Don't use default value when option is passed

[davidhewitt]

This PR changes default_value so that it does not apply for an option when the user passes the flag without a corresponding value. This case is now handled by default_missing_value.

(See also my discussion question #2083 where I understood that this is the expected behaviour.)

Consider this sample program:

#[derive(Clap, Debug)]
struct Opts {
    #[clap(long, default_value = "5")]
    batch_size: usize
}

With the current master, I get the following results of execution:

cargo run                    // OK
batch_size = 5

cargo run -- --batch-size    // <---This should not succeed, as default_missing_value is not set
batch_size = 5

cargo run -- --batch-size 6  // OK
batch_size = 6

After this patch, everything works as expected:

cargo run
batch_size = 5

cargo run -- --batch-size
error: The argument '--batch-size <batch-size>' requires a value but none was supplied

cargo run -- --batch-size 6
batch_size = 6

[davidhewitt]

Thanks for the quick review, I will address all comments tonight!

[davidhewitt]

Thanks very much for the feedback - I've addressed it all and CI appears happy. Let me know if there's any other changes you'd like to see!

[pksunkara]

bors r+

[bors]

Build succeeded:

• CI
• clap-rs.clap
• continuous-integration/travis-ci/push