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Showing content from https://timsong-cpp.github.io/cppwp/n4140/class.name below:

[class.name]

A class definition introduces a new type. [ Example:

struct X { int a; };
struct Y { int a; };
X a1;
Y a2;
int a3;

declares three variables of three different types. This implies that

a1 = a2;                        a1 = a3;                        

are type mismatches, and that

int f(X);
int f(Y);

declare an overloaded (Clause [over]) function f() and not simply a single function f() twice. For the same reason,

struct S { int a; };
struct S { int a; };            

is ill-formed because it defines S twice.  — end example ]

A class declaration introduces the class name into the scope where it is declared and hides any class, variable, function, or other declaration of that name in an enclosing scope ([basic.scope]). If a class name is declared in a scope where a variable, function, or enumerator of the same name is also declared, then when both declarations are in scope, the class can be referred to only using an elaborated-type-specifier ([basic.lookup.elab]). [ Example:

struct stat {
  };

stat gstat;                                                     
int stat(struct stat*);         
void f() {
  struct stat* ps;                                                stat(ps);                     }

 — end example ] A declaration consisting solely of class-key identifier; is either a redeclaration of the name in the current scope or a forward declaration of the identifier as a class name. It introduces the class name into the current scope. [ Example:

struct s { int a; };

void g() {
  struct s;                                                       s* p;                           struct s { char* p; };          struct s;                     }

 — end example ] [ Note: Such declarations allow definition of classes that refer to each other. [ Example:

class Vector;

class Matrix {
    friend Vector operator*(const Matrix&, const Vector&);
};

class Vector {
    friend Vector operator*(const Matrix&, const Vector&);
};

Declaration of friends is described in [class.friend], operator functions in [over.oper].  — end example ]  — end note ]

[ Note: An elaborated-type-specifier ([dcl.type.elab]) can also be used as a type-specifier as part of a declaration. It differs from a class declaration in that if a class of the elaborated name is in scope the elaborated name will refer to it.  — end note ] [ Example:

struct s { int a; };

void g(int s) {
  struct s* p = new struct s;     p->a = s;                     }

 — end example ]

[ Note: The declaration of a class name takes effect immediately after the identifier is seen in the class definition or elaborated-type-specifier. For example,

class A * A;

first specifies A to be the name of a class and then redefines it as the name of a pointer to an object of that class. This means that the elaborated form class A must be used to refer to the class. Such artistry with names can be confusing and is best avoided.  — end note ]

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