Don't compute new modifier for merge instructions in collapsing

CHANGELOG.md

@@ -1,7 +1,10 @@
 # UNRELEASED
 
-- Profile:
-  - Remove dependency on turn types and turn modifier in the process_turn function in the `car.lua` profile. Guidance instruction types are not used to influence turn penalty anymore so this will break backward compatibility between profile version 3 and 4.
+  - Changes from 5.13:
+    - Profile:
+      - Remove dependency on turn types and turn modifier in the process_turn function in the `car.lua` profile. Guidance instruction types are not used to influence turn penalty anymore so this will break backward compatibility between profile version 3 and 4.
+    - Bugfixes:
+      - Fixed #4670: Fix bug where merge instructions got the wrong direction modifier
 
 # 5.13.0
   - Changes from 5.12:

features/guidance/merge.feature

@@ -145,5 +145,5 @@ Feature: Merging
             | cf    |      | 1B           | motorway_link | yes    |
 
         When I route I should get
-            | waypoints | route      | turns                        |
-            | e,d       | ,A100,A100 | depart,merge straight,arrive |
+            | waypoints | route      | turns                           |
+            | e,d       | ,A100,A100 | depart,merge slight left,arrive |

src/engine/guidance/collapse_turns.cpp

@@ -186,16 +186,18 @@ void AdjustToCombinedTurnStrategy::operator()(RouteStep &step_at_turn_location,
 {
     const auto angle = findTotalTurnAngle(step_at_turn_location, transfer_from_step);
 
-    // Forks point to left/right. By doing a combined angle, we would risk ending up with
-    // unreasonable fork instrucitons. The direction of a fork only depends on the forking location,
+    // Forks and merges point to left/right. By doing a combined angle, we would risk ending up with
+    // unreasonable fork instrucitons. The direction of a fork or a merge only depends on the
+    // location,
     // not further angles coming up
     //
     //          d
     //       .  c
     // a - b
     //
     // could end up as `fork left` for `a-b-c`, instead of fork-right
-    const auto new_modifier = hasTurnType(step_at_turn_location, TurnType::Fork)
+    const auto new_modifier = hasTurnType(step_at_turn_location, TurnType::Fork) ||
+                                      hasTurnType(step_at_turn_location, TurnType::Merge)
                                   ? step_at_turn_location.maneuver.instruction.direction_modifier
                                   : getTurnDirection(angle);