From 4200b00973516f0a374a6864c3f98184582b1296 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Sat, 18 Jun 2022 17:58:24 +0300
Subject: [PATCH] Add converting constructor to `result` (refs #82)

---
 include/boost/system/result.hpp   |  37 ++++++
 test/CMakeLists.txt               |   1 +
 test/Jamfile.v2                   |   1 +
 test/result_convert_construct.cpp | 202 ++++++++++++++++++++++++++++++
 4 files changed, 241 insertions(+)
 create mode 100644 test/result_convert_construct.cpp

diff --git a/include/boost/system/result.hpp b/include/boost/system/result.hpp
index 26baa6d..8ff55fb 100644
--- a/include/boost/system/result.hpp
+++ b/include/boost/system/result.hpp
@@ -174,6 +174,43 @@ public:
     {
     }
 
+    // converting
+    template<class T2, class E2, class En = typename std::enable_if<
+        std::is_convertible<T2, T>::value &&
+        std::is_convertible<E2, E>::value
+        >::type>
+    BOOST_CXX14_CONSTEXPR result( result<T2, E2> const& r2 )
+        noexcept(
+            std::is_nothrow_constructible<T, T2 const&>::value &&
+            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 )
+        {
+            v_.template emplace<0>( *r2 );
+        }
+    }
+
+    template<class T2, class E2, class En = typename std::enable_if<
+        std::is_convertible<T2, T>::value &&
+        std::is_convertible<E2, E>::value
+        >::type>
+    BOOST_CXX14_CONSTEXPR result( result<T2, E2>&& r2 )
+        noexcept(
+            std::is_nothrow_constructible<T, T2&&>::value &&
+            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 )
+        {
+            v_.template emplace<0>( std::move( *r2 ) );
+        }
+    }
+
     // queries
 
     constexpr bool has_value() const noexcept
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 9c09d7d..3d08049 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -142,3 +142,4 @@ boost_test(TYPE run SOURCES result_eq.cpp)
 boost_test(TYPE run SOURCES result_range_for.cpp)
 boost_test(TYPE run SOURCES result_value_construct2.cpp)
 boost_test(TYPE run SOURCES result_error_construct2.cpp)
+boost_test(TYPE run SOURCES result_convert_construct.cpp)
diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index 684992d..b67088d 100644
--- a/test/Jamfile.v2
+++ b/test/Jamfile.v2
@@ -169,3 +169,4 @@ run result_range_for.cpp : : : $(CPP11) ;
 run result_value_construct2.cpp : : : $(CPP11) ;
 run result_error_construct2.cpp : : : $(CPP11) ;
 run result_errc_construct.cpp : : : $(CPP11) ;
+run result_convert_construct.cpp : : : $(CPP11) ;
diff --git a/test/result_convert_construct.cpp b/test/result_convert_construct.cpp
new file mode 100644
index 0000000..be6faad
--- /dev/null
+++ b/test/result_convert_construct.cpp
@@ -0,0 +1,202 @@
+// Copyright 2017, 2021, 2022 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/result.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+
+using namespace boost::system;
+
+struct X
+{
+    static int instances;
+
+    int v_;
+
+    X(): v_() { ++instances; }
+
+    X( int v ): v_( v ) { ++instances; }
+
+    X( X const& r ): v_( r.v_ ) { ++instances; }
+
+    X& operator=( X const& ) = delete;
+
+    ~X() { --instances; }
+};
+
+bool operator==( X const & x1, X const & x2 )
+{
+    return x1.v_ == x2.v_;
+}
+
+std::ostream& operator<<( std::ostream& os, X const & x )
+{
+    os << "X:" << x.v_;
+    return os;
+}
+
+int X::instances = 0;
+
+int main()
+{
+    {
+        result<int> r( 5 );
+        result<long> r2 = r;
+
+        BOOST_TEST( r2 ) && BOOST_TEST_EQ( *r2, 5 );
+    }
+
+    {
+        result<int> const r( 6 );
+        result<long> r2 = r;
+
+        BOOST_TEST( r2 ) && BOOST_TEST_EQ( *r2, 6 );
+    }
+
+    {
+        result<long> r2 = result<int>( 7 );
+
+        BOOST_TEST( r2 ) && BOOST_TEST_EQ( *r2, 7 );
+    }
+
+    BOOST_TEST_EQ( X::instances, 0 );
+
+    {
+        result<int> r( 5 );
+        result<X> r2 = r;
+
+        BOOST_TEST( r2 ) && BOOST_TEST_EQ( *r2, 5 );
+        BOOST_TEST_EQ( X::instances, 1 );
+    }
+
+    BOOST_TEST_EQ( X::instances, 0 );
+
+    {
+        result<int> const r( 6 );
+        result<X> r2 = r;
+
+        BOOST_TEST( r2 ) && BOOST_TEST_EQ( *r2, 6 );
+        BOOST_TEST_EQ( X::instances, 1 );
+    }
+
+    BOOST_TEST_EQ( X::instances, 0 );
+
+    {
+        result<X> r2 = result<int>( 7 );
+
+        BOOST_TEST( r2 ) && BOOST_TEST_EQ( *r2, 7 );
+        BOOST_TEST_EQ( X::instances, 1 );
+    }
+
+    BOOST_TEST_EQ( X::instances, 0 );
+
+    {
+        auto ec = make_error_code( errc::invalid_argument );
+
+        result<int> r( ec );
+        result<long> r2 = r;
+
+        BOOST_TEST( !r2 );
+        BOOST_TEST_EQ( r2.error(), ec );
+    }
+
+    {
+        auto ec = make_error_code( errc::invalid_argument );
+
+        result<int> r( ec );
+        result<long> r2 = r;
+
+        BOOST_TEST( !r2 );
+        BOOST_TEST_EQ( r2.error(), ec );
+    }
+
+    {
+        auto ec = make_error_code( errc::invalid_argument );
+
+        result<long> r2 = result<int>( ec );
+
+        BOOST_TEST( !r2 );
+        BOOST_TEST_EQ( r2.error(), ec );
+    }
+
+    BOOST_TEST_EQ( X::instances, 0 );
+
+    {
+        result<char const*, int> r( "test" );
+        result<std::string, X> r2 = r;
+
+        BOOST_TEST( r2 ) && BOOST_TEST_EQ( *r2, std::string( "test" ) );
+        BOOST_TEST_EQ( X::instances, 0 );
+    }
+
+    BOOST_TEST_EQ( X::instances, 0 );
+
+    {
+        result<char const*, int> const r( "test" );
+        result<std::string, X> r2 = r;
+
+        BOOST_TEST( r2 ) && BOOST_TEST_EQ( *r2, std::string( "test" ) );
+        BOOST_TEST_EQ( X::instances, 0 );
+    }
+
+    BOOST_TEST_EQ( X::instances, 0 );
+
+    {
+        result<std::string, X> r2 = result<char const*, int>( "test" );
+
+        BOOST_TEST( r2 ) && BOOST_TEST_EQ( *r2, std::string( "test" ) );
+        BOOST_TEST_EQ( X::instances, 0 );
+    }
+
+    BOOST_TEST_EQ( X::instances, 0 );
+
+    {
+        result<char const*, int> r( 5 );
+        result<std::string, 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<char const*, int> const r( 6 );
+        result<std::string, X> r2 = r;
+
+        BOOST_TEST( !r2 );
+        BOOST_TEST_EQ( r2.error(), X(6) );
+        BOOST_TEST_EQ( X::instances, 1 );
+    }
+
+    {
+        result<std::string, X> r2 = result<char const*, int>( 7 );
+
+        BOOST_TEST( !r2 );
+        BOOST_TEST_EQ( r2.error(), X(7) );
+        BOOST_TEST_EQ( X::instances, 1 );
+    }
+
+    BOOST_TEST_EQ( X::instances, 0 );
+
+    {
+        BOOST_TEST_TRAIT_TRUE((std::is_constructible<result<long>, result<int>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_convertible<result<int>, result<long>>));
+
+        BOOST_TEST_TRAIT_FALSE((std::is_constructible<result<int>, result<void*>>));
+        BOOST_TEST_TRAIT_FALSE((std::is_convertible<result<void*>, result<int>>));
+
+        BOOST_TEST_TRAIT_FALSE((std::is_constructible<result<int>, result<void>>));
+        BOOST_TEST_TRAIT_FALSE((std::is_convertible<result<void>, result<int>>));
+
+        BOOST_TEST_TRAIT_FALSE((std::is_constructible<result<void>, result<int>>));
+        BOOST_TEST_TRAIT_FALSE((std::is_convertible<result<int>, result<void>>));
+
+        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*>>));
+    }
+
+    return boost::report_errors();
+}