Template variables inside filters are not parsed correctly

[Thejaswi Puthraya]

Whenever a variable is used in a filter's (have checked for slice only...might be applicable for other filters also) arguments it is wrongly parsed and we get a TemplateSyntaxError which says "slice requires 1 arguments, 0 provided".
From the traceback here is what I received on the output:
The token being

u'some_item.var2|slice:":{{ forloop.counter0'

Notice the missing "}}"

Here is the test code:

{% for some_item in many_items %}
  {{ some_item.var1 }} {{ some_item.var2|slice:":{{ forloop.counter0 }}" }}
{% endfor %}

[Malcolm Tredinnick]

Template variables cannot be used inside other template variables like this. The parsing becomes too complex and it's almost always possible to do what you want in some other way. We aim to keep things simple at that level and variable indirection like this is out of scope.