Keys for substitution in Template file function must begin with letter, not number

[andrew-lee-1089]

Terraform Version

0.12.18

Terraform Configuration Files

With template "metadata_template.json":

{
  "node_local": ${0_VALUE_FROM_NODE_NAMES},
  "node_remote": ${1_VALUE_FROM_NODE_NAMES},
}

and terraform config:

  "resource": {
    "local_file": {
      "user_data_a_json": {
        "content": "${templatefile(\"metadata_template.json\",  {\"0_VALUE_FROM_NODE_NAMES\": \"${jsonencode(var.node_names[0])}\", \"1_VALUE_FROM_NODE_NAMES\": \"${jsonencode(var.node_names[1])}\",
"})}",
        "filename": "${var.node_names[0]}_userdata.json"
      },

I get a

Error: Error in function call

  on main.tf.json line 59, in resource.local_file.user_data_a_json:
  59:         "content": "${templatefile(.....

Call to function "templatefile" failed:
metadata_template.json:14,20-44: Extra
characters after interpolation expression; Expected a closing brace to end the
interpolation expression, but found extra characters..

Error, however if I rename "0_VALUE_FROM_NODE_NAMES" to "FIRST_VALUE_FROM_NODE_NAMES" or "THE_0TH_VALUE_FROM_NODE_NAMES", this works fine, so clearly having numbers at beginning of strings in keys (in a map) is not allowed.

I'm okay with this not being fixed, it's a perfectly acceptable restriction, with a trivial workaround, so long as it is documented in https://www.terraform.io/docs/configuration/functions/templatefile.html or similar. I found no documentation warning on this.

This is very low priority.

Additional Context

I'm using JSON instead of HCL as this is a machine generated file

[andrew-lee-1089]

Great, thanks!

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