Clean up C version of instance check and add change note.

docs/changes.rst

@@ -12,6 +12,12 @@ Version 1.14.0
   The replacement function is exposed as `wrapt.formatargspec()` if need it
   for your own code.
 
+* When using a decorator on a class, `isinstance()` checks wouldn't previously
+  work as expected and you had to manually use `Type.__wrapped__` to access
+  the real type when doing instance checks. The `__instancecheck__` hook is now
+  implemented such that you don't have to use `Type.__wrapped__` instead of
+  `Type` as last argument to `isinstance()`.
+
 **New Features**
 
 * Binary wheels provided on PyPi for `aarch64` Linux systems and macOS

src/wrapt/_wrappers.c

@@ -2551,21 +2551,18 @@ static PyObject *WraptFunctionWrapperBase_instancecheck(
     PyObject *result = NULL;
 
     int check = 0;
+
     if (!self->object_proxy.wrapped) {
         PyErr_SetString(PyExc_ValueError, "wrapper has not been initialized");
         return NULL;
     }
 
-    object = PyObject_GetAttrString(self, "__wrapped__");
-    if (!object) {
-        PyErr_Clear();
-    }
+    check = PyObject_IsInstance(instance, self->object_proxy.wrapped);
 
-    check = PyObject_IsInstance(instance, object);
-    Py_XDECREF(object);
     if (check < 0) {
         return NULL;
     }
+
     result = check ? Py_True : Py_False;
 
     Py_INCREF(result);