The validate modifier should also catch all exceptions.

[Jadarma]

The documentation on handling errors says:

convert and the other parameter transformations will wrap exceptions thrown inside them in a UsageError, so if you define a custom transformation, you don’t have to worry about an exception escaping to the user.

Since validate is not technically a transformation, the documentation does not lie, but the usage is unintuitive.
I foolishly assumed the require function is the same as the Kotlin stdlib, however it is an overload in the context of the OptionValidator.
I have some type that requires some more complex validator that is too big to fit nicely in the option declaration site, so it is extracted in a separate function:

val foo by option("--foo").validate { customValidationLogic(it) }

However, when validation fails here, since the custom logic does not throw BadParameterValue, it crashes the program assuming it's a programming error.

As a workaround for the time being, one must either do validation logic inside a convert lambda, or manually wrap exceptions, I defined my own util for this:

inline fun <AllT, EachT, ValueT> OptionWithValues<AllT, EachT, ValueT>.validateCatching(
    crossinline validator: OptionValidator<AllT & Any>,
): OptionDelegate<AllT> = copy(
    transformValue,
    transformEach,
    transformAll,
    validator = {
        if (it != null) runCatching { validator(it) }.getOrElse { cause ->
            throw BadParameterValue(cause.message.orEmpty(), option)
        }
    },
)

I believe the .validate {} lambda should match the behavior of the other transformers, to make it easier to use validation logic from functions that might come from external libraries, for example.