Interpolation fails for computed lists on mailgun_domain

[hashibot]

This issue was originally opened by @steve-jansen as hashicorp/terraform#5367. It was migrated here as part of the provider split. The original body of the issue is below.

---

Possibly related to hashicorp/terraform#2157.

The maingun_domain receiving_records and sending_records TypeLists cannot be used with interpolation functions like length().

Steps to reproduce:

variable "mailgun_api_key" {
    type = "string"
}

provider "aws" {
    region = "us-east-1"
}

provider "mailgun" {
    api_key = "${var.mailgun_api_key}"
}

resource "mailgun_domain" "main" {
    name = "test.example.com"
    spam_action = "disabled"
    smtp_password = "foobar"
}

resource "aws_route53_zone" "main" {
   name = "example.com"
}

resource "aws_route53_record" "main" {
     zone_id = "${aws_route53_zone.main.zone_id}"
     name = "${mailgun_domain.main.name}"
     type = "${element(mailgun_domain.main.receiving_records.*.record_type, count.index)}"
     ttl = "300"
     count = "${length(mailgun_domain.main.receiving_records)}"
     records = [
         "${element(mailgun_domain.main.receiving_records.*.priority, count.index)} ${element(mailgun_domain.main.receiving_records.*.value, count.index)}"
    ]
}

Results in a plan error:

$ terraform -version
Terraform v0.6.12
$ terraform plan -var mailgun_api_key=${MAILGUN_API_KEY}
There are warnings and/or errors related to your configuration. Please
fix these before continuing.

Errors:

  * 1 error(s) occurred:

* module root: 1 error(s) occurred:

* aws_route53_record.main: resource count can't reference resource variable: mailgun_domain.main.receiving_records

The Hashicorp blog article "Deploying Discourse with Terraform" by @pearkes notably does not use length(), but, instead assumes mailgun will reply with 2 receiving domains and 3 sending domains. Admittedly, this approach could be due to the age of the article (Terraform 0.2 days), or it could be due to this limitation with interpolation against a list of maps.

• Test against latest release
• Search for possible duplicate reports
• Include steps to reproduce