pure vs immutable

[Jack Applegame via Digitalmars-d-learn]

I believe that object constructed with pure constructor should be 
implicitly convertible to immutable. It is, but only for default 
constructor.

class Foo {
     string s;
     this() pure {
         s = "fpp";
     }
     this(string p) pure { s = p; }
}

void main() {
     auto foo1 = new immutable Foo();      // compiles
     auto foo2 = new immutable Foo("bar"); // fails
}