Fix reflection of Standard Library containers in C++14 loophole (fixes #33)

include/boost/pfr/detail/core14_loophole.hpp

@@ -68,7 +68,16 @@ template <class T> constexpr T& unsafe_declval_like() noexcept {
 // The definitions of friend functions.
 template <class T, class U, std::size_t N, bool B>
 struct fn_def_lref {
-    friend auto loophole(tag<T,N>) { return boost::pfr::detail::unsafe_declval_like< std::remove_all_extents_t<U> >(); }
+    friend auto loophole(tag<T,N>) {
+        // Standard Library containers do not SFINAE on invalid copy constructor. Because of that std::vector<std::unique_ptr<int>> reports that it is copyable,
+        // which leads to an instantiation error at this place.
+        //
+        // To workaround the issue, we check that the type U is movable, and move it in that case.
+        using no_extents_t = std::remove_all_extents_t<U>;
+        return static_cast< std::conditional_t<std::is_move_constructible<no_extents_t>::value, no_extents_t&&, no_extents_t&> >(
+            boost::pfr::detail::unsafe_declval_like<no_extents_t>()
+        );
+    }
 };
 template <class T, class U, std::size_t N, bool B>
 struct fn_def_rref {

test/Jamfile.v2

@@ -139,8 +139,9 @@ test-suite pfr
     [ run precise/error_pfr_c1202.cpp               : : : $(CLASSIC_PREC_DEF) : precise_c1202_issue21 ]
     [ compile-fail precise/non_aggregate.cpp        : $(CLASSIC_PREC_DEF) : precise_non_aggregate ]
 
-    # This test may compile of fail depending on C++ Standard version.
-    #[ compile-fail precise/issue30.cpp              : : : $(CLASSIC_PREC_DEF) : precise_issue30 ]
+    # The following tests may compile of fail depending on C++ Standard version.
+    #[ compile-fail precise/issue30.cpp              : $(CLASSIC_PREC_DEF) : precise_issue30 ]
+    #[ compile-fail precise/issue33.cpp              : $(CLASSIC_PREC_DEF) : precise_issue33 ]
 
 
     ##### Loophole tests running precise and flat specific tests
@@ -161,6 +162,7 @@ test-suite pfr
     [ run precise/get_non_default_constructible.cpp : : : $(LOOPHOLE_PREC_DEF) : precise_lh_get_non_default_constructible ]
     [ run precise/error_pfr_c1202.cpp               : : : $(LOOPHOLE_PREC_DEF) : precise_lh_c1202_issue21 ]
     [ run precise/issue30.cpp                       : : : $(LOOPHOLE_PREC_DEF) : precise_lh_issue30 ]
+    [ run precise/issue33.cpp                       : : : $(LOOPHOLE_PREC_DEF) : precise_lh_issue33 ]
     [ compile-fail precise/non_aggregate.cpp        : $(LOOPHOLE_PREC_DEF) : precise_lh_non_aggregate ]
 
 

test/precise/issue33.cpp

@@ -0,0 +1,26 @@
+// Copyright (c) 2018-2019 Antony Polukhin
+//
+// 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)
+
+// Test case for https://github.com/apolukhin/magic_get/issues/33
+
+#include <iostream>
+#include <vector>
+#include <boost/pfr.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+struct TestStruct {
+    std::vector<std::unique_ptr<int>> vec;
+};
+
+int main() {
+    TestStruct temp;
+    temp.vec.emplace_back();
+
+    boost::pfr::for_each_field(temp, [](const auto& value) {
+        BOOST_TEST_EQ(value.size(), 1);
+    });
+
+    return boost::report_errors();
+}