// N4606 Committee Draft, Standard for Programming Language C++
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf 
#define msg "p228-2.cpp(106) 8.6.3 References [dcl.init.ref]"

#include <iostream>
int g(int) noexcept;
void f() {
  int i;
  int& r = i; // r refers to i
  r = 1; // the value of i becomes 1
  int* p = &r; // p points to i
  int& rr = r; // rr refers to what r refers to, that is, to i
  int (&rg)(int) = g; // rg refers to the function g
  rg(i); // calls function g
  int a[3];
  int (&ra)[3] = a; // ra refers to the array a
  ra[1] = i; // modifies a[1]
}

//int& r1; // error: initializer missing
extern int& r2; // OK

double d = 2.0;
double& rd = d; // rd refers to d
const double& rcd = d; // rcd refers to d
struct A { };
struct B : A { operator int&(); } b;
A& ra = b; // ra refers to A subobject in b
const A& rca = b; // rca refers to A subobject in b
int& ir = B(); // ir refers to the result of B::operator int&

//double& rd2 = 2.0; // error: not an lvalue and reference not const
int i = 2;
//double& rd3 = i; // error: type mismatch and reference not const

struct A2 { };
struct B2 : A2 { } b2;
extern B2 f2();
const A2& rca2 = f2(); // bound to the A2 subobject of the B2 rvalue.
A2&& rra = f2(); // same as above

struct X {
  operator B();
  operator int&();
} x;

const A& r = x; // bound to the A subobject of the result of the conversion
int i2 = 42;
int&& rri = static_cast<int&&>(i2); // bound directly to i2
B&& rrb = x; // bound directly to the result of operator B

struct Banana { };
struct Enigma { operator const Banana(); };
struct Alaska { operator Banana&(); };
void enigmatic() {
  typedef const Banana ConstBanana;
  //Banana &&banana1 = ConstBanana(); // ill-formed
  //Banana &&banana2 = Enigma(); // ill-formed
  //Banana &&banana3 = Alaska(); // ill-formed
}
const double& rcd2 = 2; // rcd2 refers to temporary with value 2.0
double&& rrd = 2; // rrd refers to temporary with value 2.0
const volatile int cvi = 1;
const int& r2 = cvi; // error: type qualifiers dropped
struct A9 { operator volatile int&(); } a9;
const int& r3 = a9; // error: type qualifiers dropped
// from result of conversion function
double d2 = 1.0;
double&& rrd2 = d2; // error: initializer is lvalue of related type
struct X9 { operator int&(); };
int&& rri2 = X(); // error: result of conversion function is lvalue of related type
int i3 = 2;
double&& rrd3 = i3; // rrd3 refers to temporary with value 2.0
int main(){
  std::cout<< msg << std::endl;
  return EXIT_SUCCESS;
}

$ ./cppgl17.sh p228-2
$ clang++ p228-2.cpp
p228-2.cpp:64:12: error: redefinition of 'r2' with a different type: 'const int &' vs 'int &'
const int& r2 = cvi; // error: type qualifiers dropped
           ^
p228-2.cpp:21:13: note: previous declaration is here
extern int& r2; // OK
            ^
p228-2.cpp:66:12: error: no viable conversion from 'struct A9' to 'const int'
const int& r3 = a9; // error: type qualifiers dropped
           ^    ~~
p228-2.cpp:65:13: note: candidate function
struct A9 { operator volatile int&(); } a9;
            ^
p228-2.cpp:69:10: error: rvalue reference to type 'double' cannot bind to lvalue of type 'double'
double&& rrd2 = d2; // error: initializer is lvalue of related type
         ^      ~~
p228-2.cpp:71:7: error: no viable conversion from 'X' to 'int'
int&& rri2 = X(); // error: result of conversion function is lvalue of related type
      ^      ~~~
p228-2.cpp:43:3: note: candidate function
  operator B();
  ^
p228-2.cpp:44:3: note: candidate function
  operator int&();
  ^
4 errors generated.

$ g++-7 p228-2.cpp
p228-2.cpp:64:12: error: conflicting declaration 'const int& r2'
 const int& r2 = cvi; // error: type qualifiers dropped
            ^~
p228-2.cpp:21:13: note: previous declaration as 'int& r2'
 extern int& r2; // OK
             ^~
p228-2.cpp:66:17: error: invalid user-defined conversion from 'A9' to 'const int&' [-fpermissive]
 const int& r3 = a9; // error: type qualifiers dropped
                 ^~
p228-2.cpp:65:13: note: candidate is: A9::operator volatile int&() <near match>
 struct A9 { operator volatile int&(); } a9;
             ^~~~~~~~
p228-2.cpp:65:13: note:   no known conversion from 'volatile int&' to 'const int&'
p228-2.cpp:66:17: error: conversion from 'volatile int' to 'const int&' discards qualifiers [-fpermissive]
 const int& r3 = a9; // error: type qualifiers dropped
                 ^~
p228-2.cpp:69:17: error: cannot bind rvalue reference of type 'double&&' to lvalue of type 'double'
 double&& rrd2 = d2; // error: initializer is lvalue of related type
                 ^~
p228-2.cpp:71:14: error: invalid user-defined conversion from 'X' to 'int&&' [-fpermissive]
 int&& rri2 = X(); // error: result of conversion function is lvalue of related type
              ^~~
p228-2.cpp:44:3: note: candidate is: X::operator int&() <near match>
   operator int&();
   ^~~~~~~~
p228-2.cpp:44:3: note:   no known conversion from 'int&' to 'int&&'