From 8383f5e20463f655ddae0e01b7535da53ecbe1f3 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Mon, 5 Jan 2026 23:18:42 +0300
Subject: [PATCH 01/15] Member pointer to index (WIP)

---
 include/boost/pfr/core.hpp        | 56 +++++++++++++++++++++++++++++++
 test/core/run/member_to_index.cpp | 26 ++++++++++++++
 2 files changed, 82 insertions(+)
 create mode 100644 test/core/run/member_to_index.cpp

diff --git a/include/boost/pfr/core.hpp b/include/boost/pfr/core.hpp
index d621f273..0a88cfa4 100644
--- a/include/boost/pfr/core.hpp
+++ b/include/boost/pfr/core.hpp
@@ -18,9 +18,11 @@
 #include <boost/pfr/detail/for_each_field.hpp>
 #include <boost/pfr/detail/make_integer_sequence.hpp>
 #include <boost/pfr/detail/tie_from_structure_tuple.hpp>
+#include <boost/pfr/detail/offset_based_getter.hpp>
 
 #include <boost/pfr/tuple_size.hpp>
 
+
 #if !defined(BOOST_PFR_INTERFACE_UNIT)
 #include <type_traits>
 #include <utility>      // metaprogramming stuff
@@ -277,6 +279,60 @@ constexpr detail::tie_from_structure_tuple<Elements...> tie_from_structure(Eleme
     return detail::tie_from_structure_tuple<Elements...>(args...);
 }
 
+template <typename T, typename M>
+constexpr std::size_t index_of(const T& value, M T::*mem_ptr) {
+    constexpr auto size = boost::pfr::tuple_size_v<T>;
+    std::size_t result = size;
+
+    const void* target_address = std::addressof(value.*mem_ptr);
+
+    boost::pfr::for_each_field(value, [&result, target_address](const auto& field, std::size_t idx) {
+        if (!std::is_same<decltype(field), const M&>::value) {
+            return;
+        }
+
+        if (result != size) {
+            // already found the answer
+            return;
+        }
+
+        const void* filed_address = std::addressof(field);
+        if (target_address == filed_address) {
+            result = idx;
+        }
+    });
+
+    return result;
+}
+
+// TODO: move into detail::
+template <class... Types>
+auto strip_references(detail::sequence_tuple::tuple<Types&...>) -> detail::sequence_tuple::tuple<Types&...>;
+
+template <typename T, typename M>
+std::size_t index_of(M T::*mem_ptr) {
+    using tuple_type = detail::tuple_of_aligned_storage_t<
+        decltype(strip_references(detail::tie_as_tuple(std::declval<T&>())))
+    >;
+    using converted_member_pointer_t = M tuple_type::*;
+
+    constexpr tuple_type t{};
+
+    // TODO: not allowed in constexpr + unspecified behavior
+    auto mem_pointer = reinterpret_cast<converted_member_pointer_t>(mem_ptr);
+
+    const void* pointer = std::addressof(t.*mem_pointer);
+    namespace sequence_tuple = boost::pfr::detail::sequence_tuple;
+    if (&sequence_tuple::get<0>(t) == pointer) {
+        return 0;
+    } else if (&sequence_tuple::get<1>(t) == pointer) {
+        return 1;
+    } else if (&sequence_tuple::get<2>(t) == pointer) {
+        return 2;
+    }
+    return 3;
+}
+
 BOOST_PFR_END_MODULE_EXPORT
 
 }} // namespace boost::pfr
diff --git a/test/core/run/member_to_index.cpp b/test/core/run/member_to_index.cpp
new file mode 100644
index 00000000..94deacb0
--- /dev/null
+++ b/test/core/run/member_to_index.cpp
@@ -0,0 +1,26 @@
+// Copyright (c) 2025-2026 Antony Polukhin
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/pfr/core.hpp>
+
+#include <cassert>
+#include <string>
+#include <vector>
+
+struct Sample {
+	std::string x;
+	std::vector<int> y;
+	std::string z;
+};
+
+int main() {
+    assert(boost::pfr::index_of(&Sample::x) == 0);
+    assert(boost::pfr::index_of(&Sample::y) == 1);
+    assert(boost::pfr::index_of(&Sample::z) == 2);
+
+    static_assert(boost::pfr::index_of(Sample{}, &Sample::x) == 0);
+    static_assert(boost::pfr::index_of(Sample{}, &Sample::y) == 1);
+    static_assert(boost::pfr::index_of(Sample{}, &Sample::z) == 2);
+}

From 18e010f989f2dcaa0532a176a7bce7d6326270dd Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Mon, 5 Jan 2026 23:27:14 +0300
Subject: [PATCH 02/15] cleanup

---
 include/boost/pfr/core.hpp        | 33 +++----------------------------
 test/core/run/member_to_index.cpp | 11 +++--------
 2 files changed, 6 insertions(+), 38 deletions(-)

diff --git a/include/boost/pfr/core.hpp b/include/boost/pfr/core.hpp
index 0a88cfa4..f7c69e85 100644
--- a/include/boost/pfr/core.hpp
+++ b/include/boost/pfr/core.hpp
@@ -18,7 +18,7 @@
 #include <boost/pfr/detail/for_each_field.hpp>
 #include <boost/pfr/detail/make_integer_sequence.hpp>
 #include <boost/pfr/detail/tie_from_structure_tuple.hpp>
-#include <boost/pfr/detail/offset_based_getter.hpp>
+#include <boost/pfr/detail/fake_object.hpp>
 
 #include <boost/pfr/tuple_size.hpp>
 
@@ -280,7 +280,8 @@ constexpr detail::tie_from_structure_tuple<Elements...> tie_from_structure(Eleme
 }
 
 template <typename T, typename M>
-constexpr std::size_t index_of(const T& value, M T::*mem_ptr) {
+constexpr std::size_t index_of(M T::*mem_ptr) {
+    constexpr const T& value = boost::pfr::detail::fake_object<T>();
     constexpr auto size = boost::pfr::tuple_size_v<T>;
     std::size_t result = size;
 
@@ -305,34 +306,6 @@ constexpr std::size_t index_of(const T& value, M T::*mem_ptr) {
     return result;
 }
 
-// TODO: move into detail::
-template <class... Types>
-auto strip_references(detail::sequence_tuple::tuple<Types&...>) -> detail::sequence_tuple::tuple<Types&...>;
-
-template <typename T, typename M>
-std::size_t index_of(M T::*mem_ptr) {
-    using tuple_type = detail::tuple_of_aligned_storage_t<
-        decltype(strip_references(detail::tie_as_tuple(std::declval<T&>())))
-    >;
-    using converted_member_pointer_t = M tuple_type::*;
-
-    constexpr tuple_type t{};
-
-    // TODO: not allowed in constexpr + unspecified behavior
-    auto mem_pointer = reinterpret_cast<converted_member_pointer_t>(mem_ptr);
-
-    const void* pointer = std::addressof(t.*mem_pointer);
-    namespace sequence_tuple = boost::pfr::detail::sequence_tuple;
-    if (&sequence_tuple::get<0>(t) == pointer) {
-        return 0;
-    } else if (&sequence_tuple::get<1>(t) == pointer) {
-        return 1;
-    } else if (&sequence_tuple::get<2>(t) == pointer) {
-        return 2;
-    }
-    return 3;
-}
-
 BOOST_PFR_END_MODULE_EXPORT
 
 }} // namespace boost::pfr
diff --git a/test/core/run/member_to_index.cpp b/test/core/run/member_to_index.cpp
index 94deacb0..3def082f 100644
--- a/test/core/run/member_to_index.cpp
+++ b/test/core/run/member_to_index.cpp
@@ -5,7 +5,6 @@
 
 #include <boost/pfr/core.hpp>
 
-#include <cassert>
 #include <string>
 #include <vector>
 
@@ -16,11 +15,7 @@ struct Sample {
 };
 
 int main() {
-    assert(boost::pfr::index_of(&Sample::x) == 0);
-    assert(boost::pfr::index_of(&Sample::y) == 1);
-    assert(boost::pfr::index_of(&Sample::z) == 2);
-
-    static_assert(boost::pfr::index_of(Sample{}, &Sample::x) == 0);
-    static_assert(boost::pfr::index_of(Sample{}, &Sample::y) == 1);
-    static_assert(boost::pfr::index_of(Sample{}, &Sample::z) == 2);
+    static_assert(boost::pfr::index_of(&Sample::x) == 0);
+    static_assert(boost::pfr::index_of(&Sample::y) == 1);
+    static_assert(boost::pfr::index_of(&Sample::z) == 2);
 }

From 5f2aca762ced20a4ac89e6947bac53cac4aa4d77 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Tue, 6 Jan 2026 19:06:12 +0300
Subject: [PATCH 03/15] github editor is a piece of bad software

---
 include/boost/pfr/core.hpp | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/boost/pfr/core.hpp b/include/boost/pfr/core.hpp
index f7c69e85..9bb07626 100644
--- a/include/boost/pfr/core.hpp
+++ b/include/boost/pfr/core.hpp
@@ -287,7 +287,18 @@ constexpr std::size_t index_of(M T::*mem_ptr) {
 
     const void* target_address = std::addressof(value.*mem_ptr);
 
-    boost::pfr::for_each_field(value, [&result, target_address](const auto& field, std::size_t idx) {
+    struct visitor {
+        template <class T>
+        void operator()(const T&) {}
+
+        void operator()(const T& field) {
+            const void* filed_address = std::addressof(field);
+            if (target_address == filed_address) {
+                result = idx;
+            }
+        }
+};
+stdboost::pfr::oost::pfr::for(ea:ch_, [&result,  [&result, tar](const auto& field, std::size_t idx) {
         if (!std::is_same<decltype(field), const M&>::value) {
             return;
         }

From 24cd193eeebdc44e5f2b047a9f7554ab61890c93 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Tue, 6 Jan 2026 19:15:12 +0300
Subject: [PATCH 04/15] Update core.hpp

---
 include/boost/pfr/core.hpp | 32 +++++++++-----------------------
 1 file changed, 9 insertions(+), 23 deletions(-)

diff --git a/include/boost/pfr/core.hpp b/include/boost/pfr/core.hpp
index 9bb07626..a6aa5e9d 100644
--- a/include/boost/pfr/core.hpp
+++ b/include/boost/pfr/core.hpp
@@ -282,39 +282,25 @@ constexpr detail::tie_from_structure_tuple<Elements...> tie_from_structure(Eleme
 template <typename T, typename M>
 constexpr std::size_t index_of(M T::*mem_ptr) {
     constexpr const T& value = boost::pfr::detail::fake_object<T>();
-    constexpr auto size = boost::pfr::tuple_size_v<T>;
-    std::size_t result = size;
-
-    const void* target_address = std::addressof(value.*mem_ptr);
 
     struct visitor {
         template <class T>
-        void operator()(const T&) {}
+        void operator()(const T&, std::size_t) {}
 
-        void operator()(const T& field) {
+        void operator()(const T& field, std::size_t idx) {
             const void* filed_address = std::addressof(field);
             if (target_address == filed_address) {
                 result = idx;
             }
         }
-};
-stdboost::pfr::oost::pfr::for(ea:ch_, [&result,  [&result, tar](const auto& field, std::size_t idx) {
-        if (!std::is_same<decltype(field), const M&>::value) {
-            return;
-        }
-
-        if (result != size) {
-            // already found the answer
-            return;
-        }
-
-        const void* filed_address = std::addressof(field);
-        if (target_address == filed_address) {
-            result = idx;
-        }
-    });
 
-    return result;
+        const void* target_address;
+        std::size_t result;
+    };
+    visitor visit{std::addressof(value.*mem_ptr), boost::pfr::tuple_size_v<T>};
+    
+    boost::pfr::for_each_field(value, visit);
+    return visit.result;
 }
 
 BOOST_PFR_END_MODULE_EXPORT

From e6bc1dd9f2fc3b01a28ae6163cb7e8f0590770d7 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Tue, 6 Jan 2026 19:17:42 +0300
Subject: [PATCH 05/15] Update core.hpp

---
 include/boost/pfr/core.hpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/boost/pfr/core.hpp b/include/boost/pfr/core.hpp
index a6aa5e9d..c9c824ab 100644
--- a/include/boost/pfr/core.hpp
+++ b/include/boost/pfr/core.hpp
@@ -287,7 +287,7 @@ constexpr std::size_t index_of(M T::*mem_ptr) {
         template <class T>
         void operator()(const T&, std::size_t) {}
 
-        void operator()(const T& field, std::size_t idx) {
+        void operator()(const M& field, std::size_t idx) {
             const void* filed_address = std::addressof(field);
             if (target_address == filed_address) {
                 result = idx;

From d96e25e829732b24d8eeab2b77f653c89611ae44 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Tue, 6 Jan 2026 19:38:42 +0300
Subject: [PATCH 06/15] Update core.hpp

---
 include/boost/pfr/core.hpp | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/boost/pfr/core.hpp b/include/boost/pfr/core.hpp
index c9c824ab..c71efc49 100644
--- a/include/boost/pfr/core.hpp
+++ b/include/boost/pfr/core.hpp
@@ -279,10 +279,7 @@ constexpr detail::tie_from_structure_tuple<Elements...> tie_from_structure(Eleme
     return detail::tie_from_structure_tuple<Elements...>(args...);
 }
 
-template <typename T, typename M>
-constexpr std::size_t index_of(M T::*mem_ptr) {
-    constexpr const T& value = boost::pfr::detail::fake_object<T>();
-
+    template <typename M>
     struct visitor {
         template <class T>
         void operator()(const T&, std::size_t) {}
@@ -297,7 +294,12 @@ constexpr std::size_t index_of(M T::*mem_ptr) {
         const void* target_address;
         std::size_t result;
     };
-    visitor visit{std::addressof(value.*mem_ptr), boost::pfr::tuple_size_v<T>};
+
+template <typename T, typename M>
+constexpr std::size_t index_of(M T::*mem_ptr) {
+    constexpr const T& value = boost::pfr::detail::fake_object<T>();
+
+    visitor<M> visit{std::addressof(value.*mem_ptr), boost::pfr::tuple_size_v<T>};
     
     boost::pfr::for_each_field(value, visit);
     return visit.result;

From 361b8e7d02cf9db710869b2601c470becf8aff47 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Tue, 6 Jan 2026 20:01:16 +0300
Subject: [PATCH 07/15] Update core.hpp

---
 include/boost/pfr/core.hpp | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/boost/pfr/core.hpp b/include/boost/pfr/core.hpp
index c71efc49..1ba3f78a 100644
--- a/include/boost/pfr/core.hpp
+++ b/include/boost/pfr/core.hpp
@@ -282,11 +282,11 @@ constexpr detail::tie_from_structure_tuple<Elements...> tie_from_structure(Eleme
     template <typename M>
     struct visitor {
         template <class T>
-        void operator()(const T&, std::size_t) {}
+        constexpr void operator()(const T&, std::size_t) {}
 
-        void operator()(const M& field, std::size_t idx) {
-            const void* filed_address = std::addressof(field);
-            if (target_address == filed_address) {
+        constexpr void operator()(const M& field, std::size_t idx) {
+            const void* field_address = std::addressof(field);
+            if (target_address == field_address) {
                 result = idx;
             }
         }

From 93d02b86ce68d20627508d1dfd7655337c34d4b2 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Tue, 6 Jan 2026 20:43:12 +0300
Subject: [PATCH 08/15] Update for_each_field.hpp

---
 include/boost/pfr/detail/for_each_field.hpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/boost/pfr/detail/for_each_field.hpp b/include/boost/pfr/detail/for_each_field.hpp
index 33b3ec1f..c2d92ed3 100644
--- a/include/boost/pfr/detail/for_each_field.hpp
+++ b/include/boost/pfr/detail/for_each_field.hpp
@@ -42,14 +42,14 @@ constexpr void for_each_field(T&& value, F&& func) {
 
     ::boost::pfr::detail::for_each_field_dispatcher(
         value,
-        [f = std::forward<F>(func)](auto&& t) mutable {
+        [&func](auto&& t) mutable {
             // MSVC related workaround. Its lambdas do not capture constexprs.
             constexpr std::size_t fields_count_val_in_lambda
                 = boost::pfr::detail::fields_count<std::remove_reference_t<T>>();
 
             ::boost::pfr::detail::for_each_field_impl(
                 t,
-                std::forward<F>(f),
+                std::forward<F>(func),
                 detail::make_index_sequence<fields_count_val_in_lambda>{},
                 std::is_rvalue_reference<T&&>{}
             );

From c951f22eb731199f72f01bddc7a634d39736d330 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Tue, 6 Jan 2026 21:10:06 +0300
Subject: [PATCH 09/15] Update member_to_index.cpp

---
 test/core/run/member_to_index.cpp | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/test/core/run/member_to_index.cpp b/test/core/run/member_to_index.cpp
index 3def082f..72b1333b 100644
--- a/test/core/run/member_to_index.cpp
+++ b/test/core/run/member_to_index.cpp
@@ -5,6 +5,7 @@
 
 #include <boost/pfr/core.hpp>
 
+#include <cassert>
 #include <string>
 #include <vector>
 
@@ -15,7 +16,14 @@ struct Sample {
 };
 
 int main() {
+#if BOOST_PFR_USE_CPP17 || BOOST_PFR_USE_CPP26
     static_assert(boost::pfr::index_of(&Sample::x) == 0);
     static_assert(boost::pfr::index_of(&Sample::y) == 1);
     static_assert(boost::pfr::index_of(&Sample::z) == 2);
+#else
+    assert(boost::pfr::index_of(&Sample::x) == 0);
+	= 0
+    assert(boost::pfr::index_of(&Sample::y) == 1);
+    assert(boost::pfr::index_of(&Sample::z) == 2);
+#endif
 }

From 9cb98e103a3c722883f96aa37255c36f6388963b Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Tue, 6 Jan 2026 21:10:24 +0300
Subject: [PATCH 10/15] Update member_to_index.cpp

---
 test/core/run/member_to_index.cpp | 1 -
 1 file changed, 1 deletion(-)

diff --git a/test/core/run/member_to_index.cpp b/test/core/run/member_to_index.cpp
index 72b1333b..f734ad96 100644
--- a/test/core/run/member_to_index.cpp
+++ b/test/core/run/member_to_index.cpp
@@ -22,7 +22,6 @@ int main() {
     static_assert(boost::pfr::index_of(&Sample::z) == 2);
 #else
     assert(boost::pfr::index_of(&Sample::x) == 0);
-	= 0
     assert(boost::pfr::index_of(&Sample::y) == 1);
     assert(boost::pfr::index_of(&Sample::z) == 2);
 #endif

From e360f0700d73f697da3a22c5bab22534d9d9c8b5 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Mon, 12 Jan 2026 20:32:43 +0300
Subject: [PATCH 11/15] cleanup

---
 include/boost/pfr.hpp             |  1 +
 include/boost/pfr/core.hpp        | 28 ------------
 include/boost/pfr/index_of.hpp    | 72 +++++++++++++++++++++++++++++++
 test/core/run/member_to_index.cpp | 13 +++---
 4 files changed, 81 insertions(+), 33 deletions(-)
 create mode 100644 include/boost/pfr/index_of.hpp

diff --git a/include/boost/pfr.hpp b/include/boost/pfr.hpp
index ce447256..555bdf91 100644
--- a/include/boost/pfr.hpp
+++ b/include/boost/pfr.hpp
@@ -14,6 +14,7 @@
 #include <boost/pfr/core_name.hpp>
 #include <boost/pfr/functions_for.hpp>
 #include <boost/pfr/functors.hpp>
+#include <boost/pfr/index_of.hpp>
 #include <boost/pfr/io.hpp>
 #include <boost/pfr/io_fields.hpp>
 #include <boost/pfr/ops.hpp>
diff --git a/include/boost/pfr/core.hpp b/include/boost/pfr/core.hpp
index 1ba3f78a..d621f273 100644
--- a/include/boost/pfr/core.hpp
+++ b/include/boost/pfr/core.hpp
@@ -18,11 +18,9 @@
 #include <boost/pfr/detail/for_each_field.hpp>
 #include <boost/pfr/detail/make_integer_sequence.hpp>
 #include <boost/pfr/detail/tie_from_structure_tuple.hpp>
-#include <boost/pfr/detail/fake_object.hpp>
 
 #include <boost/pfr/tuple_size.hpp>
 
-
 #if !defined(BOOST_PFR_INTERFACE_UNIT)
 #include <type_traits>
 #include <utility>      // metaprogramming stuff
@@ -279,32 +277,6 @@ constexpr detail::tie_from_structure_tuple<Elements...> tie_from_structure(Eleme
     return detail::tie_from_structure_tuple<Elements...>(args...);
 }
 
-    template <typename M>
-    struct visitor {
-        template <class T>
-        constexpr void operator()(const T&, std::size_t) {}
-
-        constexpr void operator()(const M& field, std::size_t idx) {
-            const void* field_address = std::addressof(field);
-            if (target_address == field_address) {
-                result = idx;
-            }
-        }
-
-        const void* target_address;
-        std::size_t result;
-    };
-
-template <typename T, typename M>
-constexpr std::size_t index_of(M T::*mem_ptr) {
-    constexpr const T& value = boost::pfr::detail::fake_object<T>();
-
-    visitor<M> visit{std::addressof(value.*mem_ptr), boost::pfr::tuple_size_v<T>};
-    
-    boost::pfr::for_each_field(value, visit);
-    return visit.result;
-}
-
 BOOST_PFR_END_MODULE_EXPORT
 
 }} // namespace boost::pfr
diff --git a/include/boost/pfr/index_of.hpp b/include/boost/pfr/index_of.hpp
new file mode 100644
index 00000000..2b2a4865
--- /dev/null
+++ b/include/boost/pfr/index_of.hpp
@@ -0,0 +1,72 @@
+// Copyright (c) 2025-2026 Antony Polukhin
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_PFR_INDEX_OF_HPP
+#define BOOST_PFR_INDEX_OF_HPP
+#pragma once
+
+#include <boost/pfr/detail/config.hpp>
+
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_PFR_INTERFACE_UNIT)
+
+#include <boost/pfr/detail/fake_object.hpp>
+#include <boost/pfr/detail/for_each_field.hpp>
+
+
+#if !defined(BOOST_PFR_INTERFACE_UNIT)
+#include <type_traits>
+#include <utility>      // metaprogramming stuff
+#endif
+
+/// \file boost/pfr/index_of.hpp
+/// \copybrief boost::pfr::index_of
+
+namespace boost { namespace pfr {
+
+namespace detail {
+
+template <typename M>
+struct address_comparing_visitor {
+    template <class T>
+    constexpr void operator()(const T&, std::size_t) noexcept {}
+
+    constexpr void operator()(const M& field, std::size_t idx) noexcept {
+        const void* field_address = std::addressof(field);
+        if (target_address == field_address) {
+            result = idx;
+        }
+    }
+
+    const void* const target_address;
+    std::size_t result;
+};
+
+}
+
+BOOST_PFR_BEGIN_MODULE_EXPORT
+
+template <typename T, typename M>
+constexpr std::size_t index_of(M T::*mem_ptr) noexcept {
+    if (mem_ptr == nullptr) {
+        return ~static_cast<std::size_t>(0);
+    }
+
+    constexpr const T& value = boost::pfr::detail::fake_object<T>();
+    detail::address_comparing_visitor<M> visitor{
+        std::addressof(value.*mem_ptr),
+        ~static_cast<std::size_t>(0)
+    };
+    ::boost::pfr::detail::for_each_field(value, visitor);
+
+    return visitor.result;
+}
+
+BOOST_PFR_END_MODULE_EXPORT
+
+}} // namespace boost::pfr
+
+#endif  // #if !defined(BOOST_USE_MODULES) || defined(BOOST_PFR_INTERFACE_UNIT)
+
+#endif // BOOST_PFR_INDEX_OF_HPP
diff --git a/test/core/run/member_to_index.cpp b/test/core/run/member_to_index.cpp
index f734ad96..22488216 100644
--- a/test/core/run/member_to_index.cpp
+++ b/test/core/run/member_to_index.cpp
@@ -3,9 +3,9 @@
 // Distributed under the Boost Software License, Version 1.0. (See accompanying
 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 
-#include <boost/pfr/core.hpp>
+#include <boost/pfr/index_of.hpp>
 
-#include <cassert>
+#include <limits>
 #include <string>
 #include <vector>
 
@@ -21,8 +21,11 @@ int main() {
     static_assert(boost::pfr::index_of(&Sample::y) == 1);
     static_assert(boost::pfr::index_of(&Sample::z) == 2);
 #else
-    assert(boost::pfr::index_of(&Sample::x) == 0);
-    assert(boost::pfr::index_of(&Sample::y) == 1);
-    assert(boost::pfr::index_of(&Sample::z) == 2);
+    if (boost::pfr::index_of(&Sample::x) != 0) return 1;
+    if (boost::pfr::index_of(&Sample::y) != 1) return 2;
+    if (boost::pfr::index_of(&Sample::z) != 2) return 3;
 #endif
+
+    decltype(&Sample::x) mem_ptr = nullptr;
+    if (boost::pfr::index_of(mem_ptr) != (std::numeric_limits<std::size_t>::max)()) return 4;
 }

From f20f0b0b79bf7961bca0c7d9c38f6dcb8a1085c7 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Mon, 12 Jan 2026 20:38:51 +0300
Subject: [PATCH 12/15] provide a portable version

---
 include/boost/pfr/index_of.hpp | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/boost/pfr/index_of.hpp b/include/boost/pfr/index_of.hpp
index 2b2a4865..54f587ec 100644
--- a/include/boost/pfr/index_of.hpp
+++ b/include/boost/pfr/index_of.hpp
@@ -48,12 +48,11 @@ struct address_comparing_visitor {
 BOOST_PFR_BEGIN_MODULE_EXPORT
 
 template <typename T, typename M>
-constexpr std::size_t index_of(M T::*mem_ptr) noexcept {
+constexpr std::size_t index_of(const T& value, M T::*mem_ptr) noexcept {
     if (mem_ptr == nullptr) {
         return ~static_cast<std::size_t>(0);
     }
 
-    constexpr const T& value = boost::pfr::detail::fake_object<T>();
     detail::address_comparing_visitor<M> visitor{
         std::addressof(value.*mem_ptr),
         ~static_cast<std::size_t>(0)
@@ -63,6 +62,12 @@ constexpr std::size_t index_of(M T::*mem_ptr) noexcept {
     return visitor.result;
 }
 
+template <typename T, typename M>
+constexpr std::size_t index_of(M T::*mem_ptr) noexcept {
+    constexpr const T& value = boost::pfr::detail::fake_object<T>();
+    return ::boost::pfr::index_of(value, mem_ptr);
+}
+
 BOOST_PFR_END_MODULE_EXPORT
 
 }} // namespace boost::pfr

From 4f285bc9d4deafc701467754fcc80e719a65dcc6 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Thu, 15 Jan 2026 19:21:32 +0300
Subject: [PATCH 13/15] wip

---
 include/boost/pfr/index_of.hpp    |  7 +++----
 test/core/run/member_to_index.cpp | 26 ++++++++++++++++++--------
 2 files changed, 21 insertions(+), 12 deletions(-)

diff --git a/include/boost/pfr/index_of.hpp b/include/boost/pfr/index_of.hpp
index 54f587ec..17660087 100644
--- a/include/boost/pfr/index_of.hpp
+++ b/include/boost/pfr/index_of.hpp
@@ -11,7 +11,6 @@
 
 #if !defined(BOOST_USE_MODULES) || defined(BOOST_PFR_INTERFACE_UNIT)
 
-#include <boost/pfr/detail/fake_object.hpp>
 #include <boost/pfr/detail/for_each_field.hpp>
 
 
@@ -48,7 +47,7 @@ struct address_comparing_visitor {
 BOOST_PFR_BEGIN_MODULE_EXPORT
 
 template <typename T, typename M>
-constexpr std::size_t index_of(const T& value, M T::*mem_ptr) noexcept {
+constexpr std::size_t index_of(M T::*mem_ptr, const T& value) noexcept {
     if (mem_ptr == nullptr) {
         return ~static_cast<std::size_t>(0);
     }
@@ -64,8 +63,8 @@ constexpr std::size_t index_of(const T& value, M T::*mem_ptr) noexcept {
 
 template <typename T, typename M>
 constexpr std::size_t index_of(M T::*mem_ptr) noexcept {
-    constexpr const T& value = boost::pfr::detail::fake_object<T>();
-    return ::boost::pfr::index_of(value, mem_ptr);
+    constexpr T value{};
+    return ::boost::pfr::index_of(mem_ptr, value);
 }
 
 BOOST_PFR_END_MODULE_EXPORT
diff --git a/test/core/run/member_to_index.cpp b/test/core/run/member_to_index.cpp
index 22488216..e50b4403 100644
--- a/test/core/run/member_to_index.cpp
+++ b/test/core/run/member_to_index.cpp
@@ -15,17 +15,27 @@ struct Sample {
 	std::string z;
 };
 
+struct TrivialSample {
+	int x;
+	short y;
+	bool z;
+};
+
 int main() {
 #if BOOST_PFR_USE_CPP17 || BOOST_PFR_USE_CPP26
-    static_assert(boost::pfr::index_of(&Sample::x) == 0);
-    static_assert(boost::pfr::index_of(&Sample::y) == 1);
-    static_assert(boost::pfr::index_of(&Sample::z) == 2);
-#else
-    if (boost::pfr::index_of(&Sample::x) != 0) return 1;
-    if (boost::pfr::index_of(&Sample::y) != 1) return 2;
-    if (boost::pfr::index_of(&Sample::z) != 2) return 3;
+    static_assert(boost::pfr::index_of(&TrivialSample::x) == 0);
+    static_assert(boost::pfr::index_of(&TrivialSample::y) == 1);
+    static_assert(boost::pfr::index_of(&TrivialSample::z) == 2);
 #endif
 
+    if (boost::pfr::index_of(&Sample::x, {}) != 0) return 1;
+    if (boost::pfr::index_of(&Sample::y, {}) != 1) return 2;
+    if (boost::pfr::index_of(&Sample::z, {}) != 2) return 3;
+
     decltype(&Sample::x) mem_ptr = nullptr;
-    if (boost::pfr::index_of(mem_ptr) != (std::numeric_limits<std::size_t>::max)()) return 4;
+    if (boost::pfr::index_of(mem_ptr, {}) != (std::numeric_limits<std::size_t>::max)()) return 4;
+
+    if (boost::pfr::index_of(&TrivialSample::x) != 0) return 11;
+    if (boost::pfr::index_of(&TrivialSample::y) != 1) return 12;
+    if (boost::pfr::index_of(&TrivialSample::z) != 2) return 13;
 }

From 5d30e2d14bfd5d963bae4faeaaf8725ac68cc3c2 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Thu, 22 Jan 2026 12:22:45 +0300
Subject: [PATCH 14/15] wip

---
 include/boost/pfr.hpp                         |  1 +
 include/boost/pfr/index_of.hpp                | 15 ++++--
 .../is_constexpr_default_constructible.hpp    | 49 +++++++++++++++++++
 test/core/run/is_constexpr_constructible.cpp  | 41 ++++++++++++++++
 test/core/run/member_to_index.cpp             | 17 +++++++
 5 files changed, 120 insertions(+), 3 deletions(-)
 create mode 100644 include/boost/pfr/is_constexpr_default_constructible.hpp
 create mode 100644 test/core/run/is_constexpr_constructible.cpp

diff --git a/include/boost/pfr.hpp b/include/boost/pfr.hpp
index 555bdf91..8f51ce84 100644
--- a/include/boost/pfr.hpp
+++ b/include/boost/pfr.hpp
@@ -17,6 +17,7 @@
 #include <boost/pfr/index_of.hpp>
 #include <boost/pfr/io.hpp>
 #include <boost/pfr/io_fields.hpp>
+#include <boost/pfr/is_constexpr_default_constructible.hpp>
 #include <boost/pfr/ops.hpp>
 #include <boost/pfr/ops_fields.hpp>
 #include <boost/pfr/tuple_size.hpp>
diff --git a/include/boost/pfr/index_of.hpp b/include/boost/pfr/index_of.hpp
index 17660087..df994e4e 100644
--- a/include/boost/pfr/index_of.hpp
+++ b/include/boost/pfr/index_of.hpp
@@ -12,11 +12,12 @@
 #if !defined(BOOST_USE_MODULES) || defined(BOOST_PFR_INTERFACE_UNIT)
 
 #include <boost/pfr/detail/for_each_field.hpp>
+#include <boost/pfr/is_constexpr_default_constructible.hpp>
 
 
 #if !defined(BOOST_PFR_INTERFACE_UNIT)
 #include <type_traits>
-#include <utility>      // metaprogramming stuff
+#include <memory>      // std::addressof
 #endif
 
 /// \file boost/pfr/index_of.hpp
@@ -63,8 +64,16 @@ constexpr std::size_t index_of(M T::*mem_ptr, const T& value) noexcept {
 
 template <typename T, typename M>
 constexpr std::size_t index_of(M T::*mem_ptr) noexcept {
-    constexpr T value{};
-    return ::boost::pfr::index_of(mem_ptr, value);
+    static_assert(
+        boost::pfr::is_constexpr_default_constructible_v<T>,
+        "====================> Boost.PFR: T should be default constructible in constant evaluations. "
+        "Either add `constexpr` to constructors of member types or use the boost::pfr::index_of() overload "
+        "with explicit `const T& value` parameter."
+    );
+
+    // boost::pfr::is_constexpr_default_constructible_v<T> gives a faint hope that the
+    // compiler would be able to optimize away the temporary `T{}`.
+    return ::boost::pfr::index_of(mem_ptr, T{});
 }
 
 BOOST_PFR_END_MODULE_EXPORT
diff --git a/include/boost/pfr/is_constexpr_default_constructible.hpp b/include/boost/pfr/is_constexpr_default_constructible.hpp
new file mode 100644
index 00000000..05976d14
--- /dev/null
+++ b/include/boost/pfr/is_constexpr_default_constructible.hpp
@@ -0,0 +1,49 @@
+// Copyright (c) 2025-2026 Antony Polukhin
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_PFR_IS_CONSTEXPR_DEFAULT_CONSTRUCTIBLE_HPP
+#define BOOST_PFR_IS_CONSTEXPR_DEFAULT_CONSTRUCTIBLE_HPP
+#pragma once
+
+#include <boost/pfr/detail/config.hpp>
+
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_PFR_INTERFACE_UNIT)
+
+#if !defined(BOOST_PFR_INTERFACE_UNIT)
+#include <type_traits>
+#endif
+
+namespace boost { namespace pfr {
+
+namespace detail {
+    template <class T, int = (static_cast<void>(T{}), 0)>
+    constexpr std::true_type is_constexpr_default_constructible(long) noexcept {
+        return {};
+    }
+
+    template <class T>
+    constexpr std::false_type is_constexpr_default_constructible(int) noexcept {
+        return {};
+    }
+}  // namespace detail
+
+BOOST_PFR_BEGIN_MODULE_EXPORT
+
+template <class T>
+struct is_constexpr_default_constructible
+    : decltype(detail::is_constexpr_default_constructible<T>(1L))
+{};
+
+template <class T>
+constexpr bool is_constexpr_default_constructible_v
+    = decltype(detail::is_constexpr_default_constructible<T>(1L))::value;
+
+BOOST_PFR_END_MODULE_EXPORT
+
+}}  // namespace boost::pfr
+
+#endif  // #if !defined(BOOST_USE_MODULES) || defined(BOOST_PFR_INTERFACE_UNIT)
+
+#endif  // BOOST_PFR_IS_CONSTEXPR_DEFAULT_CONSTRUCTIBLE_HPP
diff --git a/test/core/run/is_constexpr_constructible.cpp b/test/core/run/is_constexpr_constructible.cpp
new file mode 100644
index 00000000..2eca8665
--- /dev/null
+++ b/test/core/run/is_constexpr_constructible.cpp
@@ -0,0 +1,41 @@
+// Copyright (c) 2025-2026 Antony Polukhin
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/pfr/is_constexpr_default_constructible.hpp>
+
+#include <string>
+
+struct Yes {
+    int a;
+    double b;
+    char c;
+};
+
+struct Yes2 {
+    int a;
+    double b;
+    char c;
+    Yes d[42];
+};
+
+class MyClass{
+public:
+    MyClass();
+    ~MyClass();
+};
+
+struct No {
+    int a;
+    double b;
+    char c;
+    std::string str;
+    MyClass mc;
+};
+
+int main() {
+    static_assert(is_constexpr_default_constructible_v<Yes>);
+    static_assert(is_constexpr_default_constructible_v<Yes2>);
+    static_assert(!is_constexpr_default_constructible_v<No>);
+}
diff --git a/test/core/run/member_to_index.cpp b/test/core/run/member_to_index.cpp
index e50b4403..694c9da4 100644
--- a/test/core/run/member_to_index.cpp
+++ b/test/core/run/member_to_index.cpp
@@ -21,6 +21,19 @@ struct TrivialSample {
 	bool z;
 };
 
+template <class T>
+int test_if_constexpr_default_constructible(std::false_type) {
+    return 0;
+}
+
+template <class T>
+int test_if_constexpr_default_constructible(std::true_type) {
+    if (boost::pfr::index_of(&T::x) != 0) return 31;
+    if (boost::pfr::index_of(&T::y) != 1) return 32;
+    if (boost::pfr::index_of(&T::z) != 2) return 33;
+    return 0;
+}
+
 int main() {
 #if BOOST_PFR_USE_CPP17 || BOOST_PFR_USE_CPP26
     static_assert(boost::pfr::index_of(&TrivialSample::x) == 0);
@@ -38,4 +51,8 @@ int main() {
     if (boost::pfr::index_of(&TrivialSample::x) != 0) return 11;
     if (boost::pfr::index_of(&TrivialSample::y) != 1) return 12;
     if (boost::pfr::index_of(&TrivialSample::z) != 2) return 13;
+
+    return test_if_constexpr_default_constructible<Sample>(
+        boost::pfr::is_constexpr_default_constructible<Sample>{}
+    );
 }

From 4328226b02bde2c589fee125868a57daa466cd36 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Thu, 22 Jan 2026 12:34:49 +0300
Subject: [PATCH 15/15] fix

---
 test/core/run/is_constexpr_constructible.cpp | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/test/core/run/is_constexpr_constructible.cpp b/test/core/run/is_constexpr_constructible.cpp
index 2eca8665..5a22bf6e 100644
--- a/test/core/run/is_constexpr_constructible.cpp
+++ b/test/core/run/is_constexpr_constructible.cpp
@@ -35,7 +35,7 @@ struct No {
 };
 
 int main() {
-    static_assert(is_constexpr_default_constructible_v<Yes>);
-    static_assert(is_constexpr_default_constructible_v<Yes2>);
-    static_assert(!is_constexpr_default_constructible_v<No>);
+    static_assert(is_constexpr_default_constructible_v<Yes>, "");
+    static_assert(is_constexpr_default_constructible_v<Yes2>, "");
+    static_assert(!is_constexpr_default_constructible_v<No>, "");
 }