Adding further overrides of enable_if_initializable_helper for c++20 …

include/boost/pfr/config.hpp

@@ -88,6 +88,14 @@
 #   endif
 #endif
 
+#ifndef BOOST_PFR_USE_CPP20
+#   if __cpp_concepts >= 201907L || (defined(_MSVC_LANG) && _MSVC_LANG >= 202002L)
+#       define BOOST_PFR_USE_CPP20 1
+#   else
+#       define BOOST_PFR_USE_CPP20 0
+#   endif
+#endif
+
 #if (!BOOST_PFR_USE_CPP26 && !BOOST_PFR_USE_CPP17 && !BOOST_PFR_USE_LOOPHOLE)
 #   if (defined(_MSC_VER) && _MSC_VER < 1916) ///< in Visual Studio 2017 v15.9 PFR library with classic engine normally works
 #      define BOOST_PFR_NOT_SUPPORTED 1

include/boost/pfr/detail/fields_count.hpp

@@ -182,13 +182,36 @@ constexpr auto assert_first_not_base(long) noexcept
 
 ///////////////////// Helpers for initializable detection
 // Note that these take O(N) compile time and memory!
+// In C++20+, the N=1 probe uses braces (same as C++17: correct for empty aggregates
+// and non-default-constructible trailing members; T{ubiq} is aggregate-init, not
+// copy-construction).  N>=2 probes use parenthesized aggregate init T(ubiq...)
+// (P0960R3), which does not perform brace elision, so structs with C-style array
+// members are counted correctly (1 field, not sizeof(array)/sizeof(element)).
+#if BOOST_PFR_USE_CPP20 && !BOOST_PFR_USE_CPP26 && !BOOST_PFR_USE_CPP26_REFLECTION
+template <class T, std::size_t... I, class /*Enable*/ = std::enable_if_t<std::is_copy_constructible<T>::value && (sizeof...(I) <= 1)>>
+constexpr auto enable_if_initializable_helper(std::index_sequence<I...>) noexcept
+    -> std::add_pointer_t<decltype(T{ubiq_lref_constructor{I}...})>;
+
+template <class T, std::size_t... I, class /*Enable*/ = std::enable_if_t<!std::is_copy_constructible<T>::value && (sizeof...(I) <= 1)>>
+constexpr auto enable_if_initializable_helper(std::index_sequence<I...>) noexcept
+    -> std::add_pointer_t<decltype(T{ubiq_rref_constructor{I}...})>;
+
+template <class T, std::size_t... I, class /*Enable*/ = std::enable_if_t<std::is_copy_constructible<T>::value && (sizeof...(I) >= 2)>>
+constexpr auto enable_if_initializable_helper(std::index_sequence<I...>) noexcept
+    -> std::add_pointer_t<decltype(T(ubiq_lref_constructor{I}...))>;
+
+template <class T, std::size_t... I, class /*Enable*/ = std::enable_if_t<!std::is_copy_constructible<T>::value && (sizeof...(I) >= 2)>>
+constexpr auto enable_if_initializable_helper(std::index_sequence<I...>) noexcept
+    -> std::add_pointer_t<decltype(T(ubiq_rref_constructor{I}...))>;
+#else
 template <class T, std::size_t... I, class /*Enable*/ = std::enable_if_t<std::is_copy_constructible<T>::value>>
 constexpr auto enable_if_initializable_helper(std::index_sequence<I...>) noexcept
     -> std::add_pointer_t<decltype(T{ubiq_lref_constructor{I}...})>;
 
 template <class T, std::size_t... I, class /*Enable*/ = std::enable_if_t<!std::is_copy_constructible<T>::value>>
 constexpr auto enable_if_initializable_helper(std::index_sequence<I...>) noexcept
     -> std::add_pointer_t<decltype(T{ubiq_rref_constructor{I}...})>;
+#endif
 
 template <class T, std::size_t N, class U = std::size_t, class /*Enable*/ = decltype(detail::enable_if_initializable_helper<T>(detail::make_index_sequence<N>()))>
 using enable_if_initializable_helper_t = U;

test/core/run/struct_with_array_field.cpp

@@ -0,0 +1,23 @@
+#include <boost/pfr/core.hpp>
+
+// Structs whose only field is a C-style array.  Without the C++20 fix,
+// brace elision in T{ubiq...} lets individual array elements absorb separate
+// ubiq values, so fields_count returns the array element count instead of 1.
+//
+// Note: structs that mix an array field with other fields (e.g.
+// struct { int arr[4]; int x; }) are not tested here because the ubiq-based
+// field-count probe cannot directly initialize an array member in
+// parenthesized aggregate init (arrays are not copy-constructible), so the
+// count would be wrong in both C++17 and C++20 modes for such types.
+
+struct single_int_array  { int  data[4];  };
+struct single_char_array { char buf[16];  };
+struct single_big_array  { int  data[32]; };
+
+int main() {
+#if BOOST_PFR_USE_CPP20
+    static_assert(boost::pfr::tuple_size_v<single_int_array>  == 1, "single_int_array");
+    static_assert(boost::pfr::tuple_size_v<single_char_array> == 1, "single_char_array");
+    static_assert(boost::pfr::tuple_size_v<single_big_array>  == 1, "single_big_array");
+#endif
+}