Given this simple groovy snippet:
and looking at the AST browser, you can see that in the constructor call for Structure does not have the proper generics types. The nested <Integer> is dropped, even though the outer <java.util.List> exists.
I believe that the problem comes from AntlrPluginParser.makeType(AST), which converts an Antlr AST node into a ClassNode, but does not recur down any existing type parameters.
I am using Groovy 1.7.3, so this may be fixed in 1.7.5, but I have not seen any issue for this raised in jira, so I am guessing it is not.