templatestring Function Crashes When Map Contains a Key Mapped to null

[aclarke822]

Terraform Version

Terraform v1.11.1
on windows_amd64

Terraform Configuration Files

locals {
  map = {
    test = null
  }
  template = "$${test}"

  test = templatestring(local.template, local.map)
}

output "test" {
  value = local.test
}

Debug Output

https://gist.github.com/aclarke822/db92ba7a7a8a95851a9911db75dc7191

Expected Behavior

Terraform should handle this scenario gracefully, without crashing and output a relevant error message.

Actual Behavior

Terraform crashes.

Steps to Reproduce

1. terraform apply

Additional Context

Also tested on 1.9.8 and 1.10.5 with the same result

References

No response

Generative AI / LLM assisted development?

No response

[liamcervante]

Hi @aclarke822, thanks for filing this. I've marked this as confirmed as I can replicate it.

Interestingly enough it does return an error if you the template isn't just a single reference: template = "this: $${test}".

╷
│ Error: Error in function call
│ 
│   on main.tf line 7, in locals:
│    7:   test = templatestring(local.template, local.map)
│     ├────────────────
│     │ while calling templatestring(template, vars)
│     │ local.map is object with 1 attribute "test"
│     │ local.template is "this: ${test}"
│ 
│ Call to function "templatestring" failed: <templatestring argument>:1,9-13: Invalid template interpolation value; The expression result is null. Cannot include a null
│ value in a string template..
╵

The implementation of this function is actually directly within HCL, so I've tagged this as an upstream issue and we'll have to fix this within HCL. I think that HCL is trying to be clever in the way it interprets this, and realises it doesn't need to do any string interpolation with the original template so it appears to incorrectly skip the necessary validation. We need to make it perform the same validation even when it isn't actually performing any real templating.