fix definition

include/boost/pfr/detail/core14_classic.hpp

@@ -18,6 +18,7 @@
 #include <boost/pfr/detail/make_flat_tuple_of_references.hpp>
 #include <boost/pfr/detail/make_integer_sequence.hpp>
 #include <boost/pfr/detail/size_array.hpp>
+#include <boost/pfr/detail/size_t_.hpp>
 #include <boost/pfr/detail/rvalue_t.hpp>
 
 #ifdef __clang__
@@ -32,10 +33,7 @@ namespace boost { namespace pfr { namespace detail {
 
 ///////////////////// General utility stuff
 
-template <std::size_t Index>
-using size_t_ = std::integral_constant<std::size_t, Index >;
-
-template <class T> struct identity{
+template <class T> struct identity {
     typedef T type;
 };
 

include/boost/pfr/detail/fields_count.hpp

@@ -37,7 +37,9 @@ struct ubiq_lref_constructor {
 ///////////////////// Structure that can be converted to rvalue reference to anything
 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
+    template <class Type> /*constexpr*/ operator Type&&() const noexcept {  // Allows initialization of rvalue reference fields and move-only types
+        return detail::unsafe_declval<Type&&>();
+    };
 };
 
 

include/boost/pfr/detail/unsafe_declval.hpp

@@ -12,11 +12,20 @@
 
 namespace boost { namespace pfr { namespace detail {
 
-// For returning non default constructible types. Never used at runtime! GCC's
-// std::declval may not be used in potentionally evaluated contexts, so it does not work here.
-template <class T> constexpr T unsafe_declval() noexcept {
+// This function serves as a link-time assert. If linker requires it, then
+// `unsafe_declval()` is used at runtime.
+void report_if_you_see_link_error_with_this_function() noexcept;
+
+// For returning non default constructible types. Do NOT use at runtime!
+//
+// GCC's std::declval may not be used in potentionally evaluated contexts,
+// so we reinvent it.
+template <class T>
+constexpr T unsafe_declval() noexcept {
+    report_if_you_see_link_error_with_this_function();
+
     typename std::remove_reference<T>::type* ptr = 0;
-    ptr += 42; // killing 'null pointer dereference' heuristics of static analysis tools
+    ptr += 42; // suppresses 'null pointer dereference' warnings
     return static_cast<T>(*ptr);
 }
 

test/precise/tie_anonymous.cpp

@@ -59,7 +59,7 @@ void test_in_anon_ns() {
     BOOST_TEST_EQ(std::get<1>(v).data, 2);
 
 #ifdef __cpp_lib_optional
-    other_anon_with_optional opt{"test"};
+    other_anon_with_optional opt{"test", {}, {}, {}};
     auto opt_val = boost::pfr::structure_tie(opt);
     BOOST_TEST_EQ(std::get<0>(opt_val), "test");
 #endif
@@ -76,7 +76,7 @@ void test_in_non_non_ns() {
     BOOST_TEST_EQ(std::get<1>(v).data, 2);
 
 #ifdef __cpp_lib_optional
-    other_anon_with_optional opt{"test again"};
+    other_anon_with_optional opt{"test again", {}, {}, {}};
     auto opt_val = boost::pfr::structure_tie(opt);
     BOOST_TEST_EQ(std::get<0>(opt_val), "test again");
 #endif