override_data / override_resource: `values` naming a non-computed argument is rejected (Terraform accepts it)

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OpenTofu Version

OpenTofu v1.12.6
OpenTofu v1.13.0-beta1

Both affected. Terraform 1.8.3 and 1.16.2 accept the same configuration.

OpenTofu Configuration Files

main.tf:

terraform {
  required_providers {
    local = {
      source  = "hashicorp/local"
      version = "2.5.2"
    }
  }
}

data "local_file" "example" {
  filename = "does-not-exist.txt"
}

output "content" {
  value = data.local_file.example.content
}

main.tftest.hcl:

override_data {
  target = data.local_file.example
  values = {
    filename = "overridden.txt"
    content  = "overridden content"
  }
}

run "check" {
  command = plan

  assert {
    condition     = output.content == "overridden content"
    error_message = "expected the overridden content"
  }
}

Debug Output

Error: Invalid mock/override field `filename`

Error: Non-computed field `filename` is not allowed to be overridden

Expected Behavior

The override_data block replaces the data source entirely, so supplying a value for an argument alongside the computed attributes is accepted and the run passes. Terraform reports Success! 1 passed, 0 failed. on both 1.8.3 and 1.16.2.

Actual Behavior

OpenTofu rejects the block because filename is an argument rather than a computed attribute, and the whole test file fails.

Steps to Reproduce

1. tofu init
2. tofu test

Additional Context

I am not sure whether this is a bug or a deliberate design choice — refusing to override something the configuration already controls is defensible. Raising it because it is a behavioural difference from Terraform that breaks otherwise-portable test files, and because the diagnostic reads as a hard error rather than a documented restriction.

Our real case is a data source whose name argument is echoed back by the provider, so the override lists it next to id to describe the object as a whole. Dropping the argument from values is a viable workaround if the current behaviour is intended — in which case documenting the restriction (and ideally naming the offending field as a warning rather than failing the file) would be enough.

override_data type-checking was tightened in #1903 / #2220 (warning to error on type mismatch); this is a different rule — field kind rather than value type. I could not find an existing issue for it.

References

• #2008 — earlier override_data problems, closed
• #1903 / #2220 — warning-to-error change for incorrect override value types

Workaround

Omit the argument from values and let it resolve from the configuration as normal:

override_data {
  target = data.local_file.example
  values = {
    content = "overridden content"
  }
}

Verified on OpenTofu 1.12.6 and 1.13.0-beta1, and still passing on Terraform 1.8.3 and 1.16.2. data.local_file.example.filename still evaluates to the value in the configuration, so nothing is lost as long as the argument is one the test already controls. That is a good sign the current restriction is at least defensible — but the diagnostic gives no hint that dropping the field is the intended shape.