Terraform 0.13 - Modules, for_each and providers - terraform

Question

UPDATED
I am trying to provision multiple SQL databases in Azure using Terraform.
My child module has the following code that provisions a SQL database:
providers.tf
// default provider
provider "azurerm" {
alias = "main"
features {}
}
// The provider that can access the storage account to store diagnostics
provider "azurerm" {
alias = "storage_account"
features {}
}
sql_db.tf
resource "azurerm_mssql_database" "default" {
name = var.name
base_name = var.base_name
...
tags = var.tags
provider = azurerm.main
}
data.tf
data "azurerm_storage_account" "storage" {
name = var.storage_account_name
resource_group_name = var.storage_account_rg
provider = azurerm.storage_account
}
I am calling this module in my main.tf file as follows where I want to provision multiple SQL databases using a for_each:
module "sql_db" {
for_each = var.sql_db
source = "...../sql_db.git"
base_name = each.value.base_name
name = each.value.name
providers = {
azurerm.main = azurerm.main
azurerm.storage_account = azurerm.storage_account
}
}
provider "azurerm" {
features {}
version = "=2.20.0"
}
// default provider
provider "azurerm" {
alias = "main"
features {}
}
provider "azurerm" {
alias = "storage_account"
features {}
}
When I run plan, I get the following error:
Error: Module does not support for_each
on main.tf line 35, in module "sql_db":
35: for_each = var.sql_db
Module "sql_db" cannot be used with for_each because it contains a nested
provider configuration for "azurerm.main", at
.terraform\modules\sql_db\providers.tf:2,10-19.
This module can be made compatible with for_each by changing it to receive all
of its provider configurations from the calling module, by using the
"providers" argument in the calling module block.
Error: Module does not support for_each
on main.tf line 35, in module "sql_db":
35: for_each = var.sql_db
Module "sql_db" cannot be used with for_each because it contains a nested
provider configuration for "azurerm.storage_account", at
.terraform\modules\sql_db\providers.tf:8,10-19.
This module can be made compatible with for_each by changing it to receive all
of its provider configurations from the calling module, by using the
"providers" argument in the calling module block.

Answer 1

The simple answer is, it's not supported. From the Terraform documentation:
A module containing its own provider configurations is not compatible with the for_each, count, and depends_on arguments that were introduced in Terraform v0.13.
HashiCorp has been absolutely adamant that providers can never be declared dynamically, which is why they allow neither a for_each/count within a provider block, nor a for_each/count on a module that contains a provider block.