// N4606 Committee Draft, Standard for Programming Language C++
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf 
#define msg "p241.cpp(109)9.1 Class names [class.name]"

#include <iostream>


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

a1 = a2; // error: Y assigned to X
a1 = a3; // error: int assigned to X

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

struct S { int a; };
struct S { int a; }; // error, double definition

struct stat {
// ...
};
stat gstat; // use plain stat to
// define variable
int stat(struct stat*); // redeclare stat as function
void f() {
struct stat* ps; // struct prefix needed
// to name struct stat
stat(ps); // call stat()
}

struct s { int a; };
void g() {
struct s; // hide global struct s

// with a block-scope declaration
s* p; // refer to local struct s
struct s { char* p; }; // define local struct s
struct s; // redeclaration, has no effect
}

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

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

class A * A;

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

a1 = a2; // error: Y assigned to X
a1 = a3; // error: int assigned to X

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

struct S { int a; };
struct S { int a; }; // error, double definition

struct stat {
// ...
};
stat gstat; // use plain stat to
// define variable
int stat(struct stat*); // redeclare stat as function
void f() {
struct stat* ps; // struct prefix needed
// to name struct stat
stat(ps); // call stat()
}

struct s { int a; };
void g() {
struct s; // hide global struct s

// with a block-scope declaration
s* p; // refer to local struct s
struct s { char* p; }; // define local struct s
struct s; // redeclaration, has no effect
}

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

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

class A * A;

int main(){

  std::cout<< msg << std::endl;
  return EXIT_SUCCESS;
}

$ ./cppgl17.sh p241
$ clang++ p241.cpp
p241.cpp:14:1: error: C++ requires a type specifier for all declarations
a1 = a2; // error: Y assigned to X
^
p241.cpp:15:1: error: C++ requires a type specifier for all declarations
a1 = a3; // error: int assigned to X
^
p241.cpp:63:8: error: redefinition of 'X'
struct X { int a; };
       ^
p241.cpp:8:8: note: previous definition is here
struct X { int a; };
       ^
p241.cpp:64:8: error: redefinition of 'Y'
struct Y { int a; };
       ^
p241.cpp:9:8: note: previous definition is here
struct Y { int a; };
       ^
p241.cpp:65:3: error: redefinition of 'a1'
X a1;
  ^
p241.cpp:10:3: note: previous definition is here
X a1;
  ^
p241.cpp:66:3: error: redefinition of 'a2'
Y a2;
  ^
p241.cpp:11:3: note: previous definition is here
Y a2;
  ^
p241.cpp:67:5: error: redefinition of 'a3'
int a3;
    ^
p241.cpp:12:5: note: previous definition is here
int a3;
    ^
p241.cpp:69:1: error: C++ requires a type specifier for all declarations
a1 = a2; // error: Y assigned to X
^
p241.cpp:70:1: error: C++ requires a type specifier for all declarations
a1 = a3; // error: int assigned to X
^
p241.cpp:78:8: error: redefinition of 'stat'
struct stat {
       ^
p241.cpp:23:8: note: previous definition is here
struct stat {
       ^
p241.cpp:81:1: error: must use 'struct' tag to refer to type 'stat' in this scope
stat gstat; // use plain stat to
^
struct 
p241.cpp:28:5: note: struct 'stat' is hidden by a non-type declaration of 'stat' here
int stat(struct stat*); // redeclare stat as function
    ^
p241.cpp:81:6: error: redefinition of 'gstat'
stat gstat; // use plain stat to
     ^
p241.cpp:26:6: note: previous definition is here
stat gstat; // use plain stat to
     ^
p241.cpp:84:6: error: redefinition of 'f'
void f() {
     ^
p241.cpp:29:6: note: previous definition is here
void f() {
     ^
p241.cpp:90:8: error: redefinition of 's'
struct s { int a; };
       ^
p241.cpp:35:8: note: previous definition is here
struct s { int a; };
       ^
p241.cpp:91:6: error: redefinition of 'g'
void g() {
     ^
p241.cpp:36:6: note: previous definition is here
void g() {
     ^
p241.cpp:101:7: error: redefinition of 'Matrix'
class Matrix {
      ^
p241.cpp:46:7: note: previous definition is here
class Matrix {
      ^
p241.cpp:105:7: error: redefinition of 'Vector'
class Vector {
      ^
p241.cpp:50:7: note: previous definition is here
class Vector {
      ^
p241.cpp:110:8: error: redefinition of 's'
struct s { int a; };
       ^
p241.cpp:35:8: note: previous definition is here
struct s { int a; };
       ^
p241.cpp:111:6: error: redefinition of 'g'
void g(int s) {
     ^
p241.cpp:56:6: note: previous definition is here
void g(int s3) {
     ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
20 errors generated.

$ g++-7 p241.cpp
p241.cpp:14:1: error: 'a1' does not name a type
 a1 = a2; // error: Y assigned to X
 ^~
p241.cpp:15:1: error: 'a1' does not name a type
 a1 = a3; // error: int assigned to X
 ^~
p241.cpp:63:8: error: redefinition of 'struct X'
 struct X { int a; };
        ^
p241.cpp:8:8: note: previous definition of 'struct X'
 struct X { int a; };
        ^
p241.cpp:64:8: error: redefinition of 'struct Y'
 struct Y { int a; };
        ^
p241.cpp:9:8: note: previous definition of 'struct Y'
 struct Y { int a; };
        ^
p241.cpp:65:3: error: redefinition of 'X a1'
 X a1;
   ^~
p241.cpp:10:3: note: 'X a1' previously declared here
 X a1;
   ^~
p241.cpp:66:3: error: redefinition of 'Y a2'
 Y a2;
   ^~
p241.cpp:11:3: note: 'Y a2' previously declared here
 Y a2;
   ^~
p241.cpp:67:5: error: redefinition of 'int a3'
 int a3;
     ^~
p241.cpp:12:5: note: 'int a3' previously declared here
 int a3;
     ^~
p241.cpp:69:1: error: 'a1' does not name a type
 a1 = a2; // error: Y assigned to X
 ^~
p241.cpp:70:1: error: 'a1' does not name a type
 a1 = a3; // error: int assigned to X
 ^~
p241.cpp:78:8: error: redefinition of 'struct stat'
 struct stat {
        ^~~~
p241.cpp:23:8: note: previous definition of 'struct stat'
 struct stat {
        ^~~~
p241.cpp:81:1: error: 'stat' does not name a type
 stat gstat; // use plain stat to
 ^~~~
p241.cpp: In function 'void f()':
p241.cpp:84:6: error: redefinition of 'void f()'
 void f() {
      ^
p241.cpp:29:6: note: 'void f()' previously defined here
 void f() {
      ^
p241.cpp: At global scope:
p241.cpp:90:8: error: redefinition of 'struct s'
 struct s { int a; };
        ^
p241.cpp:35:8: note: previous definition of 'struct s'
 struct s { int a; };
        ^
p241.cpp: In function 'void g()':
p241.cpp:91:6: error: redefinition of 'void g()'
 void g() {
      ^
p241.cpp:36:6: note: 'void g()' previously defined here
 void g() {
      ^
p241.cpp: At global scope:
p241.cpp:101:7: error: redefinition of 'class Matrix'
 class Matrix {
       ^~~~~~
p241.cpp:46:7: note: previous definition of 'class Matrix'
 class Matrix {
       ^~~~~~
p241.cpp:105:7: error: redefinition of 'class Vector'
 class Vector {
       ^~~~~~
p241.cpp:50:7: note: previous definition of 'class Vector'
 class Vector {
       ^~~~~~
p241.cpp:110:8: error: redefinition of 'struct s'
 struct s { int a; };
        ^
p241.cpp:35:8: note: previous definition of 'struct s'
 struct s { int a; };
        ^
p241.cpp: In function 'void g(int)':
p241.cpp:111:6: error: redefinition of 'void g(int)'
 void g(int s) {
      ^
p241.cpp:56:6: note: 'void g(int)' previously defined here
 void g(int s3) {
      ^
p241.cpp: At global scope:
p241.cpp:116:11: error: redefinition of 'A* A'
 class A * A;
           ^
p241.cpp:61:11: note: 'A* A' previously declared here
 class A * A;
           ^