How to prevent direct public creation of a struct?

[Nick Sabalausky]

I want to do something like this:

------------------------------------------
module moduleFoo;

// Only allow certain values of "str"
template foo(string str)
{
    static assert(str == "a" || str == "b" || str == "c");
    immutable foo = Foo(str);
}

struct Foo
{
    // Only "a", "b", and "c" should be allowed,
    // checked at compile-time via "template foo".
    // Also, not modifyable.
    private string _str;
    @property string str()
    {
        return _str;
    }
}
------------------------------------------

I want struct Foo itself to be public, but I want to *force* all code 
outside moduleFoo to *create* Foo via the "foo" template. Copying should be 
allowed though. Ie:

------------------------------------------
auto a = foo!"a"; // ok
a.str = "b"; // Error: str is a read-only property
auto z = foo!"z"; // Error: fails static assert
auto a2 = a; // Copy it: ok

auto b = Foo("b"); // Error: I want to *force* the usage of "template foo"

Foo x; // Kinda ambivalent about this: I'd prefer if it were disallowed, but 
I don't think that's possible. If it's indeed impossible, I know I can just 
declare Foo.str as (string str = "a";) so that's not a big problem.
------------------------------------------

The *key* thing here that I'm not sure how to do is: How do I disallow 
this?:

auto b = Foo("b"); // Error: I want to *force* the usage of "template foo"