From f004e91c9b4171f04d606cc45b2fb76809ede143 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Mon, 12 May 2025 17:40:43 +0300
Subject: [PATCH] Return std::array<std::string_view, 0> type for
 boost::pfr::names_as_array(empty_struct{}) (fixes #195)

---
 include/boost/pfr/core_name.hpp       |  3 +--
 include/boost/pfr/detail/stdarray.hpp | 21 +++++----------------
 test/core_name/run/fields_names.cpp   |  8 ++++++++
 3 files changed, 14 insertions(+), 18 deletions(-)

diff --git a/include/boost/pfr/core_name.hpp b/include/boost/pfr/core_name.hpp
index 34cab9a..186d29b 100644
--- a/include/boost/pfr/core_name.hpp
+++ b/include/boost/pfr/core_name.hpp
@@ -83,8 +83,7 @@ auto
 names_as_array() noexcept {
     return detail::make_stdarray_from_tietuple(
         detail::tie_as_names_tuple<T>(),
-        detail::make_index_sequence< tuple_size_v<T> >(),
-        1L
+        detail::make_index_sequence< tuple_size_v<T> >()
     );
 }
 
diff --git a/include/boost/pfr/detail/stdarray.hpp b/include/boost/pfr/detail/stdarray.hpp
index f17b2c4..0f802e4 100644
--- a/include/boost/pfr/detail/stdarray.hpp
+++ b/include/boost/pfr/detail/stdarray.hpp
@@ -12,29 +12,18 @@
 #include <boost/pfr/detail/sequence_tuple.hpp>
 
 #if !defined(BOOST_PFR_INTERFACE_UNIT)
-#include <utility> // metaprogramming stuff
 #include <array>
-#include <type_traits> // for std::common_type_t
-#include <cstddef>
+#include <string_view>
+#include <utility> // metaprogramming stuff
 #endif
 
 namespace boost { namespace pfr { namespace detail {
 
-template <class... Types>
-constexpr auto make_stdarray(const Types&... t) noexcept {
-    return std::array<std::common_type_t<Types...>, sizeof...(Types)>{t...};
-}
-
 template <class T, std::size_t... I>
-constexpr auto make_stdarray_from_tietuple(const T& t, std::index_sequence<I...>, int) noexcept {
-    return detail::make_stdarray(
+constexpr auto make_stdarray_from_tietuple(const T& t, std::index_sequence<I...>) noexcept {
+    return std::array<std::string_view, sizeof...(I)>{
         boost::pfr::detail::sequence_tuple::get<I>(t)...
-    );
-}
-
-template <class T>
-constexpr auto make_stdarray_from_tietuple(const T&, std::index_sequence<>, long) noexcept {
-    return std::array<std::nullptr_t, 0>{};
+    };
 }
 
 }}} // namespace boost::pfr::detail
diff --git a/test/core_name/run/fields_names.cpp b/test/core_name/run/fields_names.cpp
index e04a44a..27212f2 100644
--- a/test/core_name/run/fields_names.cpp
+++ b/test/core_name/run/fields_names.cpp
@@ -131,6 +131,13 @@ void test_names_as_array_for_empty() {
     BOOST_TEST_EQ(value.empty(), true);
 }
 
+void test_names_as_array_iteration_for_empty() {
+    const auto names = boost::pfr::names_as_array<empty>();
+    for (std::string_view name : names) {
+        BOOST_ERROR("`names` must be empty");
+    }
+}
+
 } // namespace testing
 
 int main() {
@@ -141,6 +148,7 @@ int main() {
     testing::test_names_as_array();
     testing::test_names_as_array_without_linkage();
     testing::test_names_as_array_for_empty();
+    testing::test_names_as_array_iteration_for_empty();
 
     return boost::report_errors();
 }