Error when `var.config.*.subnet_route_table_ids` derives from a value in the same state

[korenyoni]

Found a bug? Maybe our Slack Community can help.

Describe the Bug

When var.config.*.subnet_route_table_ids is in the same state and has not yet been applied, the following occurs:

module.tgw_attachment.data.aws_ec2_transit_gateway.this[0]: Read complete after 0s [id=tgw-9435ccad528479164]
╷
│ Error: Invalid count argument
│ 
│   on .terraform/modules/tgw_attachment/modules/subnet_route/main.tf line 19, in resource "aws_route" "count":
│   19:   count                  = var.route_keys_enabled ? 0 : length(local.route_config_list)
│ 
│ The "count" value depends on resource attributes that cannot be determined
│ until apply, so Terraform cannot predict how many instances will be
│ created. To work around this, use the -target argument to first apply only
│ the resources that the count depends on.
╵

This also happens when var.route_keys_enabled is true:

module.tgw_attachment.data.aws_ec2_transit_gateway.this[0]: Read complete after 0s [id=tgw-9435ccad528479164]
╷
│ Error: Invalid for_each argument
│ 
│   on .terraform/modules/tgw_attachment/modules/subnet_route/main.tf line 8, in resource "aws_route" "keys":
│    8:   for_each               = var.route_keys_enabled ? local.route_config_map : {}
│     ├────────────────
│     │ local.route_config_map will be known only after apply
│     │ var.route_keys_enabled is true
│ 
│ The "for_each" map includes keys derived from resource attributes that
│ cannot be determined until apply, and so Terraform cannot determine the
│ full set of keys that will identify the instances of this resource.
│ 
│ When working with unknown values in for_each, it's better to define the map
│ keys statically in your configuration and place apply-time results only in
│ the map values.
│ 
│ Alternatively, you could use the -target planning option to first apply
│ only the resources that the for_each value depends on, and then apply a
│ second time to fully converge.

Expected Behavior

When var.subnet_route_table_ids derives from a value in the same state, creating a TGW attachment should still be possible.

This is coming from operations in the subnet_route submodule that are too complex on unknown values (namely setproduct()).

https://github.com/cloudposse/terraform-aws-transit-gateway/blob/dc12987db8dbe740314e62acf51c804e69d19eb8/modules/subnet_route/main.tf#L3-L4

Steps to Reproduce

Steps to reproduce the behavior:

1. Write a module with the VPC and its subnets in the same state as this module.
2. Pass the subnet route table IDs from 1 to var.subnet_route_table_ids
3. Run terraform plan

Screenshots

If applicable, add screenshots or logs to help explain your problem.

Environment (please complete the following information):

Anything that will help us triage the bug will help. Here are some ideas:

• Module version v0.9.1
• Terraform v1.3.4

Additional Context

Add any other context about the problem here.

[zack-is-cool]

FWIW, this is how their test works https://github.com/cloudposse/terraform-aws-transit-gateway/blob/main/test/src/examples_complete_test.go#L32-L55

you need to deploy the VPCs first.