From 8d463987dd6c7260f25c90a31804d180c147b5df Mon Sep 17 00:00:00 2001
From: smoogipoo <smoogipoo@smgi.me>
Date: Mon, 22 Feb 2021 13:21:50 +0900
Subject: [PATCH] Fix being able to select incompatible freemods

---
 .../Screens/OnlinePlay/OnlinePlaySongSelect.cs    | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/osu.Game/Screens/OnlinePlay/OnlinePlaySongSelect.cs b/osu.Game/Screens/OnlinePlay/OnlinePlaySongSelect.cs
index f0c77b79bf..3f30ef1176 100644
--- a/osu.Game/Screens/OnlinePlay/OnlinePlaySongSelect.cs
+++ b/osu.Game/Screens/OnlinePlay/OnlinePlaySongSelect.cs
@@ -75,9 +75,18 @@ namespace osu.Game.Screens.OnlinePlay
             Mods.Value = selectedItem?.Value?.RequiredMods.Select(m => m.CreateCopy()).ToArray() ?? Array.Empty<Mod>();
             FreeMods.Value = selectedItem?.Value?.AllowedMods.Select(m => m.CreateCopy()).ToArray() ?? Array.Empty<Mod>();
 
+            Mods.BindValueChanged(onModsChanged);
             Ruleset.BindValueChanged(onRulesetChanged);
         }
 
+        private void onModsChanged(ValueChangedEvent<IReadOnlyList<Mod>> mods)
+        {
+            FreeMods.Value = FreeMods.Value.Where(checkCompatibleFreeMod).ToList();
+
+            // Reset the validity delegate to update the overlay's display.
+            freeModSelectOverlay.IsValidMod = IsValidFreeMod;
+        }
+
         private void onRulesetChanged(ValueChangedEvent<RulesetInfo> ruleset)
         {
             FreeMods.Value = Array.Empty<Mod>();
@@ -155,6 +164,10 @@ namespace osu.Game.Screens.OnlinePlay
         /// </summary>
         /// <param name="mod">The <see cref="Mod"/> to check.</param>
         /// <returns>Whether <paramref name="mod"/> is a selectable free-mod.</returns>
-        protected virtual bool IsValidFreeMod(Mod mod) => IsValidMod(mod);
+        protected virtual bool IsValidFreeMod(Mod mod) => IsValidMod(mod) && checkCompatibleFreeMod(mod);
+
+        private bool checkCompatibleFreeMod(Mod mod)
+            => Mods.Value.All(m => m.Acronym != mod.Acronym) // Mod must not be contained in the required mods.
+               && ModUtils.CheckCompatibleSet(Mods.Value.Append(mod).ToArray()); // Mod must be compatible with all the required mods.
     }
 }