From 8f04db5df5136f9431e62a6fffce273f2ba8d6bc Mon Sep 17 00:00:00 2001
From: Salman Ahmed <frenzibyte@gmail.com>
Date: Wed, 4 May 2022 17:21:19 +0300
Subject: [PATCH] Bring back behaviour of checking incompatibility on gameplay
 validity

---
 osu.Game.Tests/Mods/ModUtilsTest.cs | 12 ++++++++++++
 osu.Game/OsuGame.cs                 | 15 +++++----------
 osu.Game/Utils/ModUtils.cs          | 15 +++++++++++----
 3 files changed, 28 insertions(+), 14 deletions(-)

diff --git a/osu.Game.Tests/Mods/ModUtilsTest.cs b/osu.Game.Tests/Mods/ModUtilsTest.cs
index e858abfe44..4c126f0a3b 100644
--- a/osu.Game.Tests/Mods/ModUtilsTest.cs
+++ b/osu.Game.Tests/Mods/ModUtilsTest.cs
@@ -134,6 +134,18 @@ namespace osu.Game.Tests.Mods
 
         private static readonly object[] invalid_mod_test_scenarios =
         {
+            // incompatible pair.
+            new object[]
+            {
+                new Mod[] { new OsuModDoubleTime(), new OsuModHalfTime() },
+                new[] { typeof(OsuModDoubleTime), typeof(OsuModHalfTime) }
+            },
+            // incompatible pair with derived class.
+            new object[]
+            {
+                new Mod[] { new OsuModNightcore(), new OsuModHalfTime() },
+                new[] { typeof(OsuModNightcore), typeof(OsuModHalfTime) }
+            },
             // system mod.
             new object[]
             {
diff --git a/osu.Game/OsuGame.cs b/osu.Game/OsuGame.cs
index 29985b0893..e9fe8c43de 100644
--- a/osu.Game/OsuGame.cs
+++ b/osu.Game/OsuGame.cs
@@ -577,16 +577,11 @@ namespace osu.Game
             if (SelectedMods.Disabled)
                 return;
 
-            var validMods = mods.NewValue;
-
-            if (!ModUtils.CheckCompatibleSet(validMods, out var incompatible))
-                validMods = validMods.Except(incompatible).ToArray();
-
-            if (!ModUtils.CheckValidForGameplay(validMods, out var invalid))
-                validMods = validMods.Except(invalid).ToArray();
-
-            // ensure we always have a valid set of mods.
-            SelectedMods.Value = validMods;
+            if (!ModUtils.CheckValidForGameplay(mods.NewValue, out var invalid))
+            {
+                // ensure we always have a valid set of mods.
+                SelectedMods.Value = mods.NewValue.Except(invalid).ToArray();
+            }
         }
 
         #endregion
diff --git a/osu.Game/Utils/ModUtils.cs b/osu.Game/Utils/ModUtils.cs
index e19020a959..c18125054a 100644
--- a/osu.Game/Utils/ModUtils.cs
+++ b/osu.Game/Utils/ModUtils.cs
@@ -112,7 +112,7 @@ namespace osu.Game.Utils
         /// <param name="invalidMods">Invalid mods, if any were found. Will be null if all mods were valid.</param>
         /// <returns>Whether the input mods were all valid. If false, <paramref name="invalidMods"/> will contain all invalid entries.</returns>
         public static bool CheckValidForGameplay(IEnumerable<Mod> mods, [NotNullWhen(false)] out List<Mod>? invalidMods)
-            => checkValid(mods, m => m.Type != ModType.System && m.HasImplementation && !(m is MultiMod), out invalidMods);
+            => checkValid(mods, m => m.Type != ModType.System && m.HasImplementation && !(m is MultiMod), out invalidMods, true);
 
         /// <summary>
         /// Checks that all <see cref="Mod"/>s in a combination are valid as "required mods" in a multiplayer match session.
@@ -121,7 +121,7 @@ namespace osu.Game.Utils
         /// <param name="invalidMods">Invalid mods, if any were found. Will be null if all mods were valid.</param>
         /// <returns>Whether the input mods were all valid. If false, <paramref name="invalidMods"/> will contain all invalid entries.</returns>
         public static bool CheckValidRequiredModsForMultiplayer(IEnumerable<Mod> mods, [NotNullWhen(false)] out List<Mod>? invalidMods)
-            => checkValid(mods, m => m.IsPlayable(ModUsage.MultiplayerGlobal), out invalidMods);
+            => checkValid(mods, m => m.IsPlayable(ModUsage.MultiplayerGlobal), out invalidMods, true);
 
         /// <summary>
         /// Checks that all <see cref="Mod"/>s in a combination are valid as "free mods" in a multiplayer match session.
@@ -130,13 +130,20 @@ namespace osu.Game.Utils
         /// <param name="invalidMods">Invalid mods, if any were found. Will be null if all mods were valid.</param>
         /// <returns>Whether the input mods were all valid. If false, <paramref name="invalidMods"/> will contain all invalid entries.</returns>
         public static bool CheckValidFreeModsForMultiplayer(IEnumerable<Mod> mods, [NotNullWhen(false)] out List<Mod>? invalidMods)
-            => checkValid(mods, m => m.IsPlayable(ModUsage.MultiplayerLocal), out invalidMods);
+            => checkValid(mods, m => m.IsPlayable(ModUsage.MultiplayerLocal), out invalidMods, false);
 
-        private static bool checkValid(IEnumerable<Mod> mods, Predicate<Mod> valid, [NotNullWhen(false)] out List<Mod>? invalidMods)
+        private static bool checkValid(IEnumerable<Mod> mods, Predicate<Mod> valid, [NotNullWhen(false)] out List<Mod>? invalidMods, bool checkCompatibility)
         {
             mods = mods.ToArray();
             invalidMods = null;
 
+            if (checkCompatibility)
+            {
+                // exclude multi mods from compatibility checks.
+                // the loop below automatically marks all multi mods as not valid for gameplay anyway.
+                CheckCompatibleSet(mods.Where(m => !(m is MultiMod)), out invalidMods);
+            }
+
             foreach (var mod in mods)
             {
                 if (!valid(mod))