From c360ff1b1c066eb778799d9a0a92492eb9f7e51d Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Tue, 12 Sep 2023 04:48:45 +0300
Subject: [PATCH] Add a converting constructor to result<void>

---
 include/boost/system/result.hpp   | 17 ++++++++
 test/result_convert_construct.cpp | 64 +++++++++++++++++++++++++++++++
 2 files changed, 81 insertions(+)

diff --git a/include/boost/system/result.hpp b/include/boost/system/result.hpp
index 98fb55b..e6a4339 100644
--- a/include/boost/system/result.hpp
+++ b/include/boost/system/result.hpp
@@ -538,6 +538,23 @@ public:
     {
     }
 
+    // converting
+    template<class E2, class En = typename std::enable_if<
+        std::is_convertible<E2, E>::value
+        >::type>
+    BOOST_CXX14_CONSTEXPR result( result<void, E2> const& r2 )
+        noexcept(
+            std::is_nothrow_constructible<E, E2>::value &&
+            std::is_nothrow_default_constructible<E2>::value &&
+            std::is_nothrow_copy_constructible<E2>::value )
+        : v_( in_place_error, r2.error() )
+    {
+        if( r2 )
+        {
+            this->emplace();
+        }
+    }
+
     // queries
 
     constexpr bool has_value() const noexcept
diff --git a/test/result_convert_construct.cpp b/test/result_convert_construct.cpp
index 47c9c78..39bbcb6 100644
--- a/test/result_convert_construct.cpp
+++ b/test/result_convert_construct.cpp
@@ -183,6 +183,67 @@ int main()
 
     //
 
+    {
+        result<void, int> r;
+        result<void, X> r2 = r;
+
+        BOOST_TEST( r2 );
+        BOOST_TEST_EQ( X::instances, 0 );
+    }
+
+    BOOST_TEST_EQ( X::instances, 0 );
+
+    {
+        result<void, int> const r;
+        result<void, X> r2 = r;
+
+        BOOST_TEST( r2 );
+        BOOST_TEST_EQ( X::instances, 0 );
+    }
+
+    BOOST_TEST_EQ( X::instances, 0 );
+
+    {
+        result<void, X> r2 = result<void, int>();
+
+        BOOST_TEST( r2 );
+        BOOST_TEST_EQ( X::instances, 0 );
+    }
+
+    BOOST_TEST_EQ( X::instances, 0 );
+
+    {
+        result<void, int> r( 5 );
+        result<void, X> r2 = r;
+
+        BOOST_TEST( !r2 );
+        BOOST_TEST_EQ( r2.error(), X(5) );
+        BOOST_TEST_EQ( X::instances, 1 );
+    }
+
+    BOOST_TEST_EQ( X::instances, 0 );
+
+    {
+        result<void, int> const r( 6 );
+        result<void, X> r2 = r;
+
+        BOOST_TEST( !r2 );
+        BOOST_TEST_EQ( r2.error(), X(6) );
+        BOOST_TEST_EQ( X::instances, 1 );
+    }
+
+    {
+        result<void, X> r2 = result<void, int>( 7 );
+
+        BOOST_TEST( !r2 );
+        BOOST_TEST_EQ( r2.error(), X(7) );
+        BOOST_TEST_EQ( X::instances, 1 );
+    }
+
+    BOOST_TEST_EQ( X::instances, 0 );
+
+    //
+
     {
         int x = 5;
 
@@ -225,6 +286,9 @@ int main()
         BOOST_TEST_TRAIT_FALSE((std::is_constructible<result<int, void*>, result<int, int>>));
         BOOST_TEST_TRAIT_FALSE((std::is_convertible<result<int, int>, result<int, void*>>));
 
+        BOOST_TEST_TRAIT_FALSE((std::is_constructible<result<void, void*>, result<void, int>>));
+        BOOST_TEST_TRAIT_FALSE((std::is_convertible<result<void, int>, result<void, void*>>));
+
         BOOST_TEST_TRAIT_TRUE((std::is_constructible<result<int const&>, result<int&>>));
         BOOST_TEST_TRAIT_TRUE((std::is_convertible<result<int&>, result<int const&>>));