Describe the problem that you experienced

https://stackoverflow.com/questions/79882116/how-to-write-custom-schema-validator-functions-for-union-types

Enter the URL of the topic with the problem

https://angular.dev/guide/forms/signals/validation

Describe what you were looking for in the documentation

Would be helpful to describe applyWhenValue() also in the guide, not only in the reference part, s.th. like https://stackoverflow.com/a/79882667/1480587

Describe the actions that led you to experience the problem

No response

Describe what you want to experience that would fix the problem

No response

Add a screenshot if that helps illustrate the problem

No response

If this problem caused an exception or error, please paste it here

If the problem is browser-specific, please specify the device, OS, browser, and version

Provide any additional information here in as much as detail as you can