Support plan

• is this issue currently blocking your project? (yes/no): no
• is this issue affecting a production system? (yes/no): no

Context

• node version: 14.14
• module version with issue: 17.1.1
• last module version without issue: ?
• environment (e.g. node, browser, native): node
• used with (e.g. hapi application, another framework, standalone, ...): Express
• any other relevant information:

What are you trying to achieve or the steps to reproduce?

When I'm using the any.when() function with the shortcut for switch case:
From (https://joi.dev/api/?v=17.3.0#anywhencondition-options):

const schema = Joi.object({
    a: Joi.number().required(),
    b: Joi.number()
        .when('a', [
            { is: 0, then: 1 },
            { is: 1, then: 2 },
            { is: 2, then: 3, otherwise: 4 }
        ])
});

What was the result you got?

tsc throw me an error saying that the second param options any.when cannot be an [].

Overload 1 of 2, '(ref: string | Reference, options: WhenOptions): AlternativesSchema', gave the following error.
    Type '({ is: myKey; then: StringSchema; } | { is: myKey; then: StringSchema; otherwise: Schema; })[]' has no properties in common with type 'WhenOptions'.
  Overload 2 of 2, '(ref: Schema, options: WhenSchemaOptions): AlternativesSchema', gave the following error.
    Argument of type 'string' is not assignable to parameter of type 'Schema'.

What result did you expect?

Can transpile from TS to JS without any error.

I think I spoted the problem in the index.d.ts:
https://github.com/sideway/joi/blob/master/lib/index.d.ts#L1161
Or
https://github.com/sideway/joi/blob/master/lib/index.d.ts#L2184