default_value_if should work with flags that don't take a value

[lefth]

Please complete the following tasks

• I have searched the discussions
• I have searched the existing issues

Clap Version

3.0.7

Describe your use case

I want to have various specific flags imply a general flag, for example --mystery or --suspense implies --fiction. I don't want to require an extra "=true" on the flags.

Describe the solution you'd like

Clap can almost support this use case: fiction should have default_value_ifs stating that any mystery or suspense argument gives the default value of true, and takes_value is false. However, default_value_if and default_value_ifs seem to need takes_value. They set it implicitly, and when it is turned off, the implication does not happen.

Or if the right answer is to use min_value(0), that should be documented (see below).

Alternatives, if applicable

An alternative solution is to override parse, parse_from, try_parse, and try_parse_from (and call <Self as Parser>::*parse*) and call a validation function to set implied values.

Another alternative solution would be to let the struct or app define a post-parse hook (the validation function mentioned above).

If "true" and "false" are not valid positional values, the easiest workaround is to use min_values(0) instead of takes_value(false). If this is the best solution for now, I suggest adding a note to the takes_value docs such as: "Note that if takes_value(false) cannot be used due to a default value being set by another argument's presence, min_values(0) is a reasonable substitute."

Additional Context

This issue is shown in rust playground here: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=4855a88381f65cef8d07f7eab4d41e78

[epage]

So there are ways around this in the builder API but this was reported against the derive API.

I have been considering re-doing flags so that they set a value, rather than relying on them being preset or not which would resolve this (and simplify several other APIs).