fields_count doesn't work for wrapper with unique_ptr

[MBkkt]

Setup

• compiler: clang-19
• c++ standard: 26
• libc++: libc++, abi v2, patched (without constexpr dtor for unique_ptr)
• pfr

#define BOOST_PFR_USE_STD_MAKE_INTEGRAL_SEQUENCE 1
#include <boost/pfr.hpp>
#include <boost/pfr/detail/for_each_field.hpp>
#include <boost/pfr/tuple_size.hpp>

static_assert(BOOST_PFR_USE_CPP17);
static_assert(BOOST_PFR_USE_LOOPHOLE == 0);
static_assert(BOOST_PFR_USE_STD_MAKE_INTEGRAL_SEQUENCE);
static_assert(BOOST_PFR_HAS_GUARANTEED_COPY_ELISION);
static_assert(BOOST_PFR_ENABLE_IMPLICIT_REFLECTION);
static_assert(BOOST_PFR_CORE_NAME_ENABLED);
#ifndef BOOST_PFR_FUNCTION_SIGNATURE
static_assert(false);
#endif
#ifndef BOOST_PFR_CORE_NAME_PARSING
static_assert(false);
#endif
static_assert(BOOST_PFR_ENABLED);

Test

template<typename T>
struct Field {
  using Type = T;

  template<typename... Args>
    requires(requires { T{std::declval<Args&&>()...}; })
  constexpr Field(Args&&... args) : _value{std::forward<Args>(args)...} {
  }

  constexpr auto& operator*(this auto& self) { return self._value; }
  constexpr auto* operator->(this auto& self) {
    return std::addressof(self._value);
  }

  constexpr bool operator==(const Field& rhs) const = default;
  constexpr auto operator<=>(const Field& rhs) const = default;

 private:
  [[no_unique_address]] T _value;
};

struct CustomFields2 {
  Field<int> some_int;
  Field<std::vector<int>> some_vector;
  Field<std::unique_ptr<int>> some_unique_ptr;
  Field<std::shared_ptr<int>> some_shared_ptr;

  bool operator==(const CustomFields2&) const = default;
};

TEST(SerializerTest, testCustomFields) {
  CustomFields2 cf2;
  boost::pfr::for_each_field_with_name(cf2, [](const auto&, const auto&) {});
}

Error

looks pretty oblivious

In file included from /home/mironov/projects/serenedb/tests/basics/serializer_test.cpp:15:
In file included from /home/mironov/projects/serenedb/libs/basics/serializer.h:26:
In file included from /home/mironov/projects/serenedb/third_party/boost/boost/pfr.hpp:13:
In file included from /home/mironov/projects/serenedb/third_party/boost/boost/pfr/core.hpp:12:
In file included from /home/mironov/projects/serenedb/third_party/boost/boost/pfr/detail/core.hpp:17:
In file included from /home/mironov/projects/serenedb/third_party/boost/boost/pfr/detail/core17.hpp:12:
In file included from /home/mironov/projects/serenedb/third_party/boost/boost/pfr/detail/fields_count.hpp:13:
/home/mironov/projects/serenedb/third_party/boost/boost/pfr/detail/unsafe_declval.hpp:34:12: error: static_cast from 'typename std::remove_reference<unique_ptr<int>>::type' (aka 'std::unique_ptr<int>') to 'std::unique_ptr<int>' uses deleted function
   34 |     return static_cast<T>(*ptr);
      |            ^~~~~~~~~~~~~~~~~~~~
/home/mironov/projects/serenedb/third_party/boost/boost/pfr/detail/fields_count.hpp:55:24: note: in instantiation of function template specialization 'boost::pfr::detail::unsafe_declval<std::unique_ptr<int>>' requested here
   55 |         return detail::unsafe_declval<Type>();
      |                        ^
/home/mironov/projects/serenedb/tests/basics/serializer_test.cpp:702:44: note: in instantiation of function template specialization 'boost::pfr::detail::ubiq_rref_constructor::operator unique_ptr<std::unique_ptr<int>>' requested here
  702 |   constexpr Field(Args&&... args) : _value{std::forward<Args>(args)...} {
      |                                            ^
/home/mironov/projects/serenedb/build/third_party/llvm/runtimes/include/c++/v1/__memory/unique_ptr.h:138:59: note: candidate constructor (the implicit copy constructor) has been implicitly deleted
  138 | class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS unique_ptr {
      |                                                           ^
1 error generated.
ninja: build stopped: cannot make progress due to previous errors.

Patch to fix issue

diff --git a/third_party/boost/boost/pfr/detail/fields_count.hpp b/third_party/boost/boost/pfr/detail/fields_count.hpp
index c3397b3b9..159af79e8 100644
--- a/third_party/boost/boost/pfr/detail/fields_count.hpp
+++ b/third_party/boost/boost/pfr/detail/fields_count.hpp
@@ -52,7 +52,7 @@ struct ubiq_lref_constructor {
 struct ubiq_rref_constructor {
     std::size_t ignore;
     template <class Type> /*constexpr*/ operator Type() const && noexcept {  // Allows initialization of rvalue reference fields and move-only types
-        return detail::unsafe_declval<Type>();
+        return detail::unsafe_declval<Type&&>();
     }
 };
 
@@ -144,7 +144,7 @@ struct ubiq_rref_base_asserting {
     template <class Type> /*constexpr*/ operator Type() const &&  // Allows initialization of rvalue reference fields and move-only types
         noexcept(detail::static_assert_non_inherited<Derived, Type>())  // force the computation of assert function
     {
-        return detail::unsafe_declval<Type>();
+        return detail::unsafe_declval<Type&&>();
     }
 };