Define optional argument with exactly 1 value, + default value

[nmoreaud]

Hello,

I try to define an optional parameter with a value + default value, but it doesn't work as I expect.
I use version 3.0.

What I'd like to obtain:

• Optional [] in usage section: Usage: [--format1 VAR] --format2 VAR --format3 VAR [--format4 VAR]
  ok for format1 and format4
• Exception if argument is called without value: ok for --format2 and --format3
• No mention of [nargs=0..1] in argument section:

--format1                  Image format [jpg|png] [nargs=0..1] [default: "jpg"]
--format2                  Image format r1 [jpg|png] [default: "jpg"]                            => ok
--format3                  Image format r [jpg|png] [nargs=0..1] [default: "jpg"]
--format4                  Image format 1 [jpg|png] [default: "jpg"]                             => ok

• No mention of [required] in argument section: ok for all
• Listed in "Optional arguments" section: ok for all

Could you help me?

Here is a sample program:

        command.add_argument("--format1")
            .default_value(string("jpg"))
            .help("Image format [jpg|png]");

        command.add_argument("--format2")
            .default_value(string("jpg"))
            .required()
            .nargs(1)
            .help("Image format r1 [jpg|png]");

        command.add_argument("--format3")
            .default_value(string("jpg"))
            .required()
            .help("Image format r [jpg|png]");

        command.add_argument("--format4")
            .default_value(string("jpg"))
            .nargs(1)
            .help("Image format 1 [jpg|png]");