AA specialization not works

[JDavidLS]

string[] na = ["yo","tu","el"];

string[string] aa;
aa["yo"] = "qwer";
aa["tu"] = "qwer";
...

string toString(T)(Tt) { ...}           //OK
string toString(T:T[])(Tt) { ... }     // WORKS

string toString(T:T[C], C)(Tt) { ... }     // NOT WORKS
string toString(T:V[C], V,C)(Tt) { ... }     // NOT WORKS
string toString(V,C,T:V[C])(Tt) { ... }     // NOT WORKS

As I can use the type of key of an associative Array in a function?