How to validate an input variable based one another input variable?

I have two sets of input data , like this:

default_groups = [

policy_attachment = {
  Admins = [

Using the new custom validation, I tried this:

variable "policy_attachment" {
  default = {}
  type    = map(list(string))

  validation {
    condition = ! contains([
      for key in keys(var.policy_attachment) :
      contains(var.default_groups, key)
    ], false)
    error_message = "Role MUST exist for policy attachment."

to validate any key in policy_attachment must exists in default_groups list but getting the error:

The condition for variable “policy_attachment” can only refer to the variable
itself, using var.policy_attachment.

Is there a way to achieve that at all? Thanks in advance for any assistance.