This came up here, but I wanted to split it out to track it explicitly.

I think that reporting the indexes of arguments within args would be a helpful addition, because it lets you build a bunch of more complicated logic on top of parseArgs without needing to do a full re-parse.

For example, this would make it straightforward to:

• resolve conflicts between --foo and --no-foo as last-wins
• throw on duplicate flags (check if the flag occurs at multiple indexes)
• if someday defaults are added, check to see if an option was provided by a user or came from the default (Restructure result conventions (again) #70 (comment))
• enforce that value-taking arguments are all specified using --foo=bar rather than --foo bar (check each to see if args[index].includes('='))
• enforce that two flags are specified in a specific order (--enable-experimental-options before --some-unstable-option, e.g.)