Why do STL-Datastructures need fully defined types?

The short answer is: because the standard says so. More generally, depending on the implementation, and what each type and function does, instantiating a template may require a complete definition. The authors of the standard either didn't want to, or didn't have time to analyse and specify in detail in what cases they didn't want to require complete definitions, and settled for a blanket statement.

Note too that when the standard was written, there was relatively little experience with STL, and one could be sure that there wasn't some clever optimization which would require an instance of the argument type within a class; rather than risk banning such an optimization, it seemed safer to require a complete type.

At least one problem your compiler faces with incomplete types is that it might be unable to fully determine the size of your struct Test. And thus, how could some container internally instantiate a Test?

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