From e579622a658e2e8f7c729e916c00c9142b08af0d Mon Sep 17 00:00:00 2001
From: Rich Churcher <rich.churcher@gmail.com>
Date: Thu, 1 Aug 2024 00:13:17 +1200
Subject: [PATCH] time_system is ambiguous_with event_update_system (#14544)

# Objective

Resolve possible ambiguity detection panic between `time_system` and
`event_update_system`.

Fixes #14524

## Solution

Sets `.ambiguous_with(event_update_system)` on `time_system`. This is
slightly new territory for me, so please treat with scepticism.

## Testing

As described in the issue, added
```
        .configure_schedules(ScheduleBuildSettings {
            ambiguity_detection: LogLevel::Error,
            ..default()
        })
```
to the `time` example and ran it.
---
 crates/bevy_time/src/lib.rs      | 13 ++++++++++---
 tests/ecs/ambiguity_detection.rs |  2 +-
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/crates/bevy_time/src/lib.rs b/crates/bevy_time/src/lib.rs
index 086b37471f..906d8df1c5 100644
--- a/crates/bevy_time/src/lib.rs
+++ b/crates/bevy_time/src/lib.rs
@@ -30,7 +30,9 @@ pub mod prelude {
 }
 
 use bevy_app::{prelude::*, RunFixedMainLoop};
-use bevy_ecs::event::{signal_event_update_system, EventRegistry, ShouldUpdateEvents};
+use bevy_ecs::event::{
+    event_update_system, signal_event_update_system, EventRegistry, ShouldUpdateEvents,
+};
 use bevy_ecs::prelude::*;
 use bevy_utils::{tracing::warn, Duration, Instant};
 pub use crossbeam_channel::TrySendError;
@@ -62,8 +64,13 @@ impl Plugin for TimePlugin {
                 .register_type::<Timer>();
         }
 
-        app.add_systems(First, time_system.in_set(TimeSystem))
-            .add_systems(RunFixedMainLoop, run_fixed_main_schedule);
+        app.add_systems(
+            First,
+            time_system
+                .in_set(TimeSystem)
+                .ambiguous_with(event_update_system),
+        )
+        .add_systems(RunFixedMainLoop, run_fixed_main_schedule);
 
         // Ensure the events are not dropped until `FixedMain` systems can observe them
         app.add_systems(FixedPostUpdate, signal_event_update_system);
diff --git a/tests/ecs/ambiguity_detection.rs b/tests/ecs/ambiguity_detection.rs
index 98c5c2fb77..08e5015691 100644
--- a/tests/ecs/ambiguity_detection.rs
+++ b/tests/ecs/ambiguity_detection.rs
@@ -69,7 +69,7 @@ pub fn main() {
 
     let total_ambiguities = ambiguities.total();
     assert_eq!(
-        total_ambiguities, 82,
+        total_ambiguities, 81,
         "Main app does not have an expected conflicting systems count, \
         you might consider verifying if it's normal, or change the expected number.\n\
         Details:\n{:#?}",