public async Task MapFirstValueAsync_SourceIsDefault_ExpectDefault()
{
var source = new TaggedUnion <object, RefType?>();
var mappedValue = PlusFifteenIdRefType;
var actual = await source.MapFirstValueAsync(_ => ValueTask.FromResult(mappedValue));
var expected = default(TaggedUnion <RefType, RefType?>);
Assert.AreEqual(expected, actual);
}