Allow an argument to be both a positional and an option

[figsoda]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Clap Version

4.0.18

Describe your use case

As far as I'm aware, short and long overrides the implicit positionalness and there is no way to have both on

Describe the solution you'd like

for example, if I want to have prog a b c, prog -f a b c, and prog --foo a b c to be equivalent

#[derive(Parser)]
struct Opts {
    #[arg(short, long, position, num_args = ..)]
    foo: Option<Vec<String>>,
}

Alternatives, if applicable

Using replace from unstable-replace, but I have to deal with the quirks of replace and also use an unstable feature

#[derive(Parser)]
#[command(replace("-f", [""; 0]), replace("-foo", [""; 0]))] // not sure how to create explicitly typed slices so I had to create this abomination
struct Opts {
    #[arg(short, long, num_args = ..)]
    foo: Option<Vec<String>>,
}

Or having 2 separated fields and combine them manually, which seems overly complicated

#[derive(Parser)]
struct Opts {
    #[arg(short, long, num_args = ..)]
    foo: Option<Vec<String>>,
    foo2: Option<Vec<String>>,
}

Additional Context

No response

[epage]

I feel like trying to support both without a very strong motivation will add complexity to clap for a case that should be pretty rare in CLIs. I also feel that this isn't a pattern we should be encouraging by making things easier.

Users have the building blocks they need to implement this themselves by putting the two arguments into a group which will be made easier with #2621.

Without more, I'm going ahead and closing this as unplanned.