(cli): cdk diff pessimizes replacement if expression changes

[asnaseer-resilient]

Describe the bug

I am trying to use cloudformation_include to import a serverless framework deployed RDS stack into the CDK v2.25.0. I followed the instructions in this article https://snow-dev.com/posts/migrate-from-serverless-to-cdk.html

However, when I do a cdk diff on this I seem to get an indication that the DB will be replaced, i.e. I get this within the output:

Resources
[~] AWS::RDS::DBCluster RdsTemplate/InsightsDbCluster InsightsDbCluster replace
 └─ [~] DatabaseName (requires replacement)
     └─ [~] .Fn::Join:
         └─ @@ -1,9 +1,11 @@
            [ ] [
            [ ]   "_",
            [-]   {
            [-]     "Fn::Split": [
            [-]       "-",
            [-]       "sn-control-insights-rds-dev-insights"
            [-]     ]
            [-]   }
            [+]   [
            [+]     "sn",
            [+]     "control",
            [+]     "insights",
            [+]     "rds",
            [+]     "dev",
            [+]     "insights"
            [+]   ]
            [ ] ]
[~] AWS::RDS::DBInstance RdsTemplate/InsightsDbInstance1 InsightsDbInstance1 replace
 └─ [~] DBClusterIdentifier (requires replacement)
     └─ [~] .Ref:
         ├─ [-] InsightsDbCluster
         └─ [+] InsightsDbCluster (replaced)
[~] AWS::RDS::DBInstance RdsTemplate/InsightsDbInstance2 InsightsDbInstance2 replace
 └─ [~] DBClusterIdentifier (requires replacement)
     └─ [~] .Ref:
         ├─ [-] InsightsDbCluster
         └─ [+] InsightsDbCluster (replaced)

The template JSON file that I supplied defines the DatabaseName as:

        "DatabaseName": {
          "Fn::Join": [
            "_",
            {
              "Fn::Split": [
                "-",
                "sn-control-insights-rds-dev-insights"
              ]
            }
          ]
        },

It looks like the CDK synthesises this as:

      DatabaseName:
        Fn::Join:
          - _
          - - sn
            - control
            - insights
            - rds
            - dev
            - insights

which (I think) then causes it to incorrectly believe that the RDS will need to be redeployed.

If I run cdk deploy it correctly preserves the RDS. So it looks like it is only cdk diff that is reporting this incorrectly.

Expected Behavior

I expected cdk diff to not report that the RDS will be replaced.

Current Behavior

cdk diff incorrectly reports that the RDS will be replaced (as described above). However, cdk deploy behaves correctly in that it preserves the existing RDS and all the data within it.

Reproduction Steps

All details are provided above.

Possible Solution

No response

Additional Information/Context

No response

CDK CLI Version

2.25.0

Framework Version

No response

Node.js Version

v14.17.5

OS

macOS Monterey Version 12.4

Language

Typescript

Language Version

TypeScript v4.6.4

Other information

Originally reported in cdk slack channel - https://cdk-dev.slack.com/archives/C018XT6REKT/p1657784736307229

Adam Ruka said this in this slack channel:
I've seen this with Fn::Select before, and we fixed it in #19680, but looks like we need to do the same for Fn::Split!

[skinny85]

This is very similar to the problem we had with CDK short-circuiting Fn::Select in #19680, but this time with Fn::Split. The solution will also probably be similar.