Allow conversion to PyObject only when supported (e.g. not for templated cypclasses)

Cython/Compiler/Nodes.py

@@ -1637,7 +1637,8 @@ class CppClassNode(CStructOrUnionDefNode, BlockNode):
         cclass_bases = TupleNode(self.pos, args=[])
 
         if self.templates:
-            print("Quick warning: Python wrappers for templated cypclasses are not supported yet")
+            # Python wrapper for templated cypclasses not supported yet
+            # this is signaled to the compiler by not doing what is below
             return
 
         if self.attributes is not None:

Cython/Compiler/PyrexTypes.py

@@ -3963,7 +3963,7 @@ class CypClassType(CppClassType):
     #  wrapper_type       PyExtensionType or None      the type of the cclass wrapper
 
     is_cyp_class = 1
-    to_py_function = "__Pyx_PyObject_FromCyObject"
+    to_py_function = None
 
     def __init__(self, name, scope, cname, base_classes, templates=None, template_type=None, nogil=0, lock_mode=None, activable=False):
         CppClassType.__init__(self, name, scope, cname, base_classes, templates, template_type, nogil)
@@ -4026,6 +4026,13 @@ class CypClassType(CppClassType):
         self._left_path_to_base[base] = path
         return path
 
+    # allow conversion to Python only when wrapping is supported    
+    def create_to_py_utility_code(self, env):
+        if not self.wrapper_type:
+            return False
+        self.to_py_function = "__Pyx_PyObject_FromCyObject"
+        return True
+    
     def empty_declaration_code(self):
         if self._empty_declaration is None:
             self._empty_declaration = self.declaration_code('', deref=1)