LINQ Extensions to use in tandem with C# 8 Nullable Reference Types.

NullableEnumerableExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace System.Linq
{
	public struct RequireClass<T> where T : class { }
	public struct RequireStruct<T> where T : struct { }

	public static class NullableEnumerableExtensions
	{
		public static IEnumerable<TSource> WhereNotNull<TSource>(this IEnumerable<TSource?> source)
			where TSource : class
		{
			return source.Where(x => !(x is null)).Select(x => x!);
		}

		public static IEnumerable<TSource> WhereNotNull<TSource>(this IEnumerable<TSource?> source)
			where TSource : struct
		{
			return source.Where(x => !(x is null)).Select(x => x.GetValueOrDefault());
		}


		public static TSource SingleOrDefault<TSource>(this IEnumerable<TSource> source) where TSource : class => throw new NotSupportedException();

		public static TSource SingleOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) where TSource : class => throw new NotSupportedException();

		public static TSource? SingleOrNull<TSource>(this IEnumerable<TSource> source, RequireClass<TSource> _ = default)
			where TSource : class
		{
			return Enumerable.SingleOrDefault(source.Select(x => (TSource?)x));
		}

		public static TSource? SingleOrNull<TSource>(this IEnumerable<TSource> source, RequireStruct<TSource> _ = default)
			where TSource : struct
		{
			return Enumerable.SingleOrDefault(source.Select(x => (TSource?)x));
		}

		public static TSource? SingleOrNull<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, RequireClass<TSource> _ = default)
			where TSource : class
		{
			return Enumerable.SingleOrDefault(source.Where(predicate));
		}

		public static TSource? SingleOrNull<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, RequireStruct<TSource> _ = default)
			where TSource : struct
		{
			return Enumerable.SingleOrDefault(source.Where(predicate));
		}


		public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source) where TSource : class => throw new NotSupportedException();

		public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) where TSource : class => throw new NotSupportedException();

		public static TSource? FirstOrNull<TSource>(this IEnumerable<TSource> source, RequireClass<TSource> _ = default)
			where TSource : class
		{
			return Enumerable.FirstOrDefault(source.Select(x => (TSource?)x));
		}

		public static TSource? FirstOrNull<TSource>(this IEnumerable<TSource> source, RequireStruct<TSource> _ = default)
			where TSource : struct
		{
			return Enumerable.FirstOrDefault(source.Select(x => (TSource?)x));
		}

		public static TSource? FirstOrNull<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, RequireClass<TSource> _ = default)
			where TSource : class
		{
			return Enumerable.FirstOrDefault(source.Where(predicate));
		}

		public static TSource? FirstOrNull<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, RequireStruct<TSource> _ = default)
			where TSource : struct
		{
			return Enumerable.FirstOrDefault(source.Where(predicate));
		}
	}
}