prevent 2**N optimisation from breaking 2.0**N

Cython/Compiler/ExprNodes.py

@@ -9555,6 +9555,7 @@ class PowNode(NumBinopNode):
     def py_operation_function(self, code):
         if (self.type.is_pyobject and
                 self.operand1.constant_result == 2 and
+                isinstance(self.operand1.constant_result, (int, long)) and
                 self.operand2.type is py_object_type):
             code.globalstate.use_utility_code(UtilityCode.load_cached('PyNumberPow2', 'Optimize.c'))
             if self.inplace:

tests/run/powop.pyx

@@ -91,4 +91,7 @@ def optimised_pow2(n):
     Traceback (most recent call last):
     TypeError: unsupported operand type(s) for ** or pow(): 'int' and 'str'
     """
+    if isinstance(n, (int, long)) and 0 <= n < 1000:
+        assert isinstance(2.0 ** n, float), 'float %s' % n
+        assert isinstance(2 ** n, (int, long)), 'int %s' % n
     return 2 ** n