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    //! This module contains traits that are usable with the `#[derive(...)].`
    //! macros in [`clap_derive`].

    use crate::builder::PossibleValue;
    use crate::{ArgMatches, Command, Error};

    use std::ffi::OsString;

    /// Parse command-line arguments into `Self`.
    ///
    /// The primary one-stop-shop trait used to create an instance of a `clap`
    /// [`Command`], conduct the parsing, and turn the resulting [`ArgMatches`] back
    /// into concrete instance of the user struct.
    ///
    /// This trait is primarily a convenience on top of [`FromArgMatches`] +
    /// [`CommandFactory`] which uses those two underlying traits to build the two
    /// fundamental functions `parse` which uses the `std::env::args_os` iterator,
    /// and `parse_from` which allows the consumer to supply the iterator (along
    /// with fallible options for each).
    ///
    /// See also [`Subcommand`] and [`Args`].
    ///
    /// See the [derive reference](crate::_derive) for attributes and best practices.
    ///
    /// **NOTE:** Deriving requires the [`derive` feature flag][crate::_features]
    ///
    /// # Examples
    ///
    /// The following example creates a `Context` struct that would be used
    /// throughout the application representing the normalized values coming from
    /// the CLI.
    ///
    #[cfg_attr(not(feature = "derive"), doc = " ```ignore")]
    #[cfg_attr(feature = "derive", doc = " ```")]
    /// /// My super CLI
    /// #[derive(clap::Parser)]
    /// #[command(name = "demo")]
    /// struct Context {
    ///     /// More verbose output
    ///     #[arg(long)]
    ///     verbose: bool,
    ///     /// An optional name
    ///     #[arg(short, long)]
    ///     name: Option<String>,
    /// }
    /// ```
    ///
    /// The equivalent [`Command`] struct + `From` implementation:
    ///
    /// ```rust
    /// # use clap::{Command, Arg, ArgMatches, ArgAction};
    /// Command::new("demo")
    ///     .about("My super CLI")
    ///     .arg(Arg::new("verbose")
    ///         .long("verbose")
    ///         .action(ArgAction::SetTrue)
    ///         .help("More verbose output"))
    ///     .arg(Arg::new("name")
    ///         .long("name")
    ///         .short('n')
    ///         .help("An optional name")
    ///         .action(ArgAction::Set));
    ///
    /// struct Context {
    ///     verbose: bool,
    ///     name: Option<String>,
    /// }
    ///
    /// impl From<ArgMatches> for Context {
    ///     fn from(m: ArgMatches) -> Self {
    ///         Context {
    ///             verbose: *m.get_one::<bool>("verbose").expect("defaulted_by_clap"),
    ///             name: m.get_one::<String>("name").cloned(),
    ///         }
    ///     }
    /// }
    /// ```
    ///
    pub trait Parser: FromArgMatches + CommandFactory + Sized {
        /// Parse from `std::env::args_os()`, exit on error
        fn parse() -> Self {
            let mut matches = <Self as CommandFactory>::command().get_matches();
            let res = <Self as FromArgMatches>::from_arg_matches_mut(&mut matches)
                .map_err(format_error::<Self>);
            match res {
                Ok(s) => s,
                Err(e) => {
                    // Since this is more of a development-time error, we aren't doing as fancy of a quit
                    // as `get_matches`
                    e.exit()
                }
            }
        }

        /// Parse from `std::env::args_os()`, return Err on error.
        fn try_parse() -> Result<Self, Error> {
            let mut matches = ok!(<Self as CommandFactory>::command().try_get_matches());
            <Self as FromArgMatches>::from_arg_matches_mut(&mut matches).map_err(format_error::<Self>)
        }

        /// Parse from iterator, exit on error
        fn parse_from<I, T>(itr: I) -> Self
        where
            I: IntoIterator<Item = T>,
            T: Into<OsString> + Clone,
        {
            let mut matches = <Self as CommandFactory>::command().get_matches_from(itr);
            let res = <Self as FromArgMatches>::from_arg_matches_mut(&mut matches)
                .map_err(format_error::<Self>);
            match res {
                Ok(s) => s,
                Err(e) => {
                    // Since this is more of a development-time error, we aren't doing as fancy of a quit
                    // as `get_matches_from`
                    e.exit()
                }
            }
        }

        /// Parse from iterator, return Err on error.
        fn try_parse_from<I, T>(itr: I) -> Result<Self, Error>
        where
            I: IntoIterator<Item = T>,
            T: Into<OsString> + Clone,
        {
            let mut matches = ok!(<Self as CommandFactory>::command().try_get_matches_from(itr));
            <Self as FromArgMatches>::from_arg_matches_mut(&mut matches).map_err(format_error::<Self>)
        }

        /// Update from iterator, exit on error
        fn update_from<I, T>(&mut self, itr: I)
        where
            I: IntoIterator<Item = T>,
            T: Into<OsString> + Clone,
        {
            let mut matches = <Self as CommandFactory>::command_for_update().get_matches_from(itr);
            let res = <Self as FromArgMatches>::update_from_arg_matches_mut(self, &mut matches)
                .map_err(format_error::<Self>);
            if let Err(e) = res {
                // Since this is more of a development-time error, we aren't doing as fancy of a quit
                // as `get_matches_from`
                e.exit()
            }
        }

        /// Update from iterator, return Err on error.
        fn try_update_from<I, T>(&mut self, itr: I) -> Result<(), Error>
        where
            I: IntoIterator<Item = T>,
            T: Into<OsString> + Clone,
        {
            let mut matches =
                ok!(<Self as CommandFactory>::command_for_update().try_get_matches_from(itr));
            <Self as FromArgMatches>::update_from_arg_matches_mut(self, &mut matches)
                .map_err(format_error::<Self>)
        }
    }

    /// Create a [`Command`] relevant for a user-defined container.
    ///
    /// Derived as part of [`Parser`].
    pub trait CommandFactory: Sized {
        /// Build a [`Command`] that can instantiate `Self`.
        ///
        /// See [`FromArgMatches::from_arg_matches_mut`] for instantiating `Self`.
        fn command() -> Command;
        /// Build a [`Command`] that can update `self`.
        ///
        /// See [`FromArgMatches::update_from_arg_matches_mut`] for updating `self`.
        fn command_for_update() -> Command;
    }

    /// Converts an instance of [`ArgMatches`] to a user-defined container.
    ///
    /// Derived as part of [`Parser`], [`Args`], and [`Subcommand`].
    pub trait FromArgMatches: Sized {
        /// Instantiate `Self` from [`ArgMatches`], parsing the arguments as needed.
        ///
        /// Motivation: If our application had two CLI options, `--name
        /// <STRING>` and the flag `--debug`, we may create a struct as follows:
        ///
        #[cfg_attr(not(feature = "derive"), doc = " ```ignore")]
        #[cfg_attr(feature = "derive", doc = " ```no_run")]
        /// struct Context {
        ///     name: String,
        ///     debug: bool
        /// }
        /// ```
        ///
        /// We then need to convert the `ArgMatches` that `clap` generated into our struct.
        /// `from_arg_matches` serves as the equivalent of:
        ///
        #[cfg_attr(not(feature = "derive"), doc = " ```ignore")]
        #[cfg_attr(feature = "derive", doc = " ```no_run")]
        /// # use clap::ArgMatches;
        /// # struct Context {
        /// #   name: String,
        /// #   debug: bool
        /// # }
        /// impl From<ArgMatches> for Context {
        ///    fn from(m: ArgMatches) -> Self {
        ///        Context {
        ///            name: m.get_one::<String>("name").unwrap().clone(),
        ///            debug: *m.get_one::<bool>("debug").expect("defaulted by clap"),
        ///        }
        ///    }
        /// }
        /// ```
        fn from_arg_matches(matches: &ArgMatches) -> Result<Self, Error>;

        /// Instantiate `Self` from [`ArgMatches`], parsing the arguments as needed.
        ///
        /// Motivation: If our application had two CLI options, `--name
        /// <STRING>` and the flag `--debug`, we may create a struct as follows:
        ///
        #[cfg_attr(not(feature = "derive"), doc = " ```ignore")]
        #[cfg_attr(feature = "derive", doc = " ```no_run")]
        /// struct Context {
        ///     name: String,
        ///     debug: bool
        /// }
        /// ```
        ///
        /// We then need to convert the `ArgMatches` that `clap` generated into our struct.
        /// `from_arg_matches_mut` serves as the equivalent of:
        ///
        #[cfg_attr(not(feature = "derive"), doc = " ```ignore")]
        #[cfg_attr(feature = "derive", doc = " ```no_run")]
        /// # use clap::ArgMatches;
        /// # struct Context {
        /// #   name: String,
        /// #   debug: bool
        /// # }
        /// impl From<ArgMatches> for Context {
        ///    fn from(m: ArgMatches) -> Self {
        ///        Context {
        ///            name: m.get_one::<String>("name").unwrap().to_string(),
        ///            debug: *m.get_one::<bool>("debug").expect("defaulted by clap"),
        ///        }
        ///    }
        /// }
        /// ```
        fn from_arg_matches_mut(matches: &mut ArgMatches) -> Result<Self, Error> {
            Self::from_arg_matches(matches)
        }

        /// Assign values from `ArgMatches` to `self`.
        fn update_from_arg_matches(&mut self, matches: &ArgMatches) -> Result<(), Error>;

        /// Assign values from `ArgMatches` to `self`.
        fn update_from_arg_matches_mut(&mut self, matches: &mut ArgMatches) -> Result<(), Error> {
            self.update_from_arg_matches(matches)
        }
    }

    /// Parse a set of arguments into a user-defined container.
    ///
    /// Implementing this trait lets a parent container delegate argument parsing behavior to `Self`.
    /// with:
    /// - `#[command(flatten)] args: ChildArgs`: Attribute can only be used with struct fields that impl
    ///   `Args`.
    /// - `Variant(ChildArgs)`: No attribute is used with enum variants that impl `Args`.
    ///
    /// See the [derive reference](crate::_derive) for attributes and best practices.
    ///
    /// **NOTE:** Deriving requires the [`derive` feature flag][crate::_features]
    ///
    /// # Example
    ///
    #[cfg_attr(not(feature = "derive"), doc = " ```ignore")]
    #[cfg_attr(feature = "derive", doc = " ```")]
    /// #[derive(clap::Parser)]
    /// struct Args {
    ///     #[command(flatten)]
    ///     logging: LogArgs,
    /// }
    ///
    /// #[derive(clap::Args)]
    /// struct LogArgs {
    ///     #[arg(long, short = 'v', action = clap::ArgAction::Count)]
    ///     verbose: i8,
    /// }
    /// ```
    pub trait Args: FromArgMatches + Sized {
        /// Report the [`ArgGroup::id`][crate::ArgGroup::id] for this set of arguments
        fn group_id() -> Option<crate::Id> {
            None
        }
        /// Append to [`Command`] so it can instantiate `Self`.
        ///
        /// See also [`CommandFactory`].
        fn augment_args(cmd: Command) -> Command;
        /// Append to [`Command`] so it can update `self`.
        ///
        /// This is used to implement `#[command(flatten)]`
        ///
        /// See also [`CommandFactory`].
        fn augment_args_for_update(cmd: Command) -> Command;
    }

    /// Parse a sub-command into a user-defined enum.
    ///
    /// Implementing this trait lets a parent container delegate subcommand behavior to `Self`.
    /// with:
    /// - `#[command(subcommand)] field: SubCmd`: Attribute can be used with either struct fields or enum
    ///   variants that impl `Subcommand`.
    /// - `#[command(flatten)] Variant(SubCmd)`: Attribute can only be used with enum variants that impl
    ///   `Subcommand`.
    ///
    /// See the [derive reference](crate::_derive) for attributes and best practices.
    ///
    /// **NOTE:** Deriving requires the [`derive` feature flag][crate::_features]
    ///
    /// # Example
    ///
    #[cfg_attr(not(feature = "derive"), doc = " ```ignore")]
    #[cfg_attr(feature = "derive", doc = " ```")]
    /// #[derive(clap::Parser)]
    /// struct Args {
    ///     #[command(subcommand)]
    ///     action: Action,
    /// }
    ///
    /// #[derive(clap::Subcommand)]
    /// enum Action {
    ///     Add,
    ///     Remove,
    /// }
    /// ```
    pub trait Subcommand: FromArgMatches + Sized {
        /// Append to [`Command`] so it can instantiate `Self`.
        ///
        /// See also [`CommandFactory`].
        fn augment_subcommands(cmd: Command) -> Command;
        /// Append to [`Command`] so it can update `self`.
        ///
        /// This is used to implement `#[command(flatten)]`
        ///
        /// See also [`CommandFactory`].
        fn augment_subcommands_for_update(cmd: Command) -> Command;
        /// Test whether `Self` can parse a specific subcommand
        fn has_subcommand(name: &str) -> bool;
    }

    /// Parse arguments into enums.
    ///
    /// When deriving [`Parser`], a field whose type implements `ValueEnum` can have the attribute
    /// `#[arg(value_enum)]` which will
    /// - Call [`EnumValueParser`][crate::builder::EnumValueParser]
    /// - Allowing using the `#[arg(default_value_t)]` attribute without implementing `Display`.
    ///
    /// See the [derive reference](crate::_derive) for attributes and best practices.
    ///
    /// **NOTE:** Deriving requires the [`derive` feature flag][crate::_features]
    ///
    /// # Example
    ///
    #[cfg_attr(not(feature = "derive"), doc = " ```ignore")]
    #[cfg_attr(feature = "derive", doc = " ```")]
    /// #[derive(clap::Parser)]
    /// struct Args {
    ///     #[arg(value_enum)]
    ///     level: Level,
    /// }
    ///
    /// #[derive(clap::ValueEnum, Clone)]
    /// enum Level {
    ///     Debug,
    ///     Info,
    ///     Warning,
    ///     Error,
    /// }
    /// ```
    pub trait ValueEnum: Sized + Clone {
        /// All possible argument values, in display order.
        fn value_variants<'a>() -> &'a [Self];

        /// Parse an argument into `Self`.
        fn from_str(input: &str, ignore_case: bool) -> Result<Self, String> {
            Self::value_variants()
                .iter()
                .find(|v| {
                    v.to_possible_value()
                        .expect("ValueEnum::value_variants contains only values with a corresponding ValueEnum::to_possible_value")
                        .matches(input, ignore_case)
                })
                .cloned()
                .ok_or_else(|| format!("Invalid variant: {}", input))
        }

        /// The canonical argument value.
        ///
        /// The value is `None` for skipped variants.
        fn to_possible_value(&self) -> Option<PossibleValue>;
    }

    impl<T: Parser> Parser for Box<T> {
        fn parse() -> Self {
            Box::new(<T as Parser>::parse())
        }

        fn try_parse() -> Result<Self, Error> {
            <T as Parser>::try_parse().map(Box::new)
        }

        fn parse_from<I, It>(itr: I) -> Self
        where
            I: IntoIterator<Item = It>,
            It: Into<OsString> + Clone,
        {
            Box::new(<T as Parser>::parse_from(itr))
        }

        fn try_parse_from<I, It>(itr: I) -> Result<Self, Error>
        where
            I: IntoIterator<Item = It>,
            It: Into<OsString> + Clone,
        {
            <T as Parser>::try_parse_from(itr).map(Box::new)
        }
    }

    impl<T: CommandFactory> CommandFactory for Box<T> {
        fn command<'help>() -> Command {
            <T as CommandFactory>::command()
        }
        fn command_for_update<'help>() -> Command {
            <T as CommandFactory>::command_for_update()
        }
    }

    impl<T: FromArgMatches> FromArgMatches for Box<T> {
        fn from_arg_matches(matches: &ArgMatches) -> Result<Self, Error> {
            <T as FromArgMatches>::from_arg_matches(matches).map(Box::new)
        }
        fn from_arg_matches_mut(matches: &mut ArgMatches) -> Result<Self, Error> {
            <T as FromArgMatches>::from_arg_matches_mut(matches).map(Box::new)
        }
        fn update_from_arg_matches(&mut self, matches: &ArgMatches) -> Result<(), Error> {
            <T as FromArgMatches>::update_from_arg_matches(self, matches)
        }
        fn update_from_arg_matches_mut(&mut self, matches: &mut ArgMatches) -> Result<(), Error> {
            <T as FromArgMatches>::update_from_arg_matches_mut(self, matches)
        }
    }

    impl<T: Args> Args for Box<T> {
        fn augment_args(cmd: Command) -> Command {
            <T as Args>::augment_args(cmd)
        }
        fn augment_args_for_update(cmd: Command) -> Command {
            <T as Args>::augment_args_for_update(cmd)
        }
    }

    impl<T: Subcommand> Subcommand for Box<T> {
        fn augment_subcommands(cmd: Command) -> Command {
            <T as Subcommand>::augment_subcommands(cmd)
        }
        fn augment_subcommands_for_update(cmd: Command) -> Command {
            <T as Subcommand>::augment_subcommands_for_update(cmd)
        }
        fn has_subcommand(name: &str) -> bool {
            <T as Subcommand>::has_subcommand(name)
        }
    }

    fn format_error<I: CommandFactory>(err: crate::Error) -> crate::Error {
        let mut cmd = I::command();
        err.format(&mut cmd)
    }