// N4606 Committee Draft, Standard for Programming Language C++
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf 
#define msg "p251.cpp(116)9.2.5 Nested class declarations [class.nest]"

#include <iostream>
int x;
int y;
struct enclose {
  int x;
  static int s;
  struct inner {
    void f(int i) {
      int a = sizeof(x); // OK: operand of sizeof is an unevaluated operand
      x = i; // error: assign to enclose::x
      s = i; // OK: assign to enclose::s
      ::x = i; // OK: assign to global x
      y = i; // OK: assign to global y
    }
    void g(enclose* p, int i) {
      p->x = i; // OK: assign to enclose::x
    }
  };
};
inner* p = 0; // error: inner not in scope

struct enclose2 {
  struct inner {
    static int x;
    void f(int i);
  };
};
int enclose2::inner::x = 1;
void enclose2::inner::f(int i) { /* ... */ }

class E {
    class I1; // forward declaration of nested class
    class I2;
    class I1 { }; // definition of nested class
};
class E::I2 { }; // definition of nested class

int main(){
  std::cout<< msg << std::endl;
  return EXIT_SUCCESS;
}

$ ./cppgl17.sh p251
$ clang++ p251.cpp
p251.cpp:14:1: error: use of non-static data member 'x' of 'enclose' from nested type 'inner'
x = i; // error: assign to enclose::x
^
p251.cpp:24:1: error: unknown type name 'inner'; did you mean 'enclose::inner'?
inner* p = 0; // error: inner not in scope
^~~~~
enclose::inner
p251.cpp:11:8: note: 'enclose::inner' declared here
struct inner {
       ^
2 errors generated.

$ g++-7 p251.cpp
p251.cpp: In member function 'void enclose::inner::f(int)':
p251.cpp:14:1: error: invalid use of non-static data member 'enclose::x'
 x = i; // error: assign to enclose::x
 ^
p251.cpp:9:5: note: declared here
 int x;
     ^
p251.cpp: At global scope:
p251.cpp:24:1: error: 'inner' does not name a type; did you mean 'signed'?
 inner* p = 0; // error: inner not in scope
 ^~~~~
 signed

// N4606 Committee Draft, Standard for Programming Language C++
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf 
#define msg "p251.cpp(116)9.2.5 Nested class declarations [class.nest]"

#include <iostream>
int x;
int y;
struct enclose {
  int x;
  static int s;
  struct inner {
    void f(int i) {
      int a = sizeof(x); // OK: operand of sizeof is an unevaluated operand
//      x = i; // error: assign to enclose::x
      s = i; // OK: assign to enclose::s
      ::x = i; // OK: assign to global x
      y = i; // OK: assign to global y
  std::cout<<"s="<<s << std::endl;
  std::cout<<"y="<<y << std::endl;
  std::cout<<"enclose.x="<<enclose.x << std::endl;
    }
    void g(enclose* p, int i) {
      p->x = i; // OK: assign to enclose::x
    }
  };
};
//inner* p = 0; // error: inner not in scope

struct enclose2 {
  struct inner {
    static int x;
    void f(int i);
  };
};
int enclose2::inner::x = 1;
void enclose2::inner::f(int i) { /* ... */
  std::cout<<"i="<<i << std::endl;
 }

class E {
    class I1; // forward declaration of nested class
    class I2;
    class I1 { }; // definition of nested class
};
class E::I2 { }; // definition of nested class

int main(){
  int i=3;
  struct enclose se;
  struct enclose2 se2;
  se.inner.f(i);
  se2.inner.f(i);
  std::cout<< msg << std::endl;
  return EXIT_SUCCESS;
}