public void MapValueAsync_MapSecondAsyncFuncIsNull_ExpectArgumentNullException(
TaggedUnion <RefType, StructType> source)
{
var ex = Assert.ThrowsAsync <ArgumentNullException>(
async() => _ = await source.MapValueAsync <uint, DateTime>(_ => ValueTask.FromResult <uint>(1), null !));
Assert.AreEqual("mapSecondAsync", ex !.ParamName);
}