'index' property in objects shouldn't be implicitly overwritten in lambda functions

[LUATEC]

Bicep version Bicep CLI version 0.29.47

Describe the bug

• As a user who defines an object with a property called 'index'
• I need to know whether the 'index' property will be overwritten by a dynamic property that shadows this name
• And I need documentation to be updated with relevant reserved words
• So that I know why code does not behave as expected with user-defined values when the 'index' property provided is implicitly overwritten in lambda functions and can remedy the issue
• And I can avoid naming parameters with such words that have a special function in Bicep

When I define a new object with a parameter called 'index' and assign a value to this parameter, when I use this in a lambda function such as 'map', I expect: a) a warning to appear that inform me that the parameter 'index' shadows a reserved word used in Bicep lambda functions b) the 'index' parameter is not overwritten unless I explicitly define a parameter called 'index' and pass it into the lambda expression

Instead, the value of the object's 'index' parameter is currently implicitly overwritten to another value.

To Reproduce Steps to reproduce the behavior:

1. Create a new object with a parameter called 'index' with arbitrary user-defined values.
2. Use this object in a lambda function such as 'map' without defining a parameter called 'index' in the lambda expression
3. Verify the value of 'index' is overwritten when accessed by <object-name>.index or <object-name>['index']

Additional context This bug potentially relates to this issue.

[anthony-c-martin]

@LUATEC would you mind sharing a Bicep code sample to illustrate the problem?

[LUATEC]

@anthony-c-martin - sure!

Below's the gist of what I had. IIRC, it would try to associate different Managed Environments with the same Subnet ID. When the word index is changed out for idx, it works as expected.

var props1 = [
  {index: 0, name: 'foo', …}
  {index: 1, name: 'bar', …}
]

param props2 = map( props1, prop =>
  union(prop, {'0': {baz: 1, …}, '1': {baz: 2}, …}['${prop.index}'])
)

resource subnets 'Microsoft.Network/virtualNetworks/subnets@2023-11-01' existing = [
  for prop in props2: {
    name: props.name
    …
  }
]

var nameSubnetIdKVPs = [
  for prop in props2: {
    '${prop.name}': subnets[props.index].id
  }
]

var nameSubnetIdDict = reduce(nameSubnetIdKVPs, {}, (cur, next) => union(cur, next))

resource managedEnvironments 'Microsoft.App/managedEnvironments@2024-03-01' = [
  for prop in props2: {
    name: prop.name
    …
    properties: {
      vnetConfiguration: {
        infrastructureSubnetId: nameSubnetIdDict[prop.name]
        …
      }
    }
  }
]