Fluent APIs are not allowed in non-builder types

[adamsitnik]

Based on the feedback from @bartonjs

[KalleOlaviNiemitalo]

Should the ArgumentValidation.AcceptExistingOnly and OptionValidation.AcceptExistingOnly methods likewise return void?

[jozkee]

You might as well remove the test

[jozkee]

nit

Suggested change

	Argument<string[]> argument = new ();
	Argument<string[]> argument = new();

[jozkee]

Should the ArgumentValidation.AcceptExistingOnly and OptionValidation.AcceptExistingOnly methods likewise return void?

Up to Adam's choice, but that could be addressed in a follow-up PR if required.

[adamsitnik]

Should the ArgumentValidation.AcceptExistingOnly and OptionValidation.AcceptExistingOnly methods likewise return void?

That is a very good question. To be honest I don't like AcceptLegalFilePathsOnly and AcceptLegalFileNamesOnly as they don't restrict the argument/option type in any way. Example:

Option<bool> flag = new ("--flag");
flag.AcceptLegalFilePathsOnly(); // it should work only with files, not things like boolean

ArgumentValidation.AcceptExistingOnly and OptionValidation.AcceptExistingOnly restrict the types, but our BCL guidelines are against introducing extension methods for types that we own (they should be just instance methods). When @bartonjs and @jonsequitur are back from holidays I am going to discuss it with them.