Add operator|( result, nullary-returning-result )

include/boost/system/result.hpp

@@ -890,6 +890,8 @@ public:
 namespace detail
 {
 
+// is_value_convertible_to
+
 template<class T, class U> struct is_value_convertible_to: std::is_convertible<T, U>
 {
 };
@@ -901,6 +903,11 @@ template<class T, class U> struct is_value_convertible_to<T, U&>:
 {
 };
 
+// is_result
+
+template<class T> struct is_result: std::false_type {};
+template<class T, class E> struct is_result< result<T, E> >: std::true_type {};
+
 } // namespace detail
 
 // result | value
@@ -969,6 +976,59 @@ template<class T, class E, class F,
     }
 }
 
+// result | nullary-returning-result
+
+template<class T, 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<detail::is_value_convertible_to<T, typename U::value_type>::value>::type
+>
+    U operator|( result<T, E> const& r, F&& f )
+{
+    if( r )
+    {
+        return *r;
+    }
+    else
+    {
+        return std::forward<F>( f )();
+    }
+}
+
+template<class T, 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<detail::is_value_convertible_to<T, typename U::value_type>::value>::type
+>
+    U operator|( result<T, E>&& r, F&& f )
+{
+    if( r )
+    {
+        return *std::move( r );
+    }
+    else
+    {
+        return std::forward<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_void<typename U::value_type>::value>::type
+>
+    U operator|( result<void, E> const& r, F&& f )
+{
+    if( r )
+    {
+        return {};
+    }
+    else
+    {
+        return std::forward<F>( f )();
+    }
+}
+
 } // namespace system
 } // namespace boost