classmethod() should fall back to calling PyClassMethod_New() instead…

… of deciding for itself that the type is invalid. (GH-3660)

Cython/Utility/CythonFunction.c

@@ -1317,16 +1317,7 @@ static PyObject* __Pyx_Method_ClassMethod(PyObject *method) {
         // python classes
         return PyClassMethod_New(PyMethod_GET_FUNCTION(method));
     }
-    else if (PyCFunction_Check(method)) {
-        return PyClassMethod_New(method);
-    }
-#ifdef __Pyx_CyFunction_USED
-    else if (__Pyx_CyFunction_Check(method)) {
+    else {
         return PyClassMethod_New(method);
     }
-#endif
-    PyErr_SetString(PyExc_TypeError,
-                   "Class-level classmethod() can only be called on "
-                   "a method_descriptor or instance method.");
-    return NULL;
 }

tests/run/classmethod.pyx

@@ -9,6 +9,10 @@ class1
 class1
 >>> class1().bview()
 class1
+>>> class1().cview()
+class1
+>>> class1().cview("XX")
+class1XX
 
 >>> class2.view()
 class2
@@ -35,6 +39,12 @@ cimport cython
 def f_plus(cls, a):
     return cls.a + a
 
+def second_decorator(f):
+    # note - a class, not a function (didn't pass Cython's test in __Pyx_Method_ClassMethod)
+    class C:
+        def __call__(self, *args):
+            return f(*args)
+    return C()
 
 class class1:
     a = 5
@@ -48,6 +58,11 @@ class class1:
     def bview(cls):
         print cls.__name__
 
+    @classmethod
+    @second_decorator
+    def cview(cls, s=""):
+        print cls.__name__+s
+
 
 class class2(object):
     a = 6