nullable parameters in user defined functions

[cheng93]

Bicep version
Bicep CLI version 0.45.15 (6a4a640)

Describe the bug
Can't pass null as a parameter into a user defined function

InvalidTemplate - Deployment template validation failed: 'The template resource 'rg' at line '1' and column '1851' is not valid: The provided value for the 'environment' parameter of the user-defined function '__bicep.rgName' is not valid. The value '[null()]' is not part of the allowed value(s): 'dev,hub,live,prelive,uat'.. Please see https://aka.ms/arm-functions for usage details.'.

To Reproduce

// main.bicep
import { locationTypes, locationName, rgName } from '../exports.bicep'

param location locationTypes

var workload = 'test'

resource rg 'Microsoft.Resources/resourceGroups@2025-04-01' = {
 name: rgName(null, location, workload)
 location: locationName(location)
}

// exports.bicep
@export()
type environmentType = 'dev' | 'uat' | 'prelive' | 'live' | 'hub'

@export()
type locationTypes = 'frc' | 'ne' | 'uks' | 'ukw'

var locationMap = {
 frc: 'francecentral'
 ne: 'northeurope'
 uks: 'uksouth'
 ukw: 'ukwest'
}

@export()
func locationName(location locationTypes) string => locationMap[location]

@export()
func namePart(part string?) string => empty(part) ? '' : '-${part}'

@export()
func rgName(environment environmentType?, location locationTypes, workload string?) string =>
 'rg-emp${namePart(environment)}-${location}${namePart(workload)}'

Additional context
Generated ARM

{
 "$schema": "https://schema.management.azure.com/schemas/2018-05-01/subscriptionDeploymentTemplate.json#",
 "languageVersion": "2.0",
 "contentVersion": "1.0.0.0",
 "metadata": {
 "_generator": {
 "name": "bicep",
 "version": "0.45.15.27210",
 "templateHash": "5649992331914242084"
 }
 },
 "definitions": {
 "_1.environmentType": {
 "type": "string",
 "allowedValues": [
 "dev",
 "hub",
 "live",
 "prelive",
 "uat"
 ],
 "metadata": {
 "__bicep_imported_from!": {
 "sourceTemplate": "../exports.bicep"
 }
 }
 },
 "locationTypes": {
 "type": "string",
 "allowedValues": [
 "frc",
 "ne",
 "uks",
 "ukw"
 ],
 "metadata": {
 "__bicep_imported_from!": {
 "sourceTemplate": "../exports.bicep"
 }
 }
 }
 },
 "functions": [
 {
 "namespace": "__bicep",
 "members": {
 "locationName": {
 "parameters": [
 {
 "$ref": "#/definitions/locationTypes",
 "name": "location"
 }
 ],
 "output": {
 "type": "string",
 "value": "[variables('_1.locationMap')[parameters('location')]]"
 },
 "metadata": {
 "__bicep_imported_from!": {
 "sourceTemplate": "../exports.bicep"
 }
 }
 },
 "rgName": {
 "parameters": [
 {
 "$ref": "#/definitions/_1.environmentType",
 "nullable": true,
 "name": "environment"
 },
 {
 "$ref": "#/definitions/locationTypes",
 "name": "location"
 },
 {
 "type": "string",
 "nullable": true,
 "name": "workload"
 }
 ],
 "output": {
 "type": "string",
 "value": "[format('rg-emp{0}-{1}{2}', _1.namePart(parameters('environment')), parameters('location'), _1.namePart(parameters('workload')))]"
 },
 "metadata": {
 "__bicep_imported_from!": {
 "sourceTemplate": "../exports.bicep"
 }
 }
 }
 }
 },
 {
 "namespace": "_1",
 "members": {
 "namePart": {
 "parameters": [
 {
 "type": "string",
 "nullable": true,
 "name": "part"
 }
 ],
 "output": {
 "type": "string",
 "value": "[if(empty(parameters('part')), '', format('-{0}', parameters('part')))]"
 },
 "metadata": {
 "__bicep_imported_from!": {
 "sourceTemplate": "../exports.bicep"
 }
 }
 }
 }
 }
 ],
 "parameters": {
 "location": {
 "$ref": "#/definitions/locationTypes"
 }
 },
 "variables": {
 "workload": "test",
 "_1.locationMap": {
 "frc": "francecentral",
 "ne": "northeurope",
 "uks": "uksouth",
 "ukw": "ukwest"
 }
 },
 "resources": {
 "rg": {
 "type": "Microsoft.Resources/resourceGroups",
 "apiVersion": "2025-04-01",
 "name": "[__bicep.rgName(null(), parameters('location'), variables('workload'))]",
 "location": "[__bicep.locationName(parameters('location'))]"
 }
 }
}

{
 "experimentalFeaturesEnabled": {
 "waitAndRetry": true
 },
 "extensions": {
 "microsoftGraphV1": "br:mcr.microsoft.com/bicep/extensions/microsoftgraph/v1.0:1.0.0"
 }
}

[tsmallig33]

A fix has been committed and should be included in the next backend service release.

[cheng93]

Tate Smalligan (@tsmallig33) Do you know whether the fix has been released?

[tsmallig33]

cheng93 The fix is still pending backend service release. Will update this issue once it has fully rolled out, likely in a few more weeks.