Issue found by Ali Çehreli:
http://forum.dlang.org/thread/kh3dkf$22eb$1@digitalmars.com

DMD 2.063alpha accepts this code, ignoring the immutable():


struct Foo {
    int[] arr;
    this(int[] arr_) {
        this.arr = arr_;
    }
}
void main() {
    auto arr = [1];
    auto f = immutable(Foo)(arr);
    pragma(msg, typeof(f)); // Prints: Foo
    f.arr[0]++;
}