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

[Robert-D]

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.