Support result<void> & fv, where fv returns void. Refs #119.

include/boost/system/result.hpp

@@ -1192,7 +1192,8 @@ result<U, E> operator&( result<T, E>&& r, F&& f )
 
 template<class E, class F,
     class U = decltype( std::declval<F>()() ),
-    class En = typename std::enable_if<!detail::is_result<U>::value>::type
+    class En1 = typename std::enable_if<!detail::is_result<U>::value>::type,
+    class En2 = typename std::enable_if<!std::is_void<U>::value>::type
 >
 result<U, E> operator&( result<void, E> const& r, F&& f )
 {
@@ -1206,6 +1207,23 @@ result<U, E> operator&( result<void, E> const& r, F&& f )
     }
 }
 
+template<class E, class F,
+    class U = decltype( std::declval<F>()() ),
+    class En = typename std::enable_if<std::is_void<U>::value>::type
+>
+result<U, E> operator&( result<void, E> const& r, F&& f )
+{
+    if( r.has_error() )
+    {
+        return r.error();
+    }
+    else
+    {
+        std::forward<F>( f )();
+        return {};
+    }
+}
+
 // result & unary-returning-result
 
 template<class T, class E, class F,

test/result_and_fn1v.cpp

@@ -59,6 +59,10 @@ void fv1( int /*x*/ )
 {
 }
 
+void fv2()
+{
+}
+
 int main()
 {
     {
@@ -243,5 +247,45 @@ int main()
         BOOST_TEST( r2.has_error() );
     }
 
+    {
+        result<void> r;
+        result<void> r2 = r & fv2;
+
+        BOOST_TEST( r2.has_value() );
+    }
+
+    {
+        result<void> const r;
+        result<void> r2 = r & fv2;
+
+        BOOST_TEST( r2.has_value() );
+    }
+
+    {
+        result<void> r2 = result<void>() & fv2;
+
+        BOOST_TEST( r2.has_value() );
+    }
+
+    {
+        result<void, E> r( in_place_error );
+        result<void, E> r2 = r & fv2;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<void, E> const r( in_place_error );
+        result<void, E> r2 = r & fv2;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<void, E> r2 = result<void, E>( in_place_error ) & fv2;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
     return boost::report_errors();
 }