Enable result<void> & fr. Refs #117.

include/boost/system/result.hpp

@@ -1155,6 +1155,23 @@ U operator&( result<T, E>&& r, F&& f )
     }
 }
 
+template<class E, class F,
+    class U = decltype( std::declval<F>()() ),
+    class En1 = typename std::enable_if<detail::is_result<U>::value>::type,
+    class En2 = typename std::enable_if<std::is_convertible<E, typename U::error_type>::value>::type
+>
+U operator&( result<void, E> const& r, F&& f )
+{
+    if( r.has_error() )
+    {
+        return r.error();
+    }
+    else
+    {
+        return std::forward<F>( f )();
+    }
+}
+
 // operator&=
 
 // result &= unary-returning-value

test/result_and_fn1r.cpp

@@ -85,6 +85,21 @@ result<void, E2> fri3( int& )
     return {};
 }
 
+result<int, E2> fk()
+{
+    return 7;
+}
+
+result<int, E2> fk2()
+{
+    return E2();
+}
+
+result<void, E2> fk3()
+{
+    return {};
+}
+
 int main()
 {
     {
@@ -343,5 +358,111 @@ int main()
         BOOST_TEST( r2.has_error() );
     }
 
+    {
+        result<void, E> r;
+
+        {
+            result<int, E2> r2 = r & fk;
+            BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 7 );
+        }
+
+        {
+            result<int, E2> r2 = r & fk2;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<void, E2> r2 = r & fk3;
+            BOOST_TEST( r2.has_value() );
+        }
+    }
+
+    {
+        result<void, E> const r;
+
+        {
+            result<int, E2> r2 = r & fk;
+            BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 7 );
+        }
+
+        {
+            result<int, E2> r2 = r & fk2;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<void, E2> r2 = r & fk3;
+            BOOST_TEST( r2.has_value() );
+        }
+    }
+
+    {
+        result<int, E2> r2 = result<void, E>() & fk;
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 7 );
+    }
+
+    {
+        result<int, E2> r2 = result<void, E>() & fk2;
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<void, E2> r2 = result<void, E>() & fk3;
+        BOOST_TEST( r2.has_value() );
+    }
+
+    {
+        result<void, E> r( in_place_error );
+
+        {
+            result<int, E2> r2 = r & fk;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<int, E2> r2 = r & fk2;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<void, E2> r2 = r & fk3;
+            BOOST_TEST( r2.has_error() );
+        }
+    }
+
+    {
+        result<void, E> const r( in_place_error );
+
+        {
+            result<int, E2> r2 = r & fk;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<int, E2> r2 = r & fk2;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<void, E2> r2 = r & fk3;
+            BOOST_TEST( r2.has_error() );
+        }
+    }
+
+    {
+        result<int, E2> r2 = result<void, E>( in_place_error ) & fk;
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<int, E2> r2 = result<void, E>( in_place_error ) & fk2;
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<void, E2> r2 = result<void, E>( in_place_error ) & fk3;
+        BOOST_TEST( r2.has_error() );
+    }
+
     return boost::report_errors();
 }