From cbdb354a274c8ab51a15dd43eafd02c2b6a576f0 Mon Sep 17 00:00:00 2001
From: Edward Catmur <edward.catmur@mavensecurities.com>
Date: Wed, 17 Apr 2019 15:05:37 +0100
Subject: [PATCH 1/2] Copy variant cvref when determining result type

Ensures that we can e.g. call apply_visitor on a lvalue variant
---
 boost/variant/detail/apply_visitor_unary.hpp | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/boost/variant/detail/apply_visitor_unary.hpp b/boost/variant/detail/apply_visitor_unary.hpp
index 71610cb6..c3a741ef 100644
--- a/boost/variant/detail/apply_visitor_unary.hpp
+++ b/boost/variant/detail/apply_visitor_unary.hpp
@@ -23,6 +23,7 @@
 #   include <boost/mpl/size.hpp>
 #   include <boost/utility/declval.hpp>
 #   include <boost/core/enable_if.hpp>
+#   include <boost/type_traits/copy_cv_ref.hpp>
 #   include <boost/type_traits/remove_reference.hpp>
 #   include <boost/variant/detail/has_result_type.hpp>
 #endif
@@ -85,7 +86,7 @@ namespace detail { namespace variant {
 // This class serves only metaprogramming purposes. none of its methods must be called at runtime!
 template <class Visitor, class Variant>
 struct result_multideduce1 {
-    typedef typename Variant::types                 types;
+    typedef typename remove_reference<Variant>::type::types types;
     typedef typename boost::mpl::begin<types>::type begin_it;
     typedef typename boost::mpl::advance<
         begin_it, boost::mpl::int_<boost::mpl::size<types>::type::value - 1>
@@ -95,14 +96,14 @@ struct result_multideduce1 {
     struct deduce_impl {
         typedef typename boost::mpl::next<It>::type next_t;
         typedef typename boost::mpl::deref<It>::type value_t;
-        typedef decltype(true ? boost::declval< Visitor& >()( boost::declval< value_t >() )
+        typedef decltype(true ? boost::declval< Visitor& >()( boost::declval< copy_cv_ref_t< value_t, Variant > >() )
                               : boost::declval< typename deduce_impl<next_t>::type >()) type;
     };
 
     template <class Dummy>
     struct deduce_impl<last_it, Dummy> {
         typedef typename boost::mpl::deref<last_it>::type value_t;
-        typedef decltype(boost::declval< Visitor& >()( boost::declval< value_t >() )) type;
+        typedef decltype(boost::declval< Visitor& >()( boost::declval< copy_cv_ref_t< value_t, Variant > >() )) type;
     };
 
     typedef typename deduce_impl<begin_it>::type type;
@@ -132,7 +133,7 @@ inline decltype(auto) apply_visitor(Visitor&& visitor, Visitable&& visitable,
         boost::detail::variant::has_result_type<Visitor>
     >::type* = 0)
 {
-    boost::detail::variant::result_wrapper1<Visitor, typename remove_reference<Visitable>::type> cpp14_vis(::boost::forward<Visitor>(visitor));
+    boost::detail::variant::result_wrapper1<Visitor, Visitable> cpp14_vis(::boost::forward<Visitor>(visitor));
     return ::boost::forward<Visitable>(visitable).apply_visitor(cpp14_vis);
 }
 

From 7331d648f46b6008138cbd22087b8c5c61ff7926 Mon Sep 17 00:00:00 2001
From: Ed Catmur <ed@catmur.uk>
Date: Sun, 21 Apr 2019 00:50:00 +0100
Subject: [PATCH 2/2] Add test.

---
 libs/variant/test/const_ref_apply_visitor.cpp | 44 ++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/libs/variant/test/const_ref_apply_visitor.cpp b/libs/variant/test/const_ref_apply_visitor.cpp
index 0ec77bd4..72afff9e 100644
--- a/libs/variant/test/const_ref_apply_visitor.cpp
+++ b/libs/variant/test/const_ref_apply_visitor.cpp
@@ -224,6 +224,44 @@ void test_cpp14_visitor(const variant_type& test_var, const variant_type& test_v
 #endif
 }
 
+void test_cpp14_visitor(variant_type& test_var)
+{
+    std::cout << "Testing lvalue visitable for c++14\n";
+
+    BOOST_TEST(boost::apply_visitor([](auto& v) { return lvalue_rvalue_detector()(v); }, test_var) == "lvalue reference");
+}
+
+void test_cpp14_mutable_visitor(variant_type& test_var)
+{
+    std::cout << "Testing lvalue visitable for c++14 with inline mutable lambda\n";
+
+    BOOST_TEST(boost::apply_visitor([](auto& v) mutable -> auto { return lvalue_rvalue_detector()(v); }, test_var) == "lvalue reference");
+}
+
+void test_cpp14_visitor(variant_type& test_var, variant_type& test_var2)
+{
+    std::cout << "Testing lvalue visitable for c++14\n";
+
+    BOOST_TEST(boost::apply_visitor([](auto& v, auto& vv) { return lvalue_rvalue_detector()(v, vv); }, test_var, test_var2)
+            == "lvalue reference, lvalue reference");
+}
+
+void test_cpp14_visitor(variant_type& test_var, variant_type& test_var2, variant_type& test_var3)
+{
+#if !defined(BOOST_VARIANT_DO_NOT_USE_VARIADIC_TEMPLATES) && !defined(BOOST_NO_CXX11_HDR_TUPLE)
+    std::cout << "Testing lvalue visitable for c++14\n";
+
+    auto result = boost::apply_visitor([](auto& v, auto& t, auto& p) { return lvalue_rvalue_detector()(v, t, p); },
+                test_var, test_var2, test_var3);
+    std::cout << "result: " << result << std::endl;
+    BOOST_TEST(result == "lvalue reference, lvalue reference, lvalue reference");
+#else
+    (void)test_var;
+    (void)test_var2;
+    (void)test_var3;
+#endif
+}
+
 void test_cpp14_visitor(variant_type&& test_var)
 {
     std::cout << "Testing rvalue visitable for c++14\n";
@@ -344,8 +382,14 @@ void run_cpp14_mixed_tests()
 void run_cpp14_tests()
 {
 #ifndef BOOST_NO_CXX14_DECLTYPE_AUTO
+    variant_type const c1(10), c2(20), c3(30);
     variant_type v1(10), v2(20), v3(30);
 
+    test_cpp14_visitor(c1);
+    test_cpp14_mutable_visitor(c1);
+    test_cpp14_visitor(c2, c3);
+    test_cpp14_visitor(c1, c2, c3);
+
     test_cpp14_visitor(v1);
     test_cpp14_mutable_visitor(v1);
     test_cpp14_visitor(v2, v3);