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: 1112b89f0d

Change-Id: Ia0d366d3bff44a763aa496287814278dff732a19
(cherry picked from commit 61c47cbdc6)

test/sql/test_functions.py

@@ -1057,3 +1057,43 @@ def exec_sorted(statement, *args, **kw):
     return sorted(
         [tuple(row) for row in statement.execute(*args, **kw).fetchall()]
     )
+
+
+class RegisterTest(fixtures.TestBase, AssertsCompiledSQL):
+    __dialect__ = "default"
+
+    def setup(self):
+        self._registry = deepcopy(functions._registry)
+
+    def teardown(self):
+        functions._registry = self._registry
+
+    def test_GenericFunction_is_registered(self):
+        assert 'GenericFunction' not in functions._registry['_default']
+
+    def test_register_function(self):
+
+        # test generic function registering
+        class registered_func(GenericFunction):
+            _register = True
+
+            def __init__(self, *args, **kwargs):
+                GenericFunction.__init__(self, *args, **kwargs)
+
+        class registered_func_child(registered_func):
+            type = sqltypes.Integer
+
+        assert 'registered_func' in functions._registry['_default']
+        assert isinstance(func.registered_func_child().type, Integer)
+
+        class not_registered_func(GenericFunction):
+            _register = False
+
+            def __init__(self, *args, **kwargs):
+                GenericFunction.__init__(self, *args, **kwargs)
+
+        class not_registered_func_child(not_registered_func):
+            type = sqltypes.Integer
+
+        assert 'not_registered_func' not in functions._registry['_default']
+        assert isinstance(func.not_registered_func_child().type, Integer)