Special-case setting __new__ in class (GH-3102)

Cython/Compiler/ExprNodes.py

@@ -2297,8 +2297,10 @@ class NameNode(AtomicExprNode):
                 setter = 'PyDict_SetItem'
                 namespace = Naming.moddict_cname
             elif entry.is_pyclass_attr:
-                code.globalstate.use_utility_code(UtilityCode.load_cached("SetNameInClass", "ObjectHandling.c"))
-                setter = '__Pyx_SetNameInClass'
+                # Special-case setting __new__
+                n = "SetNewInClass" if self.name == "__new__" else "SetNameInClass"
+                code.globalstate.use_utility_code(UtilityCode.load_cached(n, "ObjectHandling.c"))
+                setter = '__Pyx_' + n
             else:
                 assert False, repr(entry)
             code.put_error_if_neg(

Cython/Utility/ObjectHandling.c

@@ -1218,6 +1218,30 @@ static PyObject *__Pyx__GetNameInClass(PyObject *nmspace, PyObject *name) {
 #define __Pyx_SetNameInClass(ns, name, value)  PyObject_SetItem(ns, name, value)
 #endif
 
+/////////////// SetNewInClass.proto ///////////////
+
+static int __Pyx_SetNewInClass(PyObject *ns, PyObject *name, PyObject *value);
+
+/////////////// SetNewInClass ///////////////
+//@requires: SetNameInClass
+
+// Special-case setting __new__: if it's a Cython function, wrap it in a
+// staticmethod. This is similar to what Python does for a Python function
+// called __new__.
+static int __Pyx_SetNewInClass(PyObject *ns, PyObject *name, PyObject *value) {
+#ifdef __Pyx_CyFunction_USED
+    int ret;
+    if (__Pyx_CyFunction_Check(value)) {
+        PyObject *staticnew = PyStaticMethod_New(value);
+        if (unlikely(!staticnew)) return -1;
+        ret = __Pyx_SetNameInClass(ns, name, staticnew);
+        Py_DECREF(staticnew);
+        return ret;
+    }
+#endif
+    return __Pyx_SetNameInClass(ns, name, value);
+}
+
 
 /////////////// GetModuleGlobalName.proto ///////////////
 //@requires: PyDictVersioning

tests/run/set_new.py

+"""
+>>> X = make_class_with_new(cynew)
+>>> X.__new__ is cynew
+True
+>>> X().__new__ is cynew
+True
+>>> def pynew(cls): return object.__new__(cls)
+>>> X = make_class_with_new(pynew)
+>>> X.__new__ is pynew
+True
+>>> X().__new__ is pynew
+True
+"""
+
+def make_class_with_new(n):
+    class X(object):
+        __new__ = n
+    return X
+
+def cynew(cls):
+    return object.__new__(cls)