6 Basic concepts [basic]

int j = 24;
int main() {
  int i = j, j;
  j = 42;
}

unsigned char x = 12;
{ unsigned char x = x; }

const int x = 12;
{ enum { x = x }; }

typedef unsigned char T;
template<class T
  = T     // lookup finds the typedef name of unsigned char
  , T     // lookup finds the template parameter
    N = 0> struct A { };

namespace N {
  int i;
  int g(int a) { return a; }
  int j();
  void q();
}
namespace { int l=1; }
// the potential scope of l is from its point of declaration to the end of the translation unit

namespace N {
  int g(char a) {   // overloads N​::​g(int)
    return l+a;     // l is from unnamed namespace
  }

  int i;            // error: duplicate definition
  int j();          // OK: duplicate function declaration

  int j() {         // OK: definition of N​::​j()
    return g(i);    // calls N​::​g(int)
  }
  int q();          // error: different return type
}

typedef int  c;
enum { i = 1 };

class X {
  char  v[i];                       // error: i refers to ​::​i but when reevaluated is X​::​i
  int  f() { return sizeof(c); }    // OK: X​::​c
  char  c;
  enum { i = 2 };
};

typedef char*  T;
struct Y {
  T  a;                             // error: T refers to ​::​T but when reevaluated is Y​::​T
  typedef long  T;
  T  b;
};

typedef int I;
class D {
  typedef I I;                      // error, even though no reordering involved
};

namespace N {
  template<class T> struct A { };               // #1
  template<class U> void f(U) { }               // #2
  struct B {
    template<class V> friend int g(struct C*);  // #3
  };
}