Fork object with option already applied

[optii]

Support plan

• which support plan is this issue covered by? (e.g. Community, Core, Plus, or Enterprise): Comunity
• is this issue currently blocking your project? (yes/no): yes
• is this issue affecting a production system? (yes/no): yes

Context

• node version: 12.14.0
• module version with issue: 17.1.0
• last module version without issue: we were previously working with joi 15 so don't think it is that relevant as fork didn't exist
• environment (e.g. node, browser, native): node
• used with (e.g. hapi application, another framework, standalone, ...): hapi application & https://github.com/hapipal/schwifty
• any other relevant information:

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

Try to run fork to apply .optional to all keys of an Joi.object (https://github.com/hapipal/schwifty/blob/cffa382e9411e404c5524c69ed39f028e24506a7/lib/model.js#L146) however
it seems that if .forkis run on a schema that already has optional applied that there is an error thrown. See reproduction below
https://runkit.com/optii/5e399335239b800013ea2b06

const joi = require("@hapi/joi")

const schema = joi.object({
  a: joi.string().optional(),
  b: joi.string()
});
 
 const bOptional = schema.fork(['b'], (schema) =>  schema.optional());
 const allOptional = schema.fork(['a', 'b'], (schema) => schema.optional());

What was the result you got?

TypeError: Cannot read property '$_mutateRebuild' of undefined

What result did you expect?

I expected for joi to ignore the fact that the schema hadn't changed or to throw a more friendlier type of error, the first option being preferred and seems in keeping with the fact that Joi.string().optional().optional() works

[gerpedrosa]

Could you apply this fix in version 16?
At the moment I can't use version 17 in my project because I need to keep support for Node.js 10