Parameter null value is not allowed

[Gijsreyn]

Prerequisites

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• Make sure you are able to repro it on the latest version
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Summary

While developing the coalesce function and writing the documentation, I stumbled on the following error from example 3:

Parser: Parameter 'customValue' not found in context.

It looks like parameter default values set to null are not allowed.

Steps to reproduce

Run the following example from the branch comparison-function-coalesce:

$in = @'
$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
parameters:
  customValue:
    type: string
    defaultValue: null
  timeout:
    type: int
    defaultValue: 0
resources:
- name: Configuration with fallbacks
  type: Microsoft.DSC.Debug/Echo
  properties:
    output:
      configValue: "[coalesce(parameters('customValue'), 'default-config')]"
      timeout:     "[coalesce(parameters('timeout'), 30)]"
'@
dsc config get --input $in

Expected behavior

The example would have returned null on `configValue`

Actual behavior

The error: Parser: Parameter 'customValue' not found in context occurs

Error details

Environment data

Name                           Value
----                           -----
PSVersion                      7.5.2
PSEdition                      Core
GitCommitId                    7.5.2
OS                             Microsoft Windows 10.0.26100
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

Version

dsc 3.2.0-preview.3

Visuals

No response

[Gijsreyn]

I think the function parser also doesn't check for null values being parsed:

match arg.kind() {
    "string" => { ... },
    "number" => { ... },
    "boolean" => { ... },
    "expression" => { ... },
    _ => {
        return Err(DscError::Parser(t!("parser.functions.unknownArgType", kind = arg.kind()).to_string()));
    }
}

The following test code failed:

#[test]
fn all_null_returns_null() {
    let mut parser = Statement::new().unwrap();
    let result = parser.parse_and_execute("[coalesce(null, null, null)]", &Context::new()).unwrap();
    assert_eq!(result, serde_json::Value::Null);
}

[SteveL-MSFT]

I checked with the ARM/Bicep team and nullable is new for language 2.0 while DSC currently only supports language 1.0:

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "languageVersion": "2.0",
  "contentVersion": "1.0.0.0",
  "metadata": {
    "_generator": {
      "name": "bicep",
      "version": "0.36.177.2456",
      "templateHash": "14631105793920681229"
    }
  },
  "parameters": {
    "foo": {
      "type": "string",
      "nullable": true
    }
  },
  "resources": {}
}

We will eventually want to support this, but I think adding 2.0 support is a big work item we'll have to defer for now