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

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"

CodePudding user response：

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.