Terraform: Reference to undeclared input variable

Terraform reports Reference to undeclared input variable when an expression uses var.NAME and the current module has no variable "NAME" block. Every module, including each child module, only sees the variables it declares itself; values in terraform.tfvars do not declare anything.

Reproduce it

cat > main.tf <<'EOF'
resource "terraform_data" "app" {
  input = var.region
}
EOF
terraform init -input=false -no-color >/dev/null
terraform validate -no-color

Error: Reference to undeclared input variable

  on main.tf line 2, in resource "terraform_data" "app":
   2:   input = var.region

An input variable with the name "region" has not been declared. This variable
can be declared with a variable "region" {} block.

Fix: declare the variable

variable "region" {
  type        = string
  description = "Region to deploy into"
  default     = "eu-west-1"
}

Fix: pass it to a child module

A variable from the root module is not visible inside ./modules/network. Declare it there and pass it explicitly:

variable "region" {
  type = string
}

module "network" {
  source = "./modules/network"
  region = var.region
}