Matching with std.concurrency receive

[Adam]

Okie, having some trouble trying to do matches against classes /
interfaces with templates using the std.concurrency's receive()
function.

Here's a simple use case I have:

import std.stdio;
import std.concurrency;
import std.variant;

immutable interface BasicType {
}

immutable class SubType : BasicType {
}

immutable class A(T : BasicType) {
}

void main() {
    Tid tid = spawn(&thread);
    immutable A!(SubType) instance = new immutable(A!(SubType))();
    send(tid, instance);
    while (true) {}
}

void thread() {
    receive(
            (A!(BasicType) message)
            {
                writeln("Received");
            },
            (Variant ant)
            {
                writeln(ant.type());
            }
           );
}

Running this outputs the Variant ant's type, so we see that it
"falls through" to that pattern. However, if I switch the match from
BasicType to SubType... it just hangs on me.

What horribly stupid thing am I doing here?