`ArgAction::Count` type error not caught at compile time

[cyqsimon]

Please complete the following tasks

• [X] I have searched the discussions
• [X] I have searched the open and rejected issues

Rust Version

rustc 1.61.0 (fe5b13d68 2022-05-18)

Clap Version

v3.2.6

Minimal reproducible code

use clap::{ArgAction, Parser};

#[derive(Debug, Clone, Parser)]
pub struct CliArgs {
    #[clap(short = 'v', action = ArgAction::Count)]
    pub verbose: i32,
}
fn main() {
    let _args = CliArgs::parse();
    println!("Hello, world!");
}

Steps to reproduce the bug with the above code

• Run cargo build and observe that the program builds.
• Run cargo run and observe that the program panics at runtime due to a type error.

Actual Behaviour

Type error causes a panic at runtime.

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `u8`,
 right: `i32`: Argument `verbose`'s selected action Count contradicts `value_parser` (ValueParser::other(i32))'

Expected Behaviour

Type error should be caught at compile time.

Alternatively, allow upcasting the u8 into user`s preferred type automatically.

[epage]

Thinking through this and the problems we normally have in moving an error to compile time, I think this is one case where we skip most of those.

[slonik-az]

The better solution is to automatically downcast to user provided type. Currently, one cannot even downcast u8 -> usize, see #4417 and #3912.

[tgross35]

I think this is also an issue in the example on the args trait page. Currently it has:

#[derive(clap::Parser)]
struct Args {
   #[command(flatten)]
   logging: LogArgs,
}

#[derive(clap::Args)]
struct LogArgs {
   #[arg(long, short = 'v', action = clap::ArgAction::Count)]
   verbose: i8,
}

And I had something based on this that didn't work until I changed i8 to u8

Edit: submitted #4610 to fix just this example