[C++] Elaborated type specifier

Reference:
https://en.cppreference.com/w/cpp/language/elaborated_type_specifier.html

https://vsdmars.blogspot.com/2014/02/c11-extended-friend-declaration.html

class T
{
public:
    class U;
private:
    int U;
};
 
int main()
{
    int T;
    T t; // error: the local variable T is found
    class T t; // OK: finds ::T, the local variable T is ignored
    T::U* u; // error: lookup of T::U finds the private data member
    class T::U* u; // OK: the data member is ignored
}

template<typename T>
struct Node
{
    struct Node* Next; // OK: lookup of Node finds the injected-class-name
    struct Data* Data; // OK: declares type Data at global scope
                       // and also declares the data member Data
    friend class ::List; // error: cannot introduce a qualified name
    enum Kind* kind; // error: cannot introduce an enum
};
 
Data* p; // OK: struct Data has been declared

template<typename T>
class Node
{
    friend class T; // error: type parameter cannot appear in an elaborated type specifier;
                    // note that similar declaration `friend T;` is OK.
};
 
class A {};
enum b { f, t };
 
int main()
{
    class A a; // OK: equivalent to 'A a;'
    enum b flag; // OK: equivalent to 'b flag;'
}

enum class E { a, b };
enum E x = E::a; // OK
enum class E y = E::b; // error: 'enum class' cannot introduce an elaborated type specifier
 
struct A {};
class A a; // OK

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