From 5933e0d2d99a30ebc57b83a8dad81934746eabc3 Mon Sep 17 00:00:00 2001
From: Dean Herbert <pe@ppy.sh>
Date: Fri, 18 Jun 2021 13:17:32 +0900
Subject: [PATCH] Change `CheckCompatibleSet` to never deselect the current
 candidat when checking incompatibility

---
 osu.Game.Tests/Mods/ModUtilsTest.cs | 30 +++++++++++++++++++++++++++--
 osu.Game/Utils/ModUtils.cs          |  3 +++
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/osu.Game.Tests/Mods/ModUtilsTest.cs b/osu.Game.Tests/Mods/ModUtilsTest.cs
index 7384471c41..9f27289d7e 100644
--- a/osu.Game.Tests/Mods/ModUtilsTest.cs
+++ b/osu.Game.Tests/Mods/ModUtilsTest.cs
@@ -21,6 +21,14 @@ namespace osu.Game.Tests.Mods
             Assert.That(ModUtils.CheckCompatibleSet(new[] { mod.Object }));
         }
 
+        [Test]
+        public void TestModIsCompatibleByItselfWithIncompatibleInterface()
+        {
+            var mod = new Mock<CustomMod1>();
+            mod.Setup(m => m.IncompatibleMods).Returns(new[] { typeof(IModCompatibilitySpecification) });
+            Assert.That(ModUtils.CheckCompatibleSet(new[] { mod.Object }));
+        }
+
         [Test]
         public void TestIncompatibleThroughTopLevel()
         {
@@ -34,6 +42,20 @@ namespace osu.Game.Tests.Mods
             Assert.That(ModUtils.CheckCompatibleSet(new Mod[] { mod2.Object, mod1.Object }), Is.False);
         }
 
+        [Test]
+        public void TestIncompatibleThroughInterface()
+        {
+            var mod1 = new Mock<CustomMod1>();
+            var mod2 = new Mock<CustomMod2>();
+
+            mod1.Setup(m => m.IncompatibleMods).Returns(new[] { typeof(IModCompatibilitySpecification) });
+            mod2.Setup(m => m.IncompatibleMods).Returns(new[] { typeof(IModCompatibilitySpecification) });
+
+            // Test both orderings.
+            Assert.That(ModUtils.CheckCompatibleSet(new Mod[] { mod1.Object, mod2.Object }), Is.False);
+            Assert.That(ModUtils.CheckCompatibleSet(new Mod[] { mod2.Object, mod1.Object }), Is.False);
+        }
+
         [Test]
         public void TestMultiModIncompatibleWithTopLevel()
         {
@@ -149,11 +171,15 @@ namespace osu.Game.Tests.Mods
                 Assert.That(invalid.Select(t => t.GetType()), Is.EquivalentTo(expectedInvalid));
         }
 
-        public abstract class CustomMod1 : Mod
+        public abstract class CustomMod1 : Mod, IModCompatibilitySpecification
         {
         }
 
-        public abstract class CustomMod2 : Mod
+        public abstract class CustomMod2 : Mod, IModCompatibilitySpecification
+        {
+        }
+
+        public interface IModCompatibilitySpecification
         {
         }
     }
diff --git a/osu.Game/Utils/ModUtils.cs b/osu.Game/Utils/ModUtils.cs
index 1c3558fc90..98766cb844 100644
--- a/osu.Game/Utils/ModUtils.cs
+++ b/osu.Game/Utils/ModUtils.cs
@@ -60,6 +60,9 @@ namespace osu.Game.Utils
                 {
                     foreach (var invalid in combination.Where(m => type.IsInstanceOfType(m)))
                     {
+                        if (invalid == mod)
+                            continue;
+
                         invalidMods ??= new List<Mod>();
                         invalidMods.Add(invalid);
                     }