Add function to check for (and return) invalid mods

osu.Game/Utils/ModUtils.cs

@@ -83,6 +83,48 @@ namespace osu.Game.Utils
                               .All(m => allowedSet.Contains(m.GetType()));
         }
 
+        /// <summary>
+        /// Check the provided combination of mods are valid for a local gameplay session.
+        /// </summary>
+        /// <param name="mods">The mods to check.</param>
+        /// <param name="invalidMods">Invalid mods, if any where found. Can 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, out Mod[]? invalidMods)
+        {
+            mods = mods.ToArray();
+
+            List<Mod>? foundInvalid = null;
+
+            void addInvalid(Mod mod)
+            {
+                foundInvalid ??= new List<Mod>();
+                foundInvalid.Add(mod);
+            }
+
+            foreach (var mod in mods)
+            {
+                bool valid = mod.Type != ModType.System
+                             && mod.HasImplementation
+                             && !(mod is MultiMod);
+
+                if (!valid)
+                {
+                    // if this mod was found as invalid, we can exclude it before potentially excluding more incompatible types.
+                    addInvalid(mod);
+                    continue;
+                }
+
+                foreach (var type in mod.IncompatibleMods)
+                {
+                    foreach (var invalid in mods.Where(m => type.IsInstanceOfType(m)))
+                        addInvalid(invalid);
+                }
+            }
+
+            invalidMods = foundInvalid?.ToArray();
+            return foundInvalid == null;
+        }
+
         /// <summary>
         /// Flattens a set of <see cref="Mod"/>s, returning a new set with all <see cref="MultiMod"/>s removed.
         /// </summary>