AWS CloudFormation function call fails: Fn::ImportValue must not depend on any resources, imported values, or Fn::GetAZs

Question

I have a cloud formation template (mainVPC) that creates few Subnets in a VPC and exports the subnets with names "PrivateSubnetA", "PrivateSubnetB" ... I have a different cloud formation template that creates DBSubnetGroup. I want to use "PrivateSubnetA", "PrivateSubnetB" as default values if user does not provide data. CloundFormation does not support imported values in parameters. So I put some default value (XXXX) and had a condition section to see if the user has provided some input

Conditions:
  userNotProvidedSubnetA: !Equals 
    - !Ref PrivateSubnetA
    - XXXX
  userNotProvidedSubnetB: !Equals 
    - !Ref PrivateSubnetB
    - XXXX

This helps me in figuring out if the user has provided data. Now I want to use default values, if the user has not provided values, else use user-provided values. below is code for that

 DBSubnetGroup:
    Type: 'AWS::RDS::DBSubnetGroup'
    Properties:
      DBSubnetGroupDescription: RDS Aurora Cluster Subnet Group
      SubnetIds:
        - !If 
          - userNotProvidedSubnetA
          - Fn::ImportValue:
                !Sub  '${fmMainVpc}-PrivateSubnetA'
          - !Ref PrivateSubnetA
        - !If 
          - userNotProvidedSubnetB
          - Fn::ImportValue:
                !Sub '${fmMainVpc}-PrivateSubnetB'
          - !Ref PrivateSubnetB

This fails with the error "Template error: the attribute in Fn::ImportValue must not depend on any resources, imported values, or Fn::GetAZs". ImportValue is not used anywhere else in the template.

Is there a way for using exported values as default values ( the default values cannot be hardcoded, they come as exported values from a run of another stack), while providing an option for the users to provide their own values (to create resources).

Thanks.

Answer 1

This can also be caused by having a reference inside Fn::ImportValue to a parameter be misnamed. For example, if I have the following parameter NetworkStackName defined and I mis-reference it in the Fn::ImportValue statement (as NetworkName), I will get this error. I would need to change the NetworkName to match the value in Parameters, NetworkStackName to fix the error.

Parameters:
  NetworkStackName:
      Type: String
      Default: happy-network-topology
Resources:
  MySQLDatabase:
    Type: AWS::RDS::DBInstance
    Properties:
      Engine: MySQL
      DBSubnetGroupName:
        Fn::ImportValue:
          !Sub "${NetworkName}-DBSubnetGroup"