With current DMD 2 (0219a5f), the following snippet does not compile:

---
template isBar(T) {
    static if (is(T _ : Bar!(v), string v)) {
        enum isBar = true;
    } else {
        enum isBar = false;
    }
}
struct Bar(string v) {}
pragma(msg, isBar!(Bar!("bar")));
---

The error message is: »Error: undefined identifier string«.

However, if »string« is replaced with »immutable(char)[]« in the is expression, it gives the intended result:

---
template isBar(T) {
    static if (is(T _ : Bar!(v), immutable(char)[] v)) {
        enum isBar = true;
    } else {
        enum isBar = false;
    }
}
struct Bar(string v) {}
pragma(msg, isBar!(Bar!("bar"))); // prints true
---