how can I ensure that a template instantiation is unique?

[Vlad Levenfeld via Digitalmars-d-learn]

I'm instantiating a couple of template structs that conflict with 
each other. I'd like them to be unique types, automatically. So I 
tried this:

template Foo (string unique_id = __FILE__~__LINE__.to!string)
{...}

but it didn't work. When I use this for templated functions, I 
get the file and line number where the function is instantiated. 
But in this case, I got the file and line number where the 
template was defined.

Is there anything I can do to make sure each instantiated type is 
unique?