Do not register the GenericFunction in sql.functions._registry

Fixed that the :class:`.GenericFunction` class was inadvertently
registering itself as one of the named functions.  Pull request courtesy
Adrien Berchet.

Fixes: #4653

Closes: #4654
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4654
Pull-request-sha: 1112b89f0d5af8cd5ba88cef744698a79dbdb963

Change-Id: Ia0d366d3bff44a763aa496287814278dff732a19

1 files changed, 12 insertions, 1 deletions

diff --git a/lib/sqlalchemy/sql/functions.py b/lib/sqlalchemy/sql/functions.py
index feb4fdb90..d0aa23988 100644
--- a/lib/sqlalchemy/sql/functions.py
+++ b/lib/sqlalchemy/sql/functions.py
@@ -603,7 +603,17 @@ class _GenericMeta(VisitableType):
             # legacy
             if "__return_type__" in clsdict:
                 cls.type = clsdict["__return_type__"]
-            register_function(identifier, cls, package)
+
+            # Check _register attribute status
+            cls._register = getattr(cls, '_register', True)
+
+            # Register the function if required
+            if cls._register:
+                register_function(identifier, cls, package)
+            else:
+                # Set _register to True to register child classes by default
+                cls._register = True
+
         super(_GenericMeta, cls).__init__(clsname, bases, clsdict)
 
 
@@ -671,6 +681,7 @@ class GenericFunction(util.with_metaclass(_GenericMeta, Function)):
     """
 
     coerce_arguments = True
+    _register = False
 
     def __init__(self, *args, **kwargs):
         parsed_args = kwargs.pop("_parsed_args", None)