Usage anonymous type and generic with nullable enabled

[iskiselev]

How should be rewritten next code to not have CS8620 (it appeared after #44339 was fixed):

  #nullable enable
        public IEnumerable<T1> GetItems<T1>(IEnumerable<T1> input)
        {
            return input.Select(
                    item => new
                    {
                        Item = item,
                        Final = false
                    }).Concat(
                    Enumerable.Repeat(
                        new
                        {
                            Item = default(T1),
                            Final = true
                        },
                        1))
                .Where(it => !it.Final).Select(it => it.Item);
        }
#nullable restore

Sample can be checked here. How it can be defined that Item can have null in original Select?

[iskiselev]

Should default(T1) be annotated with exclamation?