Unknown arguments cause incorrect command positional coerce()

[evocateur]

As evidenced in lerna/lerna#2315, when arguments are passed after -- with the default parserConfiguration (append them to argv._), yargs@14.2.0 causes an incorrect coerce("") call on a command's positional argument, as well as re-parsing any flags (which fails when strict() is enabled).

require('yargs')
  .strict()
  .option('loglevel', {
    type: 'string',
  })
  .command('repro [stuff]', 'example', yargs => {
    yargs.positional('stuff', {
      describe: 'things',
      coerce: stuff => {
        if (stuff === '') {
          throw new Error('fail');
        }
        return stuff;
      },
    });
  }, argv => {
    console.log(argv);
  })
  .parse('repro -- --loglevel=woo');

It also causes the valid --loglevel option in the example above to fail, once the coerce() method is modfied to ensure that the parameter is in fact non-empty first (or if it's an identity function stuff => stuff, or removed entirely). So that's fun.

The current workaround is to enable populate-- in yargs-parser:

  .parserConfiguration({
    "populate--": true,
  })

This seems sub-optimal.