Incorrect default value behavior with require_equals and min_values(0)

[jonhoo]

Rust Version

rustc 1.22.0-nightly (ddd123ed9 2017-09-09)

Affected Version of clap

2.26.1

Expected Behavior Summary

When a require_equals + min_values(0) argument is given on the command line without a value, the default value should be set as its value in matches.

Actual Behavior Summary

If the argument is given without a value, occurs is 1, but vals is [].

Steps to Reproduce the issue

Using the code below, try the following command lines:

$ foo
occurs: 0
value: Some(",")
$ foo -d=@
occurs: 1
value: Some("@")
$ foo -d
occurs: 1
value: None

Notice that in the last case, value should be Some(",").

Sample Code or Link to Sample Code

extern crate clap;

use clap::{App, Arg};
fn main() {
    let m = App::new("foo")
        .arg(
            Arg::with_name("delimiter")
                .short("d")
                .long("delimiter")
                .takes_value(true)
                .require_equals(true)
                .min_values(0)
                .default_value(","),
        )
        .get_matches();
    println!("occurs: {}", m.occurrences_of("delimiter"));
    println!("value: {:?}", m.value_of("delimiter"));
}

Debug output

For the -d case:

DEBUG:clap:Parser::propogate_settings: self=time, g_settings=AppFlags(
    NEEDS_LONG_HELP | NEEDS_LONG_VERSION | NEEDS_SC_HELP | UTF8_NONE | COLOR_AUTO
)
DEBUG:clap:Parser::get_matches_with;
DEBUG:clap:Parser::create_help_and_version;
DEBUG:clap:Parser::create_help_and_version: Building --help
DEBUG:clap:Parser::create_help_and_version: Building --version
DEBUG:clap:Parser::get_matches_with: Begin parsing '"-d"' ([45, 100])
DEBUG:clap:Parser::is_new_arg: arg="-d", Needs Val of=NotFound
DEBUG:clap:Parser::is_new_arg: Arg::allow_leading_hyphen(false)
DEBUG:clap:Parser::is_new_arg: - found
DEBUG:clap:Parser::is_new_arg: starts_new_arg=true
DEBUG:clap:Parser::possible_subcommand: arg="-d"
DEBUG:clap:Parser::get_matches_with: possible_sc=false, sc=None
DEBUG:clap:Parser::parse_short_arg: full_arg="-d"
DEBUG:clap:Parser::parse_short_arg:iter:d
DEBUG:clap:Parser::parse_short_arg:iter:d: Found valid opt
DEBUG:clap:Parser::parse_short_arg:iter:d: p[0]=[], p[1]=[]
DEBUG:clap:Parser::parse_opt; opt=delimiter, val=None
DEBUG:clap:Parser::parse_opt; opt.settings=ArgFlags(TAKES_VAL | DELIM_NOT_SET | REQUIRE_EQUALS)
DEBUG:clap:Parser::parse_opt; Checking for val...None
DEBUG:clap:ArgMatcher::inc_occurrence_of: arg=delimiter
DEBUG:clap:ArgMatcher::inc_occurrence_of: first instance
DEBUG:clap:Parser::groups_for_arg: name=delimiter
DEBUG:clap:Parser::groups_for_arg: No groups defined
DEBUG:clap:Parser::parse_opt: More arg vals not required...
DEBUG:clap:arg_post_processing!;
DEBUG:clap:OptBuilder::fmt:delimiter
DEBUG:clap:arg_post_processing!: Is '--delimiter=<delimiter>' in overrides...No
DEBUG:clap:OptBuilder::fmt:delimiter
DEBUG:clap:arg_post_processing!: Does '--delimiter=<delimiter>' have overrides...No
DEBUG:clap:OptBuilder::fmt:delimiter
DEBUG:clap:arg_post_processing!: Does '--delimiter=<delimiter>' have conflicts...No
DEBUG:clap:OptBuilder::fmt:delimiter
DEBUG:clap:arg_post_processing!: Does '--delimiter=<delimiter>' have requirements...No
DEBUG:clap:_handle_group_reqs!;
DEBUG:clap:Parser:get_matches_with: After parse_short_arg ValuesDone
DEBUG:clap:Validator::validate;
DEBUG:clap:Validator::validate_blacklist: blacklist=[]
DEBUG:clap:Validator::validate_required: required=[];
DEBUG:clap:Validator::validate_matched_args;
DEBUG:clap:Validator::validate_matched_args:iter:delimiter: vals=[]
DEBUG:clap:Validator::validate_arg_num_vals;
DEBUG:clap:Validator::validate_arg_num_vals: min_vals set: 0
DEBUG:clap:Validator::validate_values: arg="delimiter"
DEBUG:clap:Validator::validate_arg_requires;
DEBUG:clap:Validator::validate_arg_num_occurs: a=delimiter;
DEBUG:clap:usage::create_usage_with_title;
DEBUG:clap:usage::create_usage_no_title;
DEBUG:clap:usage::get_required_usage_from: reqs=[], extra=None
DEBUG:clap:usage::get_required_usage_from: after init desc_reqs=[]
DEBUG:clap:usage::get_required_usage_from: no more children
DEBUG:clap:usage::get_required_usage_from: final desc_reqs=[]
DEBUG:clap:usage::get_required_usage_from: args_in_groups=[]
DEBUG:clap:usage::needs_flags_tag;
DEBUG:clap:usage::needs_flags_tag:iter: f=hclap_help;
DEBUG:clap:usage::needs_flags_tag:iter: f=vclap_version;
DEBUG:clap:usage::needs_flags_tag: [FLAGS] not required
DEBUG:clap:usage::create_help_usage: usage=x [OPTIONS]

[jonhoo]

And because @kbknapp raised it during discussion: adding another dummy argument and passing it after -d (as in -d -x) does not change this outcome.