[libc++] LWG3643: Missing constexpr in std::counted_iterator (#87901)

This pull request implements LWG3643: Missing constexpr in
std::counted_iterator. Specifically, one overload of
std::counted_operator::operator++ was not marked as constexpr,
despite being eligible for it after the introduction of try-block
support in constexpr functions in C++20.

libcxx/docs/Status/Cxx23Issues.csv

@@ -150,7 +150,7 @@
 "`3619 <https://wg21.link/LWG3619>`__","Specification of ``vformat_to`` contains ill-formed ``formatted_size`` calls","February 2022","|Nothing to do|","","|format|"
 "`3621 <https://wg21.link/LWG3621>`__","Remove feature-test macro ``__cpp_lib_monadic_optional`` ","February 2022","|Complete|","15.0"
 "`3632 <https://wg21.link/LWG3632>`__","``unique_ptr`` ""Mandates: This constructor is not selected by class template argument deduction""","February 2022","|Nothing to do|",""
-"`3643 <https://wg21.link/LWG3643>`__","Missing ``constexpr`` in ``std::counted_iterator`` ","February 2022","","","|ranges|"
+"`3643 <https://wg21.link/LWG3643>`__","Missing ``constexpr`` in ``std::counted_iterator`` ","February 2022","|Complete|","19.0","|ranges|"
 "`3648 <https://wg21.link/LWG3648>`__","``format`` should not print ``bool`` with ``'c'`` ","February 2022","|Complete|","15.0","|format|"
 "`3649 <https://wg21.link/LWG3649>`__","[fund.ts.v2] Reinstate and bump ``__cpp_lib_experimental_memory_resource`` feature test macro","February 2022","",""
 "`3650 <https://wg21.link/LWG3650>`__","Are ``std::basic_string`` 's ``iterator`` and ``const_iterator`` constexpr iterators?","February 2022","|Nothing to do|",""

libcxx/include/__iterator/counted_iterator.h

@@ -129,7 +129,7 @@ public:
     return *this;
   }
 
-  _LIBCPP_HIDE_FROM_ABI decltype(auto) operator++(int) {
+  _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) operator++(int) {
     _LIBCPP_ASSERT_UNCATEGORIZED(__count_ > 0, "Iterator already at or past end.");
     --__count_;
 #  ifndef _LIBCPP_HAS_NO_EXCEPTIONS

libcxx/test/std/iterators/predef.iterators/counted.iterator/increment.pass.cpp

@@ -9,7 +9,7 @@
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
 // constexpr counted_iterator& operator++();
-// decltype(auto) operator++(int);
+// constexpr decltype(auto) operator++(int);
 // constexpr counted_iterator operator++(int)
 //   requires forward_iterator<I>;
 
@@ -62,6 +62,26 @@ concept PlusEnabled = requires(Iter& iter) {
 constexpr bool test() {
   int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
 
+  {
+    using Counted = std::counted_iterator<InputOrOutputArchetype>;
+    std::counted_iterator iter(InputOrOutputArchetype{buffer}, 8);
+
+    iter++;
+    assert((++iter).base().ptr == buffer + 2);
+
+    ASSERT_SAME_TYPE(decltype(iter++), void);
+    ASSERT_SAME_TYPE(decltype(++iter), Counted&);
+  }
+  {
+    using Counted = std::counted_iterator<cpp20_input_iterator<int*>>;
+    std::counted_iterator iter(cpp20_input_iterator<int*>{buffer}, 8);
+
+    iter++;
+    assert(++iter == Counted(cpp20_input_iterator<int*>{buffer + 2}, 6));
+
+    ASSERT_SAME_TYPE(decltype(iter++), void);
+    ASSERT_SAME_TYPE(decltype(++iter), Counted&);
+  }
   {
     using Counted = std::counted_iterator<forward_iterator<int*>>;
     std::counted_iterator iter(forward_iterator<int*>{buffer}, 8);
@@ -95,29 +115,8 @@ int main(int, char**) {
   test();
   static_assert(test());
 
-  int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
-
-  {
-    using Counted = std::counted_iterator<InputOrOutputArchetype>;
-    std::counted_iterator iter(InputOrOutputArchetype{buffer}, 8);
-
-    iter++;
-    assert((++iter).base().ptr == buffer + 2);
-
-    ASSERT_SAME_TYPE(decltype(iter++), void);
-    ASSERT_SAME_TYPE(decltype(++iter), Counted&);
-  }
-  {
-    using Counted = std::counted_iterator<cpp20_input_iterator<int*>>;
-    std::counted_iterator iter(cpp20_input_iterator<int*>{buffer}, 8);
-
-    iter++;
-    assert(++iter == Counted(cpp20_input_iterator<int*>{buffer + 2}, 6));
-
-    ASSERT_SAME_TYPE(decltype(iter++), void);
-    ASSERT_SAME_TYPE(decltype(++iter), Counted&);
-  }
 #ifndef TEST_HAS_NO_EXCEPTIONS
+  int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
   {
     using Counted = std::counted_iterator<ThrowsOnInc<int*>>;
     std::counted_iterator iter(ThrowsOnInc<int*>{buffer}, 8);