public async Task AsyncSync_sourceがnullの場合は例外が発生する()
 {
     Func <Task> act = () => ResultExtensions.GetErrorOr((Task <Result <string> >)null, _ => new Error());
     await act.Should().ThrowAsync <ArgumentNullException>();
 }
 public async Task AsyncAsync_sourceがnullの場合は例外が発生する()
 {
     Func <Task> act    = () => ResultExtensions.GetErrorOr((Task <Result>)null, () => Task.FromResult(new Error()));
     var         result = await act.Should().ThrowAsync <ArgumentNullException>();
 }