How to use () type in clap_derive

[tmccombs]

The documentation for clap_derive states that () can be used as a type for "user-defined" effect. However, if I try to use something like:

    #[arg(long, overrides_with = "hidden", hide = true)]
    pub no_hidden: (),

then I get the following error:

error[E0599]: the method `value_parser` exists for reference `&&&&&&_AutoValueParser<()>`, but its trait bounds were not satisfied
    --> src/cli.rs:107:5
     |
107  |     #[arg(long, overrides_with = "hidden", hide = true)]
     |     ^ method cannot be called on `&&&&&&_AutoValueParser<()>` due to unsatisfied trait bounds
     |
    ::: /home/thayne/.cargo/registry/src/github.com-1ecc6299db9ec823/clap-4.0.18/src/builder/value_parser.rs:2171:1
     |
2171 | pub struct _AutoValueParser<T>(std::marker::PhantomData<T>);
     | ------------------------------ doesn't satisfy `_: clap::builder::via_prelude::_ValueParserViaParse`
     |
     = note: the following trait bounds were not satisfied:
             `(): ValueEnum`
             which is required by `&&&&&_AutoValueParser<()>: clap::builder::via_prelude::_ValueParserViaValueEnum`
             `(): ValueParserFactory`
             which is required by `&&&&&&_AutoValueParser<()>: clap::builder::via_prelude::_ValueParserViaFactory`
             `(): From<OsString>`
             which is required by `&&&&_AutoValueParser<()>: clap::builder::via_prelude::_ValueParserViaFromOsString`
             `(): From<&'s std::ffi::OsStr>`
             which is required by `&&&_AutoValueParser<()>: clap::builder::via_prelude::_ValueParserViaFromOsStr`
             `(): From<std::string::String>`
             which is required by `&&_AutoValueParser<()>: clap::builder::via_prelude::_ValueParserViaFromString`
             `(): From<&'s str>`
             which is required by `&_AutoValueParser<()>: clap::builder::via_prelude::_ValueParserViaFromStr`
             `(): FromStr`
             which is required by `_AutoValueParser<()>: clap::builder::via_prelude::_ValueParserViaParse`
     = note: this error originates in the macro `clap::value_parser` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0599`.

I couldn't find any examples of using this type, so I'm not sure if I'm missing something or this is a bug in clap_derive.

[epage]

That error is because it is trying to infer the value_parser field. Explicitly setting value_parser will will resolve that.

From reading the sample you gave, I'm assuming you will need to set the arg_action(ArgAction::SetTrue) as well.

[epage]

#4459 will make it so you only need to specify the action as it has a default value parser

[tmccombs]

Thank you!