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Overview of the Issue

Using optional in variables of type object isn't working in Packer, but working in Terraform.

Reproduction Steps

# foo.pkr.hcl
variable "context" {
  type = object({
    bar_pro = optional(string, "bar")
  })
}

$> packer validate foo.pkr.hcl 
Error: Invalid type specification

  on foo.pkr.hcl line 3, in variable "context":
   3:     bar_pro = optional(string, "bar")

Optional attribute modifier is only for type constraints, not for exact types.

# main.tf has exactly same content as foo.pkr.hcl
# tf init etc ommited
$> terraform validate
Success! The configuration is valid.

Packer version

Packer v1.8.5
Terraform v1.3.6

Operating system and Environment details

Linux fedora 6.0.15-300.fc37.x86_64