[clang][NFC] Add examples from [dcl.init.aggr] to C++ conformance tests (#91435)

This patch adds examples from 2024-04-22 draft of
[[dcl.init.aggr]](http://eel.is/c++draft/dcl.init.aggr) to C++
conformance tests. Testing is done via constant evaluation and static
asserts. As far as I can see, the rest of the conformance suite is
typically testing the latest language mode at the time of writing (with
a notable exception of defect report tests), so I'm also testing in the
latest language mode.

Author: Endilll

clang/test/CXX/dcl/dcl.init/aggr.cpp

@@ -0,0 +1,294 @@
+// RUN:  %clang_cc1 -std=c++2c -verify %s
+
+namespace ex1 {
+struct C {
+  union {
+    int a;
+    const char* p;
+  };
+  int x;
+};
+
+constexpr C c = { .a = 1, .x = 3 };
+static_assert(c.a == 1);
+static_assert(c.x == 3);
+
+static constexpr C c2 = { .a = 1.0, .x = 3 };
+// expected-error@-1 {{type 'double' cannot be narrowed to 'int' in initializer list}}
+//   expected-note@-2 {{insert an explicit cast to silence this issue}}
+} // namespace ex1
+
+namespace ex2 {
+struct A {
+  int x;
+  struct B {
+    int i;
+    int j;
+  } b;
+};
+
+constexpr A a = { 1, { 2, 3 } };
+static_assert(a.x == 1);
+static_assert(a.b.i == 2);
+static_assert(a.b.j == 3);
+
+struct base1 { int b1, b2 = 42; };
+struct base2 {
+  constexpr base2() {
+    b3 = 43;
+  }
+  int b3;
+};
+struct derived : base1, base2 {
+  int d;
+};
+
+constexpr derived d1{{1, 2}, {}, 4};
+static_assert(d1.b1 == 1);
+static_assert(d1.b2 == 2);
+static_assert(d1.b3 == 43);
+static_assert(d1.d == 4);
+
+constexpr derived d2{{}, {}, 4};
+static_assert(d2.b1 == 0);
+static_assert(d2.b2 == 42);
+static_assert(d2.b3 == 43);
+static_assert(d2.d == 4);
+} // namespace ex2
+
+namespace ex3 {
+struct S {
+  int a;
+  const char* b;
+  int c;
+  int d = b[a];
+};
+
+constexpr S ss = { 1, "asdf" };
+static_assert(ss.a == 1);
+static_assert(__builtin_strcmp(ss.b, "asdf") == 0);
+static_assert(ss.c == int{});
+static_assert(ss.d == ss.b[ss.a]);
+
+struct string {
+  int d = 43;
+};
+
+struct A {
+  string a;
+  int b = 42;
+  int c = -1;
+};
+
+constexpr A a{.c = 21};
+static_assert(a.a.d == string{}.d);
+static_assert(a.b == 42);
+static_assert(a.c == 21);
+} // namespace ex3
+
+namespace ex4 {
+int x[] = { 1, 3, 5 };
+static_assert(sizeof(x) / sizeof(int) == 3);
+} // namespace ex4
+
+namespace ex5 {
+struct X { int i, j, k; };
+
+constexpr X a[] = { 1, 2, 3, 4, 5, 6 };
+constexpr X b[2] = { { 1, 2, 3 }, { 4, 5, 6 } };
+static_assert(sizeof(a) == sizeof(b));
+static_assert(a[0].i == b[0].i);
+static_assert(a[0].j == b[0].j);
+static_assert(a[0].k == b[0].k);
+static_assert(a[1].i == b[1].i);
+static_assert(a[1].j == b[1].j);
+static_assert(a[1].k == b[1].k);
+} // namespace ex5
+
+namespace ex6 {
+struct S {
+  int y[] = { 0 };
+  // expected-error@-1 {{array bound cannot be deduced from a default member initializer}}
+};
+} // namespace ex6
+
+namespace ex7 {
+struct A {
+  int i;
+  static int s;
+  int j;
+  int :17;
+  int k;
+};
+
+constexpr A a = { 1, 2, 3 };
+static_assert(a.i == 1);
+static_assert(a.j == 2);
+static_assert(a.k == 3);
+} // namespace ex7
+
+namespace ex8 {
+struct A;
+extern A a;
+struct A {
+  const A& a1 { A{a,a} };
+  const A& a2 { A{} };
+  // expected-error@-1 {{default member initializer for 'a2' needed within definition of enclosing class 'A' outside of member functions}}
+  //   expected-note@-2 {{default member initializer declared here}}
+};
+A a{a,a};
+
+struct B {
+  int n = B{}.n;
+  // expected-error@-1 {{default member initializer for 'n' needed within definition of enclosing class 'B' outside of member functions}}
+  //   expected-note@-2 {{default member initializer declared here}}
+};
+} // namespace ex8
+
+namespace ex9 {
+constexpr int x[2][2] = { 3, 1, 4, 2 };
+static_assert(x[0][0] == 3);
+static_assert(x[0][1] == 1);
+static_assert(x[1][0] == 4);
+static_assert(x[1][1] == 2);
+
+constexpr float y[4][3] = {
+  { 1 }, { 2 }, { 3 }, { 4 }
+};
+static_assert(y[0][0] == 1);
+static_assert(y[0][1] == 0);
+static_assert(y[0][2] == 0);
+static_assert(y[1][0] == 2);
+static_assert(y[1][1] == 0);
+static_assert(y[1][2] == 0);
+static_assert(y[2][0] == 3);
+static_assert(y[2][1] == 0);
+static_assert(y[2][2] == 0);
+static_assert(y[3][0] == 4);
+static_assert(y[3][1] == 0);
+static_assert(y[3][2] == 0);
+} // namespace ex9
+
+namespace ex10 {
+struct S1 { int a, b; };
+struct S2 { S1 s, t; };
+
+constexpr S2 x[2] = { 1, 2, 3, 4, 5, 6, 7, 8 };
+constexpr S2 y[2] = {
+  {
+    { 1, 2 },
+    { 3, 4 }
+  },
+  {
+    { 5, 6 },
+    { 7, 8 }
+  }
+};
+static_assert(x[0].s.a == 1);
+static_assert(x[0].s.b == 2);
+static_assert(x[0].t.a == 3);
+static_assert(x[0].t.b == 4);
+static_assert(x[1].s.a == 5);
+static_assert(x[1].s.b == 6);
+static_assert(x[1].t.a == 7);
+static_assert(x[1].t.b == 8);
+} // namespace ex10
+
+namespace ex11 {
+char cv[4] = { 'a', 's', 'd', 'f', 0 };
+// expected-error@-1 {{excess elements in array initializer}}
+} // namespace ex11
+
+namespace ex12 {
+constexpr float y[4][3] = {
+  { 1, 3, 5 },
+  { 2, 4, 6 },
+  { 3, 5, 7 },
+};
+static_assert(y[0][0] == 1);
+static_assert(y[0][1] == 3);
+static_assert(y[0][2] == 5);
+static_assert(y[1][0] == 2);
+static_assert(y[1][1] == 4);
+static_assert(y[1][2] == 6);
+static_assert(y[2][0] == 3);
+static_assert(y[2][1] == 5);
+static_assert(y[2][2] == 7);
+static_assert(y[3][0] == 0.0);
+static_assert(y[3][1] == 0.0);
+static_assert(y[3][2] == 0.0);
+
+constexpr float z[4][3] = {
+  1, 3, 5, 2, 4, 6, 3, 5, 7
+};
+static_assert(z[0][0] == 1);
+static_assert(z[0][1] == 3);
+static_assert(z[0][2] == 5);
+static_assert(z[1][0] == 2);
+static_assert(z[1][1] == 4);
+static_assert(z[1][2] == 6);
+static_assert(z[2][0] == 3);
+static_assert(z[2][1] == 5);
+static_assert(z[2][2] == 7);
+static_assert(z[3][0] == 0.0);
+static_assert(z[3][1] == 0.0);
+static_assert(z[3][2] == 0.0);
+} // namespace ex12
+
+namespace ex13 {
+struct S { } s;
+struct A {
+  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
+} // namespace ex13
+
+namespace ex14 {
+struct A {
+  int i;
+  constexpr operator int() const { return 42; };
+};
+struct B {
+  A a1, a2;
+  int z;
+};
+constexpr A a{};
+constexpr B b = { 4, a, a };
+static_assert(b.a1.i == 4);
+static_assert(b.a2.i == a.i);
+static_assert(b.z == a.operator int());
+} // namespace ex14
+
+namespace ex15 {
+union u { // #ex15-u
+  int a;
+  const char* b;
+};
+
+u a = { 1 };
+u b = a;
+u c = 1;
+// expected-error@-1 {{no viable conversion from 'int' to 'u'}}
+//   expected-note@#ex15-u {{candidate constructor (the implicit copy constructor) not viable: no known conversion from 'int' to 'const u &' for 1st argument}}
+//   expected-note@#ex15-u {{candidate constructor (the implicit move constructor) not viable: no known conversion from 'int' to 'u &&' for 1st argument}}
+u d = { 0, "asdf" };
+// expected-error@-1 {{excess elements in union initializer}}
+u e = { "asdf" };
+// expected-error@-1 {{cannot initialize a member subobject of type 'int' with an lvalue of type 'const char[5]'}}
+u f = { .b = "asdf" };
+u g = {
+  .a = 1, // #ex15-g-a
+  .b = "asdf"
+  // expected-error@-1 {{initializer partially overrides prior initialization of this subobject}}
+  //   expected-note@#ex15-g-a {{previous initialization is here}}
+};
+} // namespace ex15