value_name appends, rather than overrides

[epage]

Please complete the following tasks

• I have searched the discussions
• I have searched the existing issues

Rust Version

rustc 1.53.0 (53cb7b09b 2021-06-17)

Clap Version

3.0.0-beta.2

Minimal reproducible code

#[clap(
number_of_values = 1,
long = "using",
value_name = "pm",
value_name = "foo",
)]
using: Option,

Steps to reproduce the bug with the above code

Run it

Actual Behaviour

• pm and foo show up in --help
• Clap errors if there aren't two arguments

Expected Behaviour

• Last one wins

Additional Context

Split out of #2632. Depending on what solution we go with #2633 could be simplified

There is a separate value_names that is documented for providing multiple. That also appends

This is either a bug in the documentation or in the code.

Related: documentation also says value_name is cosmetic only but value_name, under certain situations, implies number_of_values.

Most of the time, people will not run into this. Its when programmatically creating an argument, like in the derive macro.

Debug Output

No response

[epage]

@pksunkara any opinion on whether this is a documentation or implementation bug?

[pksunkara]

Related: documentation also says value_name is cosmetic only

Docs say that the name is cosmetic. Not the function.

This is either a bug in the documentation or in the code.

While most of the other methods just extend existing data, default_value and default_values which is probably the most similar to these functions override. That is probably what we want to do here too.