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