[libc+][stack] P3372R3: constexpr <stack>

Implements `<stack>` as per P3372R3: `constexpr` containers and adaptors
- https://wg21.link/P3372R3

Closes #128672

This should be a fairly straightforward implementation, as `std::stack` is a
container adaptor, and most of the work is done in the underlying container.

Depends on #128656
Part of #127876

- https://wg21.link/P3372R3
- https://eel.is/c++draft/#containers
  - https://eel.is/c++draft/container.adaptors#stack.syn
    - https://eel.is/c++draft/stack.syn
    - https://eel.is/c++draft/stack

Author: H-G-Hristov

libcxx/.clang-format

@@ -25,6 +25,7 @@ AttributeMacros: [
                   '_LIBCPP_CONSTEXPR_SINCE_CXX17',
                   '_LIBCPP_CONSTEXPR_SINCE_CXX20',
                   '_LIBCPP_CONSTEXPR_SINCE_CXX23',
+                  '_LIBCPP_CONSTEXPR_SINCE_CXX26',
                   '_LIBCPP_CONSTEXPR',
                   '_LIBCPP_CONSTINIT',
                   '_LIBCPP_DEPRECATED_IN_CXX11',

libcxx/include/stack

@@ -24,12 +24,21 @@
 #include "nasty_containers.h"
 #include "test_container_comparisons.h"
 
-int main(int, char**) {
+TEST_CONSTEXPR_CXX26 bool test() {
   assert((test_sequence_container_adaptor_spaceship<std::stack, std::deque>()));
   assert((test_sequence_container_adaptor_spaceship<std::stack, std::list>()));
   assert((test_sequence_container_adaptor_spaceship<std::stack, std::vector>()));
   assert((test_sequence_container_adaptor_spaceship<std::stack, nasty_list>()));
   assert((test_sequence_container_adaptor_spaceship<std::stack, nasty_vector>()));
-  // `std::stack` is not constexpr, so no `static_assert` test here.
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
+
   return 0;
 }

libcxx/test/std/containers/container.adaptors/stack/compare.three_way.pass.cpp

@@ -22,7 +22,7 @@
 #  include "test_convertible.h"
 #endif
 
-int main(int, char**) {
+TEST_CONSTEXPR_CXX26 bool test() {
   typedef std::vector<int, limited_allocator<int, 10> > Container;
   typedef std::stack<int, Container> Q;
   Q q;
@@ -37,5 +37,14 @@ int main(int, char**) {
   static_assert(test_convertible<Q>(), "");
 #endif
 
+  return true;
+}
+
+int main(int, char**) {
+  test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
+
   return 0;
 }

libcxx/test/std/containers/container.adaptors/stack/stack.cons/ctor_default.pass.cpp

@@ -36,7 +36,7 @@ void test_return_type() {
 #endif
 }
 
-int main(int, char**) {
+TEST_CONSTEXPR_CXX26 bool test() {
   test_return_type<std::stack<int> >();
   test_return_type<std::stack<int, std::vector<int> > >();
 
@@ -57,5 +57,14 @@ int main(int, char**) {
   assert(q.size() == 3);
   assert(q.top() == Emplaceable(3, 4.5));
 
+  return true;
+}
+
+int main(int, char**) {
+  test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
+
   return 0;
 }

libcxx/test/std/containers/container.adaptors/stack/stack.defn/emplace.pass.cpp

@@ -15,13 +15,22 @@
 
 #include "test_macros.h"
 
-int main(int, char**) {
+TEST_CONSTEXPR_CXX26 bool test() {
   std::stack<int> q;
   assert(q.empty());
   q.push(1);
   assert(!q.empty());
   q.pop();
   assert(q.empty());
 
+  return true;
+}
+
+int main(int, char**) {
+  test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
+
   return 0;
 }

libcxx/test/std/containers/container.adaptors/stack/stack.defn/empty.pass.cpp

@@ -15,7 +15,7 @@
 
 #include "test_macros.h"
 
-int main(int, char**) {
+TEST_CONSTEXPR_CXX26 bool test() {
   std::stack<int> q;
   assert(q.size() == 0);
   q.push(1);
@@ -32,5 +32,14 @@ int main(int, char**) {
   q.pop();
   assert(q.size() == 0);
 
+  return true;
+}
+
+int main(int, char**) {
+  test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
+
   return 0;
 }

libcxx/test/std/containers/container.adaptors/stack/stack.defn/pop.pass.cpp

@@ -15,7 +15,7 @@
 
 #include "test_macros.h"
 
-int main(int, char**) {
+TEST_CONSTEXPR_CXX26 bool test() {
   std::stack<int> q;
   q.push(1);
   assert(q.size() == 1);
@@ -29,3 +29,12 @@ int main(int, char**) {
 
   return 0;
 }
+
+int main(int, char**) {
+  test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
+
+  return 0;
+}

libcxx/test/std/containers/container.adaptors/stack/stack.defn/push.pass.cpp

@@ -17,7 +17,7 @@
 #include "../../push_range_container_adaptors.h"
 #include "test_macros.h"
 
-int main(int, char**) {
+TEST_CONSTEXPR_CXX26 bool test() {
   for_all_iterators_and_allocators<int, const int*>([]<class Iter, class Sent, class Alloc>() {
     test_push_range<std::stack<int, std::deque<int, Alloc>>, Iter, Sent>();
   });
@@ -29,5 +29,14 @@ int main(int, char**) {
   test_push_range_exception_safety_throwing_copy<std::stack>();
   test_push_range_exception_safety_throwing_allocator<std::stack, std::deque, int>();
 
+  return true;
+}
+
+int main(int, char**) {
+  test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
+
   return 0;
 }

libcxx/test/std/containers/container.adaptors/stack/stack.defn/push_range.pass.cpp

@@ -18,7 +18,7 @@
 #include "test_macros.h"
 #include "MoveOnly.h"
 
-int main(int, char**) {
+TEST_CONSTEXPR_CXX26 bool test() {
   std::stack<MoveOnly> q;
   q.push(MoveOnly(1));
   assert(q.size() == 1);
@@ -30,5 +30,14 @@ int main(int, char**) {
   assert(q.size() == 3);
   assert(q.top() == MoveOnly(3));
 
+  return true;
+}
+
+int main(int, char**) {
+  test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
+
   return 0;
 }