Fn::Join no longer works with nested Fn:: methods

[elexisvenator]

Problem file: fn.ts
Problem method: FnJoin constructor
Last working version: 5bb7c93 (0.13)
Broke in version: 33c32a8 (current, 0.14.1)

Problem
FN::Join no longer work when the listOfValues parameter contains another function, such as Fn::GetAtt.
For example, this code attempts to output the array of IP addresses for an AD Resource:

import { cloudformation as DirectoryService } from '@aws-cdk/aws-directoryservice';
import { FnJoin } from '@aws-cdk/cdk';

const adResource = new DirectoryService.MicrosoftADResource(this, `MicrosoftADDirectoryService`, {
  vpcSettings: {
    vpcId: vpcId
    // 2 subnets = 2 ip addresses
    subnetIds: [
      subnetA,
      subnetB,
    ],
  },
  // other params here
});

// create an output, works in 0.13.x, not in 0.14.1
const ipAddressOutput = new Output(this, 'MicrosoftAdDnsIpAddresses', {
  value: new FnJoin(',', adResource.microsoftAdDnsIpAddresses.resolve() as string[])
}).makeImportValue().toString()

// workaround
const ipAddressOutput2 = new Output(this, 'MicrosoftAdDnsIpAddresses', {
  value: {
    'Fn::Join': [
      ',',
      adResource.microsoftAdDnsIpAddresses.resolve()
    ]
  },
}).makeImportValue().toString()

Cause(?)
It looks like the latest changes to FN::Join are designed to unpack nested joins, but I don't see how this would work if the join contained a non-join function.
There was a previous commit 5996442 that did something similar, but was rolled back for unrelated reasons. That commit seems to be doing a lot more to make sure there were only nested joins before unpacking them. Maybe that logic can be brought in?

Until then, I am handwriting Fn::Join json objects as a workaround.

[elexisvenator]

For L2 instances, there is slightly different voodoo you have to do for a workaround. The cast as unknown as string makes me sad :(

return new Ssm.ParameterResource(this, 'MicrosoftAdDnsIpAddressesParam', {
      name: this.domainSsmParameterPrefix + 'directory-dns',
      type: 'StringList',
      value: {
        'Fn::Join': [
          this.delimeter,
          microsoftAdDnsIpAddresses.resolve()
        ]
      } as unknown as string,

[rix0rrr]

Thanks for reporting, having a look

[rix0rrr]

I think the issue is in this line:

  value: new FnJoin(',', adResource.microsoftAdDnsIpAddresses.resolve() as string[])

The fact that you were able to bypass the typechecker here and make that work is actually an accident.

Related issues: #674, #744

[rix0rrr]

Your current best workaround will be this:

new Fn('Fn::Join', [', ', adResource.microsoftAdDnsIpAddresses])

Until we sort that issue.

[rix0rrr]

I'm going to close this, as your issue is a more specific case of another issue: lack of support for array tokens, and there is a workable workaround for it.

Feel free to reopen if you disagree.