[release/11.0] Remove open delegate Equals fastpath

src/coreclr/System.Private.CoreLib/src/System/Delegate.CoreCLR.cs

@@ -212,14 +212,6 @@ public sealed override unsafe bool Equals([NotNullWhen(true)] object? obj)
                 // unmanaged
                 if (IsUnmanagedFunctionPtr)
                     return other.IsUnmanagedFunctionPtr && _methodPtrAux == other._methodPtrAux;
-
-                // Under cached interface dispatch we might see the shared CID_VirtualOpenDelegateDispatch stub.
-                // Fallback to desc comparison in such case for correctness.
-#if !FEATURE_CACHED_INTERFACE_DISPATCH
-                // both delegates are open
-                if (_methodPtrAux == other._methodPtrAux)
-                    return true;
-#endif
             }
 
             // It's possible that the method pointer was JITted in one delegate but not the other.

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/DelegateTests.cs

@@ -485,6 +485,15 @@ private static void IntIntMethod(int expected, int actual)
             Assert.Equal(expected, actual);
         }
 
+        [Fact]
+        public static void DifferentMethodForDerivedBase()
+        {
+            var d1 = (Action<Derived>)Delegate.CreateDelegate(typeof(Action<Derived>), typeof(Base).GetMethod("M")!);
+            var d2 = (Action<Derived>)Delegate.CreateDelegate(typeof(Action<Derived>), typeof(Derived).GetMethod("M")!);
+            Assert.False(d1.Equals(d2));
+            Assert.False(d1.Method.Equals(d2.Method));
+        }
+
         [Fact]
         public static void SameMethodObtainedViaDelegateAndReflectionAreSameForClass()
         {
@@ -571,6 +580,9 @@ internal virtual void M1() { }
             internal virtual void M2() { }
         }
 
+        class Base { public virtual void M() { } }
+        class Derived : Base { public override void M() { } }
+
         private delegate void IntIntDelegate(int expected, int actual);
         private delegate void IntIntDelegateWithDefault(int expected, int actual = 7);