// N4606 Committee Draft, Standard for Programming Language C++
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf 
#define msg "p224.cpp(104)8.6.1 Aggregates [dcl.init.aggr]"

#include <iostream>
struct A {
  int x;
  struct B {
    int i;
    int j;
  } b;
} a = { 1, { 2, 3 } };

struct base1 { int b1, b2 = 42; };
struct base2 {
  base2() {
    b3 = 42;
  }
  int b3;
};
struct derived : base1, base2 {
  int d;
};
derived d1{{1, 2}, {}, 4};
derived d2{{}, {}, 4};

int x[] = { 1, 3, 5 };

struct A2 {
  int i;
  static int s;
  int j;
  int :17;
  int k;
} a2 = { 1, 2, 3 };

char cv[4] = { 'a', 's', 'd', 'f', 0 }; // error

struct S { int a; const char* b; int c; int d = b[a]; };
S ss = { 1, "asdf" };

struct X { int i, j, k = 42; };
X a[] = { 1, 2, 3, 4, 5, 6 };
X b[2] = { { 1, 2, 3 }, { 4, 5, 6 } };

struct A3;
extern A3 a3;
struct A3 {
  const A3& a1 { A3{a3,a3} }; // OK
  const A3& a2 { A3{} }; // error
};
A3 a{a,a}; // OK

struct S { } s;
struct A4 {
  S s1;
  int i1;
  S s2;
  int i2;
  S s3;
  int i3;
} a = {
  { }, // Required initialization
  0,
  s, // Required initialization
  0
}; // Initialization not required for A::s3 because A::i3 is also not initialized

int x7[2][2] = { 3, 1, 4, 2 };

float y7[4][3] = {
  { 1 }, { 2 }, { 3 }, { 4 }
};

float y8[4][3] = {
  { 1, 3, 5 },
  { 2, 4, 6 },
  { 3, 5, 7 },
};

float y9[4][3] = {
  1, 3, 5, 2, 4, 6, 3, 5, 7
};

struct A5 {
  int i;
  operator int();
};
struct B3 {
  A5 a1, a2;
  int z;
};
A5 a5;
B3 b5 = { 4, a5, a5 };

union u { int a; const char* b; };
u a6 = { 1 };
u b6 = a6;
u c6 = 1; // error
u d6 = { 0, "asdf" }; // error
u e6 = { "asdf" }; // error

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

$ clang++ p224.cpp
p224.cpp:37:36: error: excess elements in array initializer
char cv[4] = { 'a', 's', 'd', 'f', 0 }; // error
                                   ^
p224.cpp:43:3: error: redefinition of 'a' with a different type: 'X []' vs 'struct A'
X a[] = { 1, 2, 3, 4, 5, 6 };
  ^
p224.cpp:12:3: note: previous definition is here
} a = { 1, { 2, 3 } };
  ^
p224.cpp:49:18: warning: binding reference member 'a1' to a temporary value [-Wdangling-field]
  const A3& a1 { A3{a3,a3} }; // OK
                 ^~~~~~~~~
p224.cpp:49:13: note: reference member declared here
  const A3& a1 { A3{a3,a3} }; // OK
            ^
p224.cpp:50:21: error: default member initializer for 'a2' needed within definition of enclosing class 'A3' outside of member functions
  const A3& a2 { A3{} }; // error
                    ^
p224.cpp:50:13: note: default member initializer declared here
  const A3& a2 { A3{} }; // error
            ^
p224.cpp:52:4: error: redefinition of 'a' with a different type: 'A3' vs 'struct A'
A3 a{a,a}; // OK
   ^
p224.cpp:12:3: note: previous definition is here
} a = { 1, { 2, 3 } };
  ^
p224.cpp:54:8: error: redefinition of 'S'
struct S { } s;
       ^
p224.cpp:39:8: note: previous definition is here
struct S { int a; const char* b; int c; int d = b[a]; };
       ^
p224.cpp:62:3: error: redefinition of 'a' with a different type: 'struct A4' vs 'struct A'
} a = {
  ^
p224.cpp:12:3: note: previous definition is here
} a = { 1, { 2, 3 } };
  ^
p224.cpp:99:3: error: no viable conversion from 'int' to 'u'
u c6 = 1; // error
  ^    ~
p224.cpp:96:7: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'int' to 'const u &' for 1st argument
union u { int a; const char* b; };
      ^
p224.cpp:96:7: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'int' to 'u &&' for 1st argument
union u { int a; const char* b; };
      ^
p224.cpp:100:13: error: excess elements in union initializer
u d6 = { 0, "asdf" }; // error
            ^~~~~~
p224.cpp:101:10: error: cannot initialize a member subobject of type 'int' with an lvalue of type 'const char [5]'
u e6 = { "asdf" }; // error
         ^~~~~~
1 warning and 9 errors generated.

$ g++-7 p224.cpp
p224.cpp:37:38: error: too many initializers for 'char [4]'
 char cv[4] = { 'a', 's', 'd', 'f', 0 }; // error
                                      ^
p224.cpp:43:5: error: conflicting declaration 'X a []'
 X a[] = { 1, 2, 3, 4, 5, 6 };
     ^
p224.cpp:12:3: note: previous declaration as 'A a'
 } a = { 1, { 2, 3 } };
   ^
p224.cpp:50:21: error: constructor required before non-static data member for 'A3::a2' has been parsed
   const A3& a2 { A3{} }; // error
                     ^
p224.cpp:50:23: error: invalid initialization of reference of type 'const A3&' from expression of type '<brace-enclosed initializer list>'
   const A3& a2 { A3{} }; // error
                       ^
p224.cpp:52:4: error: conflicting declaration 'A3 a'
 A3 a{a,a}; // OK
    ^
p224.cpp:12:3: note: previous declaration as 'A a'
 } a = { 1, { 2, 3 } };
   ^
p224.cpp:54:8: error: redefinition of 'struct S'
 struct S { } s;
        ^
p224.cpp:39:8: note: previous definition of 'struct S'
 struct S { int a; const char* b; int c; int d = b[a]; };
        ^
p224.cpp:62:3: error: conflicting declaration 'A4 a'
 } a = {
   ^
p224.cpp:12:3: note: previous declaration as 'A a'
 } a = { 1, { 2, 3 } };
   ^
p224.cpp:99:8: error: conversion from 'int' to non-scalar type 'u' requested
 u c6 = 1; // error
        ^
p224.cpp:100:20: error: too many initializers for 'u'
 u d6 = { 0, "asdf" }; // error
                    ^
p224.cpp:101:17: error: invalid conversion from 'const char*' to 'int' [-fpermissive]
 u e6 = { "asdf" }; // error
                 ^