terraform 0.12 panics when calling distinct with empty list

[alex-goncharov]

Terraform Version

$ terraform -v
Terraform v0.12.0

Terraform Configuration Files

locals {
  d = distinct([])
}

Debug Output

https://gist.github.com/alex-goncharov/d51af9a2b88a089cb32154abb031081a

Crash Output

https://gist.github.com/alex-goncharov/66f756fc609371862200e69a97b5f6f2

Expected Behavior

distinct returned empty list

Actual Behavior

terraform panics

Steps to Reproduce

create file with about hcl
terraform init && terraform plan

Additional Context

References

[apparentlymart]

Sorry for this crash @alex-goncharov, and thanks for reporting it.

This seems to just be a missing case not handled in the function implementation, here:

https://github.com/hashicorp/terraform/blob/f68673fbe4ae7f97741a60979a56afbff6f3b53d/lang/funcs/collection.go#L366

cty.ListVal relies on getting a non-empty list in order to know what element type the resulting list will have, so it panics if given an empty list. This should be fixable then by adding an extra guard condition just before:

if len(list) == 0 {
    return cty.ListValEmpty(retType.ElementType), nil
}

We'll get this fixed up! Thanks again for reporting it.

[antonbabenko]

As a workaround to this issue I write it like this - compact(distinct(concat(var.maybe_empty, [""])))

https://github.com/terraform-aws-modules/terraform-aws-security-group/blob/687465a8f04af85fae13fd10d8982b12590debb7/modules/http-80/main.tf#L15

[apparentlymart]

This should be fixed by #21538.

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