Can clap provide a new method to conditionally set the valid value of arguments?

[vanguardCLY]

In the existing clap method, value_parser() can realize the setting of valid values ​​of arguments. However, in some scenarios, the valid value of a argument is not fixed, and the valid value of this argument needs to be specified conditionally based on the values ​​of other arguments. For example, there are two arguments, one is type, and the valid values ​​are a1 and a2; the other is fmt. When type=a1, the valid values ​​of fmt are b1 and b2, and when type=a2, the valid values ​​of fmt are b2 and b3. Can clap introduce an new method, say named value_parser_if(), to fulfill this functionality? Thank you!

[epage]

This reads as a potentially different way of looking at #5950.

On top of the answer given there, a value_parser is a fairly fundamental unit in clap because it provides the type for the argument. Supporting multiple value_parsers would open users up to chances for more bugs being deferred to runtime which is a frustrating experience.