CS9236 with the Sum extension method overloads.

Sum.cs

using System.Runtime.CompilerServices;
using static Enumerable;

int[][] x = [[]];

var y = x.Sum(x => x.Sum(x => 1));

#if false
// Same as System.Linq.Enumerable, this causes CS9236.
static class Enumerable
{
    public static float Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, float> selector) => throw new NotImplementedException();
    public static int Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, int> selector) => throw new NotImplementedException();
    public static long Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, long> selector) => throw new NotImplementedException();
    public static decimal? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal?> selector) => throw new NotImplementedException();
    public static double Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, double> selector) => throw new NotImplementedException();
    public static int? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, int?> selector) => throw new NotImplementedException();
    public static long? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, long?> selector) => throw new NotImplementedException();
    public static float? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, float?> selector) => throw new NotImplementedException();
    public static double? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, double?> selector) => throw new NotImplementedException();
    public static decimal Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal> selector) => throw new NotImplementedException();
}
#else
// Make the INumber overload public and decrease the priority of int's and other overloads.
static class Enumerable
{
    public static TResult Sum<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector)
        where TResult : struct, System.Numerics.INumber<TResult> => throw new NotImplementedException();

    public static TResult? Sum<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult?> selector)
        where TResult : struct, System.Numerics.INumber<TResult> => throw new NotImplementedException();

    [OverloadResolutionPriority(-1)] public static float Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, float> selector) => throw new NotImplementedException();
    [OverloadResolutionPriority(-1)] public static int Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, int> selector) => throw new NotImplementedException();
    [OverloadResolutionPriority(-1)] public static long Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, long> selector) => throw new NotImplementedException();
    [OverloadResolutionPriority(-1)] public static decimal? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal?> selector) => throw new NotImplementedException();
    [OverloadResolutionPriority(-1)] public static double Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, double> selector) => throw new NotImplementedException();
    [OverloadResolutionPriority(-1)] public static int? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, int?> selector) => throw new NotImplementedException();
    [OverloadResolutionPriority(-1)] public static long? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, long?> selector) => throw new NotImplementedException();
    [OverloadResolutionPriority(-1)] public static float? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, float?> selector) => throw new NotImplementedException();
    [OverloadResolutionPriority(-1)] public static double? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, double?> selector) => throw new NotImplementedException();
    [OverloadResolutionPriority(-1)] public static decimal Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal> selector) => throw new NotImplementedException();
}
#endif