Template constraint on alias template parameter.

[jmh530 via Digitalmars-d-learn]

The code below compiles, but I want to put an additional 
constraint on the `test` function is only called with a Foo 
struct.

I tried things like is(T == Foo) and is(T : Foo), but those don't 
work. However, something like is(T!int : Foo!int) works, but 
is(T!U == Foo!U, U) doesn't. Any idea why is(T!U == Foo!U, U) 
doesn't work?

struct Foo(T)
{
    T x;
}

void test(alias T)()
    if (__traits(isTemplate, T))
{
    import std.stdio: writeln;
    writeln("there");
}

void main()
{
    test!Foo();
}