Add a new type of KalmanFitterError: no states created at all

Core/include/Acts/Fitter/KalmanFitter.hpp

@@ -653,6 +653,12 @@ class KalmanFitter {
     /// Get the result of the fit
     auto kalmanResult = propRes.template get<KalmanResult>();
 
+    /// It could happen that the fit ends in zero processed states.
+    /// The result gets meaningless so such case is regarded as fit failure.
+    if (kalmanResult.result.ok() and not kalmanResult.processedStates) {
+      kalmanResult.result = Result<void>(KalmanFitterError::PropagationInVain);
+    }
+
     if (!kalmanResult.result.ok()) {
       return kalmanResult.result.error();
     }
@@ -736,6 +742,12 @@ class KalmanFitter {
     /// Get the result of the fit
     auto kalmanResult = propRes.template get<KalmanResult>();
 
+    /// It could happen that the fit ends in zero processed states.
+    /// The result gets meaningless so such case is regarded as fit failure.
+    if (kalmanResult.result.ok() and not kalmanResult.processedStates) {
+      kalmanResult.result = Result<void>(KalmanFitterError::PropagationInVain);
+    }
+
     if (!kalmanResult.result.ok()) {
       return kalmanResult.result.error();
     }

Core/include/Acts/Fitter/KalmanFitterError.hpp

@@ -17,7 +17,8 @@ namespace Acts {
 enum class KalmanFitterError {
   UpdateFailed = 1,
   SmoothFailed = 2,
-  OutputConversionFailed = 3
+  OutputConversionFailed = 3,
+  PropagationInVain = 4
 };
 
 namespace detail {
@@ -35,6 +36,8 @@ class KalmanFitterErrorCategory : public std::error_category {
         return "Kalman smooth failed";
       case KalmanFitterError::OutputConversionFailed:
         return "Kalman output conversion failed";
+      case KalmanFitterError::PropagationInVain:
+        return "No detector observed during the propagation";
       default:
         return "unknown";
     }