Set DefNode's decorators to None after assignment synthesis

Cython/Compiler/ParseTreeTransforms.py

@@ -1611,6 +1611,7 @@ if VALUE is not None:
                     decorator.pos,
                     function = decorator.decorator,
                     args = [rhs])
+            def_node.decorators = None
 
         assmt = Nodes.SingleAssignmentNode(
             def_node.pos,

tests/run/decorator_lambda.pyx

@@ -13,3 +13,12 @@ class TestClassDecorator(object):
     """
     def hello(self):
         return "Hello, world!"
+
+
+@decorate(lambda x: x)
+def test_function():
+    """
+    >>> test_function()
+    123
+    """
+    return 123