Terraform 0.12 - Produce map/object from nested for loop

A partial answer can be found at https://github.com/hashicorp/terraform/issues/22263. Long story short: this was a foolish attempt to begin with, a map cannot contain duplicate keys.

I am however still interested in understanding how a map of maps could be produced from a nested for loop. See second code example above, producing a list of maps.

EDIT: a full answer was given on the github issue linked above.

"This is (obviously) a useless structure, but I wanted to illustrate that it is possible:

locals {
  association-list = {
    for policy, users in var.iam-policy-users-map:
      policy => {      // can't have the nested for expression before the key!
        for u in users:
           policy => u...
      }
  }
}

Outputs:

association-list = {
  "policy1" = {
    "policy1" = [
      "user1",
    ]
  }
  "policy2" = {
    "policy2" = [
      "user1",
      "user2",
    ]
  }
}

"