how to get template parameter of a template template parameter?

Assume std::vector didnt have a value_type. Is is possible to write a template to deduce the value_type? Or more general, given a T<X>, how can I deduce X?

A very naive..

template <template <typename X> T>
void test(T<X> t) { 
     X x;
}

will probably make anybody who knows a bit about templates laugh at my foolish attempt and when instantiated like this:

int main() {
    std::vector<int> x;
    test(x);
}

create the following errors:

error: expected ‘class’ before ‘T’
 template < template<typename X> T>
                                 ^
error: ‘X’ was not declared in this scope
 void test(T<X> u) {
             ^
error: template argument 1 is invalid
 void test(T<X> u) {
              ^
In function ‘void test(int)’:
error: ‘X’ was not declared in this scope
   X x;
   ^
error: expected ‘;’ before ‘x’
   X x;
     ^
In function ‘int main()’:
error: no matching function for call to ‘test(std::vector<int>&)’
   test(x);
         ^
note: candidate is:
note: template<template<class X> class T> void test(int)
 void test(T<X> u) {
      ^
note:   template argument deduction/substitution failed:
note:   cannot convert ‘x’ (type ‘std::vector<int>’) to type ‘int’

EDIT: the first one is easy to fix, but fixing it wont affect the others...

PS: I think I have a small misunderstanding, as std::vector<int> isnt a template, but a concrete type. However, still I would like to know if there is a way to get the int from a someTemplate<int> with some template magic.