Error: cannot implicitly convert expression this.aa of type inout(string[string]) to string[string]

[Robert-D via Digitalmars-d-learn]

struct S {
string[string] aa;

    S dup() inout pure {
        return S(aa);
    }
}
void main() {
        auto s = S(["": ""]);
    s.dup();
}

Result:
 Error: cannot implicitly convert expression this.aa of type 
inout(string[string]) to string[string]


I need help with the above program.