should-work.cs

    public enum VariantType
    {
        LeftChild,
        RightChild1,
        RightChild2,
        RightGrandChild,
    }

    [PolymorphicFieldValueRecord]
    [PolymorphicDerivedType(typeof(LeftChild), VariantType.LeftChild)]
    [PolymorphicDerivedType(typeof(RightChild), VariantType.RightChild1)]
    [PolymorphicDerivedType(typeof(RightChild), VariantType.RightChild2)]
    [PolymorphicDerivedType(typeof(RightGrandChild), VariantType.RightGrandChild)]
    public record Base
    {
        public Base(VariantType variant)
        {
            Type = variant;
        }

        public NonExhaustiveEnum<VariantType> Type { get; }

        public required string RequiredBaseProperty { get; init; }

        public required FieldValue<string> OptionalBaseProperty { get; init; }
    }

    [PolymorphicFieldValueRecord]
    public record LeftChild()
        : Base(VariantType.LeftChild)
    {
        public required string RequiredLeftChildProperty { get; init; }

        public required FieldValue<string> OptionalLeftChildProperty { get; init; }
    }

    [PolymorphicFieldValueRecord]
    public record RightChild
        : Base
    {
        public RightChild(VariantType variant)
            : base(variant)
        {
        }

        public required string RequiredRightChildProperty { get; init; }

        public required FieldValue<string> OptionalRightChildProperty { get; init; }
    }

    [PolymorphicFieldValueRecord]
    public record RightGrandChild(VariantType variant)
        : RightChild(variant)
    {
        public required string RequiredRightGrandChildProperty { get; init; }

        public required FieldValue<string> OptionalRightGrandChildProperty { get; init; }
    }