Terraform for_each unknown computed value

TL;DR -

1. Using a for_each loop over a map of values where the keys are static and the values can be either static or computed, works as expected per hashicorp/terraform#4149 and hashicorp/terraform#30937
2. If you take that logic and move it into a module, it still functions as expected (without issue related to The "for_each" value depends on resource attributes that cannot be determined until apply...)
3. If you take the module from 2 above and wrap it in a module that uses a for_each loop, you get the error The "for_each" value depends on resource attributes that cannot be determined until apply... even though the map keys are all static and known

Solution

When iterating over a map of maps, if we use "eagerly evaluated" functions such as try() and can() will cause the well known The "for_each" value depends on resource attributes that cannot be determined until apply... error when a computed value is provided in a map value that is passed to a try() or can() function. To circumvent this eager/early evaluation, we can instead a nested set of lookup() which is not eager/early evaluated and avoids unknown value errors.

Fail ❌

module "example" {
  source = "../../"

  for_each = { for k, v in var.fargate_profiles : k => v if var.create }

  ...

  iam_role_additional_policies = try(each.value.iam_role_additional_policies", var.fargate_profile_defaults.iam_role_additional_policies, {})
}

Pass ✔️

module "example" {
  source = "../../"

  for_each = { for k, v in var.fargate_profiles : k => v if var.create }

  ...

  # To better understand why this `lookup()` logic is required, see:
  # https://github.com/hashicorp/terraform/issues/31646#issuecomment-1217279031
  iam_role_additional_policies = lookup(each.value, "iam_role_additional_policies", lookup(var.fargate_profile_defaults, "iam_role_additional_policies", {}))
}