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: 14.19.3
• module version with issue: 17.6.1 / 17.6.2
• last module version without issue: 17.6.0
• environment (e.g. node, browser, native): node
• used with (e.g. hapi application, another framework, standalone, ...): ?
• any other relevant information: n/a

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

let apiKey: string | undefined = "someString";
apiKey = Joi.attempt(apiKey, Joi.string().optional().label("apiKey"));

What was the result you got?

TS2322: Type 'unknown' is not assignable to type 'string | undefined'.

What result did you expect?

No TS errors.

I suspect the changes from #2797 to be the root cause.