Support result<void> &= fv. Refs #119.

include/boost/system/result.hpp

@@ -1296,6 +1296,20 @@ result<T, E>& operator&=( result<T, E>& r, F&& f )
     return r;
 }
 
+template<class E, class F,
+    class U = decltype( std::declval<F>()() ),
+    class En = typename std::enable_if<!detail::is_result<U>::value>::type
+>
+result<void, E>& operator&=( result<void, E>& r, F&& f )
+{
+    if( r )
+    {
+        std::forward<F>( f )();
+    }
+
+    return r;
+}
+
 // result &= unary-returning-result
 
 template<class T, class E, class F,

test/result_and_eq_fn1v.cpp

@@ -59,6 +59,13 @@ int& h( int& )
     return x;
 }
 
+static int fv_called;
+
+void fv()
+{
+    ++fv_called;
+}
+
 int main()
 {
     {
@@ -110,5 +117,25 @@ int main()
         BOOST_TEST( r.has_error() );
     }
 
+    {
+        result<void> r;
+        fv_called = 0;
+
+        r &= fv;
+
+        BOOST_TEST( r.has_value() );
+        BOOST_TEST_EQ( fv_called, 1 );
+    }
+
+    {
+        result<void, E> r( in_place_error );
+        fv_called = 0;
+
+        r &= fv;
+
+        BOOST_TEST( r.has_error() );
+        BOOST_TEST_EQ( fv_called, 0 );
+    }
+
     return boost::report_errors();
 }