add GetRequiredValue to avoid null checks for required options

[adamsitnik]

The existing GetValue method returns nullable T?, which often leads to plenty of nullability warnings when working with required options. GetRequiredValue returns the parsed (or configured default value) or throws.

Option<FileInfo> fileOption = new("--file") { Required = true };
RootCommand command = new() { fileOption };

command.SetAction(paseResult =>
{
-   FileInfo file = paseResult.GetValue(fileOption)!; // mind the !
+   FileInfo file = paseResult.GetRequiredValue(fileOption);
    Console.WriteLine($"File name: {file.FullName}");
});

[baronfel]

cc @jaredpar - weren't we jsut talking about some nullability analysis for this use method?

[jonsequitur]

Are we also testing Option.GetRequiredValue for these cases?

[jonsequitur]

ParserTests is a bit of a catchall. I think these would be better organized under ArgumentTests and OptionTests.

[jonsequitur]

I believe this can still return null.

[adamsitnik]

I believe this can still return null.

The only scenario I can come up with is the user defining null as the default value via DefaultValueFactory. Do you believe we should throw in such cases?

[jonsequitur]

My memory was that SymbolResult.GetResult would never return null but either I misremembered or that changed at some point. This looks great!

[jaredpar]

weren't we jsut talking about some nullability analysis for this use method?

Yes. It's the number one cause of nullable suppressions in sdk after I changed the opt in model.