public void MapAsync_MapSecondAsyncFuncIsNull_ExpectArgumentNullException(
TaggedUnion <RefType, StructType> source)
{
var ex = Assert.ThrowsAsync <ArgumentNullException>(
async() => _ = await source.MapAsync <decimal, object>(_ => Task.FromResult(21.5m), null !));
Assert.AreEqual("mapSecondAsync", ex !.ParamName);
}
