Add fast path for Overlaps when other is IReadOnlySet with fast lookup and same equality comparer #132763

src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenSetInternalBase.cs

@@ -130,6 +130,19 @@ private protected override bool OverlapsCore(IEnumerable<T> other)
         {
             Debug.Assert(_thisSet.Count != 0, "EmptyFrozenSet should have been used.");
 
+            // Fast path for Overlaps when other is IReadOnlySet with fast lookup and same equality comparer
+            if (other is IReadOnlySet<T> otherAsSet && ComparersAreCompatible(otherAsSet))
+            {
+                return otherAsSet.Count > Count
+                    ? SmallerFrozenSetOverlapsLargerCompatibleReadOnlySetImpl(this, otherAsSet)
+                    : LargerFrozenSetOverlapsSmallerCompatibleReadOnlySetImpl(otherAsSet, this);
+            }
+
+            return OverlapsEnumerableImpl(other);
+        }
+
+        private bool OverlapsEnumerableImpl(IEnumerable<T> other)
+        {
             foreach (T element in other)
             {
                 if (_thisSet.FindItemIndex(element) >= 0)
@@ -141,6 +154,32 @@ private protected override bool OverlapsCore(IEnumerable<T> other)
             return false;
         }
 
+        private static bool SmallerFrozenSetOverlapsLargerCompatibleReadOnlySetImpl(FrozenSet<T> smaller, IReadOnlySet<T> larger)
+        {
+            foreach (T element in smaller)
+            {
+                if (larger.Contains(element))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        private static bool LargerFrozenSetOverlapsSmallerCompatibleReadOnlySetImpl(IReadOnlySet<T> smaller, FrozenSet<T> larger)
+        {
+            foreach (T element in smaller)
+            {
+                if (larger.Contains(element))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
         /// <inheritdoc />
         private protected override bool SetEqualsCore(IEnumerable<T> other)
         {