Accessing template parameters from outside?

[Norbert Nemec]

As far as I can see, an IsExpression allows to check whether T is an
exact type, but is there a possibilty to check whether T is an instance
of a certain template with arbitrary parameter? Like:

	is(T:mytemplate!(???))

Even more: once I know that T is an instance of mytemplate, is there a
way to find out what the template parameters are?

Basically, what would be needed is some pattern matching mechanism like:

	if(is(T:mytemplate($N)))

where the IsExpression is true if the pattern fits, and at the same
time, the "wildcard" $N is assigned to whatever it matches.

The whole thing is inspired by the template mechanism of C++ which
allows the following:

--------------------
template<int N> class vector {
	double data[N];
}

template<typename T> class container {
	T content;
	void dump() {
		cout << "Container for unknown type\n";
	}
}

// simple specialization
template<> class container<double> {
	double content;
	void dump() {
		cout << "Container for double\n";
	}
}

// specialization for template as parameter:
template<int N> class container<vector<N> > {
	vector<N> content;
	void dump() {
		cout << "Container for vector of size " << N << "\n";
	}
}

container<int> icontainer;
icontainer.dump();
// prints: "Container for unknown type"

container<double > dcontainer;
dcontainer.dump();
// prints: "Container for double"

container<vector<3> > vcontainer;
vcontainer.dump();
// prints: "Container for vector of size 3"
---------------------

Meaning: inside the specialized template, I can, at compile time,
analyse the parameters.

The corresponding code in D would like something like:

---------------------
class vector(int N) {
	double[N] data;
};

class container(T) {
	T content;
	void dump() {
		static if(is(T:double)) {
			printf("Container for double\n");
		} else static if(is(T:vector ?????)
			printf("Container for vector %i\n",?????);
		} else {
			printf("Container for unknown type\n");
		}
	}
};
---------------------

How should I write the lines containing the "?????"? I know the example
above it stupid, but the core point itself came up when I tried several
solutions in for the units library yesterday. I found ugly workarounds,
but no real solution.