233.md

<details open><summary>Info</summary><p>

* **Did you know that C++20 made `typename` more optional?**

  * http://wg21.link/p0634

</p></details><details open><summary>Example</summary><p>

```cpp
template<class T> /*typename*/T::type return_type();   // okay
template<class T> void void_parameter(/*typename*/T::type); // error: variable or field 'parameter' declared void
template<class T> auto auto_parameter(/*typename*/T::type); // okay

template<class T>
struct traits {
  using type = /*typename*/ T::type; // okay
};
```

> https://godbolt.org/z/oMMzfjeKv

</p></details><details open><summary>Puzzle</summary><p>

* **Can you add/remove required/unnecessary `typename's`**

```cpp
template<class T>
struct traits {
  using type = typename T::type;
};

template<class T>
struct s : typename T::type {};

template<class T> typename T::type f1();
template<class T> auto f2() -> typename T::type;
auto f3(auto t) -> typename decltype(t)::type;

template<class T> auto f4(typename T::type);
template<class T> bool f5(typename T::type);
template<class T> void f6(T::type);
template<class T> auto f7(T::type) -> void;
auto f8(auto t) -> typename decltype(t)::type;
```

> https://godbolt.org/z/EEeTh5TWP

</p></details><details><summary>Solutions</summary><p>

```cpp
template<class T>
struct traits {
  using type = T::type;
};

template<class T>
struct s : T::type {};

template<class T> T::type f1();
template<class T> auto f2() -> T::type;
auto f3(auto t) -> decltype(t)::type;

template<class T> auto f4(T::type);
template<class T> bool f5(T::type);
template<class T> void f6(typename T::type);
template<class T> auto f7(typename T::type) -> void;
auto f8(auto t) -> decltype(t)::type;
```

> https://godbolt.org/z/nMsqeaev8