Exception in VS2017 Community about mappinggenerator HOT 6 CLOSED

[Pure]
public class SegmentedString {
    private readonly string @string = string.Empty;
    private readonly string begin = string.Empty;
    private readonly string middle = string.Empty;
    private readonly string end = string.Empty;

    public SegmentedString(Option<string> @string)
        : this(string.Empty, @string, string.Empty) {
    }

    public SegmentedString(Option<string> begin, Option<string> middle, Option<string> end) {
        this.begin = begin.IfNone(string.Empty);
        this.middle = middle.IfNone(string.Empty);
        this.end = end.IfNone(string.Empty);

        this.@string = this.begin + this.middle + this.end;
    }

    public Option<SegmentedString> Begin => Option<SegmentedString>.Some(Option<string>.Some(this.begin));

    public Option<SegmentedString> Middle => Option<SegmentedString>.Some(Option<string>.Some(this.middle));

    public Option<SegmentedString> End => Option<SegmentedString>.Some(Option<string>.Some(this.end));

    public static implicit operator SegmentedString(Option<string> @string) => new SegmentedString(string.Empty, @string, string.Empty);

    public static implicit operator SegmentedString(string @string) => new SegmentedString(string.Empty, Option<string>.Some(@string), string.Empty);

    public static implicit operator SegmentedString((Option<string> begin, Option<string> middle, Option<string> end) @string) => new SegmentedString(@string.begin, @string.middle, @string.end);

    public static implicit operator SegmentedString(Option<ValueTuple<string, string, string>> @string) => @string;

    public static implicit operator string(SegmentedString segmentedStringEx) => segmentedStringEx.ToString();

    public override string ToString() => this.@string;
}