Allow conversion from PyObject to CyObject

Cython/Compiler/PyrexTypes.py

@@ -3998,6 +3998,21 @@ class CypClassType(CppClassType):
             return False
         self.to_py_function = "__Pyx_PyObject_FromCyObject"
         return True
+
+    def create_from_py_utility_code(self, env):
+        if not self.wrapper_type:
+            return False
+        wrapper_objstruct = self.wrapper_type.objstruct_cname
+        underlying_type_name = self.cname
+        self.from_py_function = "__Pyx_PyObject_AsCyObject<%s, %s>" % (wrapper_objstruct, underlying_type_name)
+        return True
+    
+    def from_py_call_code(self, source_code, result_code, error_pos, code,
+                          from_py_function=None, error_condition=None):
+        extra_args = [self.wrapper_type.typeptr_cname if self.wrapper_type else None]
+        return self._assign_from_py_code(
+            source_code, result_code, error_pos, code, from_py_function, error_condition, extra_args=extra_args)
+
     
     def empty_declaration_code(self):
         if self._empty_declaration is None:

Cython/Utility/CyObjects.cpp

@@ -185,6 +185,31 @@
         return ob;
     }
 
+    /*
+     * Cast from CyObject to PyObject:
+     *  - borrow an Python reference
+     *  - return a new atomic reference
+     * 
+     * template:
+     *  - W: the type of the extension type wrapper
+     *  - U: the type of the underlying cypclass
+     *  - T: pointer to the PyTypeObject instance of the wrapper
+     */
+    template <typename W, typename U>
+    static inline U* __Pyx_PyObject_AsCyObject(PyObject * ob, PyTypeObject * type) {
+        // the PyObject is not of the expected type
+        if (ob->ob_type != type)
+            return NULL;
+        W * wrapper = (W *)ob;
+        U * underlying = dynamic_cast<U *>(wrapper->nogil_cyobject);
+        // no underlying cyobject: shouldn't happen, playing it safe for now
+        if (underlying == NULL)
+            return NULL;
+        // return a new atomic reference
+        underlying->CyObject_INCREF();
+        return underlying;
+    }
+
     /* Cast argument to CyObject* type. */
     #define _CyObject_CAST(op) op