Rosyln error CS9036 not described

[RufusJWB]

It seems that the compiler error CS9036 is nowhere described. It would be great, if you could provide a documentation about it. Thank you!

This refers to the following scenario razor.fyi

class C
{
    public string? Prop { get; set; }
}
class Program
{
    public required C C { get; set; }
    static void Main()
    {
        var program = new Program()
        {
            // error CS9036: Required member 'Program.C' must be assigned a value, it cannot use a nested member or collection initializer.
            C = { Prop = "a" }
        };
    }
}

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Associated WorkItem - 434158