prevent "2**intsubtype" from being incorrectly optimised if intsubtype overrides __rpow__()

Cython/Utility/Optimize.c

@@ -438,13 +438,13 @@ static PyObject* __Pyx__PyNumber_PowerOf2(PyObject *two, PyObject *exp, PyObject
 
 static PyObject* __Pyx__PyNumber_PowerOf2(PyObject *two, PyObject *exp, PyObject *none, int inplace) {
 // in CPython, 1<<N is substantially faster than 2**N
-// TODO: disable this in Py3.5 if http://bugs.python.org/issue21420 gets accepted
+// see http://bugs.python.org/issue21420
 #if CYTHON_COMPILING_IN_CPYTHON
     Py_ssize_t shiftby;
-    if (likely(PyLong_Check(exp))) {
+    if (likely(PyLong_CheckExact(exp))) {
         shiftby = PyLong_AsSsize_t(exp);
 #if PY_MAJOR_VERSION < 3
-    } else if (likely(PyInt_Check(exp))) {
+    } else if (likely(PyInt_CheckExact(exp))) {
         shiftby = PyInt_AsLong(exp);
 #endif
     } else {

tests/run/powop.pyx

@@ -67,6 +67,34 @@ def int_pow(short a, short b):
     return a**b
 
 
+class I(int):
+    """
+    Copied from CPython's test_descr.py
+
+    >>> I(2) ** I(3)
+    I(8)
+    >>> 2 ** I(3)
+    I(8)
+    >>> I(3).pow2()
+    I(8)
+    """
+    def __repr__(self):
+        return 'I(%s)' % int(self)
+    def __pow__(self, other, mod=None):
+        if mod is None:
+            return I(pow(int(self), int(other)))
+        else:
+            return I(pow(int(self), int(other), int(mod)))
+    def __rpow__(self, other, mod=None):
+        if mod is None:
+            return I(pow(int(other), int(self), mod))
+        else:
+            return I(pow(int(other), int(self), int(mod)))
+
+    def pow2(self):
+        return 2 ** self
+
+
 def optimised_pow2(n):
     """
     >>> optimised_pow2(0)