[xbt] Add missing const in Result::is_valid

diff --git a/include/xbt/future.hpp b/include/xbt/future.hpp
index 8372290..122d590 100644 (file)
--- a/include/xbt/future.hpp
+++ b/include/xbt/future.hpp
@@ -73,7 +73,7 @@ public:
     return *this;
   }
 
-  bool is_valid()
+  bool is_valid() const
   {
     return status_ != ResultStatus::invalid;
   }