Transform AWS::LanguageExtensions failed with: Fn::If layout is incorrect

[miguel-aws]

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Tell us about the bug

Trying to use Fn::If within Fn::ForEach to reference a condition but CloudFormation returns Transform AWS::LanguageExtensions failed with: Fn::If layout is incorrect

Docs state that Fn::If is supported within Fn::ForEach.

See Test cases section for scenarios and replication template.

Expected behavior

Template validates correctly, the conditions are recognized, and proceeds to create the resources.

Observed behavior

Stack events report Transform AWS::LanguageExtensions failed with: Fn::If layout is incorrect

Test cases

Referencing ${Identifier} variable with !Sub to form the condition name causes the error.

        !If
          - !Sub "${ServerType}CreateSecondaryVolume"
          - - DeviceName: xvdf
              Ebs:
                DeleteOnTermination: true
                Encrypted: true
                VolumeSize: 100
                VolumeType: gp3
                Iops: 3000
          - !Ref AWS::NoValue

If we hardcode the condition name (i.e. BackupCreateSecondaryVolume), the stack goes through with no problems,

        !If
          - BackupCreateSecondaryVolume
          - - DeviceName: xvdf
              Ebs:
                DeleteOnTermination: true
                Encrypted: true
                VolumeSize: 100
                VolumeType: gp3
                Iops: 3000
          - !Ref AWS::NoValue

Template

AWSTemplateFormatVersion: '2010-09-09'
Transform: 'AWS::LanguageExtensions'
Mappings:
  ServerTypeMapping:
    Backup:
      CreateSecondaryVolume: true
    SQL:
      CreateSecondaryVolume: false
Conditions:
  BackupCreateSecondaryVolume: !Equals [!FindInMap [ServerTypeMapping, Backup, CreateSecondaryVolume], true]
  SQLCreateSecondaryVolume: !Equals [!FindInMap [ServerTypeMapping, SQL, CreateSecondaryVolume], true]
Resources:
  'Fn::ForEach::ServerTypes':
  - ServerType
  - [Backup, SQL]
  - LaunchTemplate${ServerType}:
      Type: AWS::EC2::LaunchTemplate
      Properties:
        LaunchTemplateData:
          BlockDeviceMappings:
            !If
              - !Sub "${ServerType}CreateSecondaryVolume"
              - - DeviceName: xvdf
                  Ebs:
                    DeleteOnTermination: true
                    Encrypted: true
                    VolumeSize: 100
                    VolumeType: gp3
                    Iops: 3000
              - !Ref AWS::NoValue
        LaunchTemplateName:
          Fn::Join: ["-", [!Sub "${AWS::StackName}", !Sub "${ServerType}"]]

[arthurboghossian]

@mparana CloudFormation currently doesn't support an intrinsic function for the 1st parameter (the condition name) of the Fn::If intrinsic function.

For example, calling CreateStack on the template below would result in CreateStack failing with Template error: Fn::If requires a list argument with the first element being a condition:

AWSTemplateFormatVersion: 2010-09-09
Conditions:
  Cond:
    !Equals ["0", "1"]
Resources:
  Topic:
    Type: AWS::SNS::Topic
    Properties:
      TopicName: !If
        - !Sub 'Cond'
        - TrueTopic
        - FalseTopic

So allowing intrinsic functions for the condition name would be a new feature, which would be part of the AWS::LanguageExtensions Transform.

If possible, can you please create a feature request GitHub issue within this repo?

[miguel-aws]

@arthurboghossian Thank you for the insights.

I created the feature request here: #137