shared and idup

[Low Functioning via Digitalmars-d-learn]

shared int[] foo;
auto bar() {
        foo ~= 42;
        return foo.idup;
}
Error: cannot implicitly convert element type shared(int) to 
immutable in foo.idup

Is this not correct? If I instead dup'd an array of ints (or some 
other non-reference elements) and cast to immutable, would I be 
in danger of undefined behavior?