Fn::ImportValue does not like Fn::Sub in its short notation

user_404615 0
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Why is following format not working giving error "the attribute in Fn::ImportValue must be a string or a function that returns a string"?

    Source: !Join [":", [!ImportValue [!Sub "${ProductEnvironment}-${ProductName}-IngressGreengrassProxyFunctionArn"], !ImportValue [!Sub "${ProductEnvironment}-${ProductName}-IngressGreengrassProxyFunctionAlias"]]]

but it works if I rewrite it as:

    Source: !Join
      - ":"
      - - Fn::ImportValue: !Sub "${ProductEnvironment}-${ProductName}-IngressGreengrassProxyFunctionArn"
        - Fn::ImportValue: !Sub "${ProductEnvironment}-${ProductName}-IngressGreengrassProxyFunctionAlias"
amazon-cloudformation amazon-web-services
1 Answers

The docs explain:

You can't use the short form of !ImportValue when it contains a !Sub.

AWS does not provide details of its internal implementations, why they implemented ImportValue this way.

user_2488230
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