Improve diagnostic for attempt to use get_name with non-external (#146

)

Co-authored-by: Antony Polukhin <[email protected]>

example/quick_examples.cpp

@@ -109,6 +109,8 @@ void test_examples() {
   // Disabled from testing since it's unportable
 #if 0
   {
+// Keep in mind that it's unportable code
+// You should move this structure somewhere outside of function scope
 //[pfr_quick_examples_get_name
     // Get name of field by index
 

include/boost/pfr/detail/core_name20_static.hpp

@@ -167,7 +167,7 @@ consteval auto name_of_field() noexcept {
         && std::string_view{
             detail::name_of_field_impl<
                 core_name_skip, detail::make_clang_wrapper(std::addressof(
-                    fake_object<core_name_skip>.size_at_begin
+                    detail::fake_object<core_name_skip>().size_at_begin
                 ))
             >().data()
         } == "size_at_begin",
@@ -187,7 +187,7 @@ consteval auto name_of_field() noexcept {
 template <class T, std::size_t I>
 inline constexpr auto stored_name_of_field = detail::name_of_field<T,
     detail::make_clang_wrapper(std::addressof(detail::sequence_tuple::get<I>(
-        detail::tie_as_tuple(detail::fake_object<T>)
+        detail::tie_as_tuple(detail::fake_object<T>())
     )))
 >();
 

include/boost/pfr/detail/fake_object.hpp

@@ -16,8 +16,19 @@
 
 namespace boost { namespace pfr { namespace detail {
 
+// This variable serves as a compile-time assert. If you see any error here, then
+// you're probably using `boost::pfr::get_name()` or `boost::pfr::names_as_array()` with a non-external linkage type.
 template <class T>
-extern const T fake_object;
+extern const T passed_type_has_no_external_linkage;
+
+// For returning non default constructible types, it's exclusively used in member name retrieval.
+//
+// Neither std::declval nor boost::pfr::detail::unsafe_declval are usable there.
+// Limitation - T should have external linkage.
+template <class T>
+constexpr const T& fake_object() noexcept {
+    return passed_type_has_no_external_linkage<T>;
+}
 
 }}} // namespace boost::pfr::detail
 

test/core_name/print_name.cpp

@@ -29,7 +29,7 @@ using namespace boost::pfr;
 template <class T, std::size_t I>
 inline constexpr auto no_check_stored_name_of_field = detail::name_of_field_impl<T,
     detail::make_clang_wrapper(std::addressof(detail::sequence_tuple::get<I>(
-        detail::tie_as_tuple(detail::fake_object<T>)
+        detail::tie_as_tuple(detail::fake_object<T>())
     )))
 >();