Input walk improperly triggering errors on some computed resource attribute references

[phinze]

This is a generalized problem during the "input walk" where Terraform searches for variables needing user input, but it's expressed by the following scenario:

Given:

# main.tf
resource "aws_instance" "foo" { }
resource "aws_instance" "bar" { }

module "child" {
  source = "./child"
  in = "one,${aws_instance.foo.id},${aws_instance.bar.id}"
}

# ./child/main.tf
variable "in" {}

resource "aws_instance" "mod" {
  value = "${var.in}"
}

Where aws_instance.foo and aws_instance.mod exist, but aws_instance.bar does not, a terraform plan will result in the following error:

1 error(s) occurred:

                * Resource 'aws_instance.bar' does not have attribute 'id' for variable 'aws_instance.bar.id'

We just need to add walkInput to the list of operations that can tolerate a computed resource attribute.

[ghost]

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