingCount--)
        {
            await completionSemaphore.WaitAsync().ConfigureAwait(false);
            while (true)
            {
                i = pendingTaskList.FindIndex(si, t => t is { IsCompleted: true });
                if (i >= 0)
                    break;
                si = 0;
            }
            var completedTask = pendingTaskList[i]!;
            pendingTaskList[i] = null;
            if (i + 1 == pendingTaskList.Count)
            {
                var rsi = pendingTaskList.Count;
                while (rsi > 0 && pendingTaskList[rsi - 1] is null)
                    rsi--;
                pendingTaskList.RemoveRange(rsi, pendingTaskList.Count - rsi);
                si = 0;
            }
            else
            {
                si = i + 1;
            }
            (i, var result) = await completedTask.ConfigureAwait(false);
            results[i] = result;
        }
        
        return results;
        
        async Task<(int, (T, Task<TResult>))> RunAsync(int i, T item)
        {
            try
            {	      
                if (concurrencySemaphore is not null)
                    await concurrencySemaphore.WaitAsync().ConfigureAwait(false);
                var task = function(item, cancellationToken);
                _ = await Task.WhenAny(task, IncompleteTask).ConfigureAwait(false);
                return (i, (item, task));
            }
            finally
            {
                if (concurrencySemaphore is not null)
                    concurrencySemaphore.Release();
                completionSemaphore.Release();
            }
        }
    }
}