Add static test to detect ability to integrate PFR as fallback into the Fusion

test/Jamfile.v2

@@ -53,6 +53,8 @@ test-suite pfr_tests
 
     [ run offset_based_getter.cpp ]
 
+    [ run can_be_as_fallback_in_the_fusion.cpp ]
+
     [ run test_tuple_sizes_on.cpp : : : <define>BOOST_PFR_RUN_TEST_ON=char : test_tuple_sizes_on_chars ]
     [ run test_tuple_sizes_on.cpp : : : <define>BOOST_PFR_RUN_TEST_ON=int : test_tuple_sizes_on_ints ]
     [ run test_tuple_sizes_on.cpp : : : <define>BOOST_PFR_RUN_TEST_ON=short : test_tuple_sizes_on_shorts ]

test/can_be_as_fallback_in_the_fusion.cpp

@@ -0,0 +1,33 @@
+// Copyright (c) 2022 Denis Mikhailov
+//
+// 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 <type_traits>
+#include <boost/pfr/traits.hpp>
+
+struct Aggregate {};
+using NonAggregate = int;
+
+namespace boost { namespace pfr {
+    struct boost_fusion_tag;
+}}
+
+template<class T, class E=void>
+struct tag_of_fallback : std::false_type {
+};
+
+template<class T>
+struct tag_of_fallback<T, std::enable_if_t<std::is_same<T,T>::value>>
+{
+    using type = std::conditional_t<
+        boost::pfr::is_implicitly_reflectable<T, boost::pfr::boost_fusion_tag>::value
+      , std::true_type
+      , std::false_type
+    >;
+};
+
+static_assert(tag_of_fallback<Aggregate>::type{} == true, "");
+static_assert(tag_of_fallback<NonAggregate>::type{} == false, "");
+
+int main() { }