aws_subnet: ipv6_cidr_block fails on empty ""

[kennethgds]

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Terraform CLI and Terraform AWS Provider Version

Terraform v0.12.23
+ provider registry.terraform.io/hashicorp/aws v3.3.0

Affected Resource(s)

• aws_subnet

Terraform Configuration Files

resource "aws_subnet" "public" {
  count = length(var.public_cidrs)

  vpc_id            = aws_vpc.vpc.id
  availability_zone = element(data.aws_availability_zones.available.names, count.index)
  cidr_block        = var.public_cidrs[count.index]
  ipv6_cidr_block   = aws_vpc.vpc.assign_generated_ipv6_cidr_block ? cidrsubnet(aws_vpc.vpc.ipv6_cidr_block, 8, count.index) : ""

  map_public_ip_on_launch         = true
  assign_ipv6_address_on_creation = aws_vpc.vpc.assign_generated_ipv6_cidr_block

  tags = merge(
    map("Name", format("%s-public", aws_vpc.vpc.tags["Name"])),
    map("network", "public", "availability-zone", element(data.aws_availability_zones.available.names, count.index)),
    var.tags
  )
}

Expected Behavior

Terraform runs with no plan changes

Actual Behavior

Terraform errored out with

Error: "" is not a valid CIDR block: invalid CIDR address:

Basically this line

ipv6_cidr_block  = aws_vpc.vpc.assign_generated_ipv6_cidr_block ? cidrsubnet(aws_vpc.vpc.ipv6_cidr_block, 8, count.index) : ""

Steps to Reproduce

1. terraform plan

Important Factoids

Note: Terraform plan runs fine on 3.2.0 AWS provider version, Something broke on 3.3.0 version

[ewbankkit]

It looks like the change was introduced in #12303.
This is similar to #13003 and some others.
Relates: #13943.

[ewbankkit]

@kennethgds Thanks for raising this issue.
As v3.3.0 of the AWS Provider supports only Terraform 0.12+, you can replace

ipv6_cidr_block   = aws_vpc.vpc.assign_generated_ipv6_cidr_block ? cidrsubnet(aws_vpc.vpc.ipv6_cidr_block, 8, count.index) : ""

with

ipv6_cidr_block   = aws_vpc.vpc.assign_generated_ipv6_cidr_block ? cidrsubnet(aws_vpc.vpc.ipv6_cidr_block, 8, count.index) : null

[kennethgds]

Thanks for the quick reply on this @ewbankkit

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!