Let cclass wrapper tp_new build the underlying cypclass

Cython/Compiler/ModuleNode.py

@@ -1565,6 +1565,29 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
             "static PyObject *%s(PyTypeObject *t, %sPyObject *a, %sPyObject *k) {" % (
                 slot_func, unused_marker, unused_marker))
 
+        # for cyp wrappers, just allocate the cyobject and return the wrapper
+        # let the wrapped __init__ handle initialisation
+        if type.is_cyp_wrapper:
+            from .CypclassWrapper import generate_cypclass_wrapper_allocation
+
+            code.putln("if (t != %s) {" % type.typeptr_cname)
+            code.putln(
+                "PyErr_SetString(PyExc_TypeError,\"Cannot build a subtype of %s with %s.__new__\");"
+                % (scope.qualified_name, scope.qualified_name)
+            )
+            code.putln("return NULL;")
+            code.putln("}")
+            code.putln("CyObject * self = %s();" % type.wrapped_alloc)
+            generate_cypclass_wrapper_allocation(code, type)
+            code.putln(
+                "PyObject* wrapper = reinterpret_cast<PyObject *>(static_cast<%s *>(self));"
+                % Naming.cypclass_wrapper_layout_type
+            )
+            code.putln("Py_INCREF(wrapper);")
+            code.putln("return wrapper;")
+            code.putln("}")
+            return
+
         need_self_cast = (type.vtabslot_cname or
                           (py_buffers or memoryview_slices or py_attrs) or
                           cpp_class_attrs or cyp_class_attrs)

Cython/Compiler/Nodes.py

@@ -5588,6 +5588,9 @@ class CypclassWrapperDefNode(CClassDefNode):
         self.wrapped_cypclass.entry.type.wrapper_type = self.entry.type
         # > remember the declaration of the wrapped type
         self.entry.type.wrapped_cname = self.wrapped_cypclass.entry.type.empty_declaration_code()
+        # > remember the cname of the wrapped type allocation function
+        alloc_entry = self.wrapped_cypclass.entry.type.scope.lookup_here("<alloc>")
+        self.entry.type.wrapped_alloc = alloc_entry.func_cname
 
 
 class PropertyNode(StatNode):

Cython/Compiler/PyrexTypes.py

@@ -1501,12 +1501,14 @@ class PyExtensionType(PyObjectType):
     #  defered_declarations [thunk]      Used to declare class hierarchies in order
     #  check_size       'warn', 'error', 'ignore'    What to do if tp_basicsize does not match
     #  wrapped_cname    string or None   The full namespace declaration of the wrapped type when this is a cyp_wrapper
+    #  wrapped_alloc    string or None   The cname of the wrapped type allocation function when this is a cyp_wrapper
 
     is_extension_type = 1
     has_attributes = 1
     early_init = 1
 
     wrapper_cname = None
+    wrapped_alloc = None
 
     objtypedef_cname = None
 

Cython/Compiler/TypeSlots.py

@@ -433,7 +433,8 @@ class ConstructorSlot(InternalMethodSlot):
                 and not scope.has_pyobject_attrs
                 and not scope.has_memoryview_attrs
                 and not scope.has_cpp_class_attrs
-                and not (self.slot_name == 'tp_new' and scope.parent_type.vtabslot_cname)):
+                and not (self.slot_name == 'tp_new' and scope.parent_type.vtabslot_cname)
+                and not (self.slot_name == 'tp_new' and scope.parent_type.is_cyp_wrapper)):
             entry = scope.lookup_here(self.method) if self.method else None
             if not (entry and entry.is_special):
                 return False

Cython/Utility/CyObjects.cpp

@@ -209,16 +209,16 @@
     static inline U* __Pyx_PyObject_AsCyObject(PyObject * ob, PyTypeObject * type) {
         // the PyObject is not of the expected type
         if (ob->ob_type != type) {
-            PyErr_SetString(PyExc_TypeError, "Conversion Error: Could not convert to CyObject");
+            PyErr_Format(PyExc_TypeError, "Cannot convert PyObject %s to CyObject %s", ob->ob_type->tp_name, type->tp_name);
             return NULL;
         }
 
         CyPyObject * wrapper = (CyPyObject *)ob;
         U * underlying = dynamic_cast<U *>(static_cast<CyObject *>(wrapper));
 
-        // no underlying cyobject: shouldn't happen, playing it safe for now
+        // failed dynamic cast: should not happen
         if (underlying == NULL) {
-            PyErr_SetString(PyExc_TypeError, "Conversion Error: CyObject wrapper has no underlying CyObject");
+            PyErr_Format(PyExc_TypeError, "Could not convert %s PyObject wrapper to its underlying CyObject", type->tp_name);
             return NULL;
         }