Do not apply decorators twice

This fixes https://github.com/cython/cython/issues/1679

Cython/Compiler/Nodes.py

@@ -2646,6 +2646,9 @@ class DefNode(FuncDefNode):
 
     child_attrs = ["args", "star_arg", "starstar_arg", "body", "decorators", "return_type_annotation"]
 
+    is_staticmethod = False
+    is_classmethod = False
+
     lambda_name = None
     reqd_kw_flags_cname = "0"
     is_wrapper = 0
@@ -2758,7 +2761,6 @@ class DefNode(FuncDefNode):
         return True
 
     def analyse_declarations(self, env):
-        self.is_classmethod = self.is_staticmethod = False
         if self.decorators:
             for decorator in self.decorators:
                 func = decorator.decorator

Cython/Compiler/ParseTreeTransforms.py

@@ -1336,8 +1336,18 @@ class DecoratorTransform(ScopeTrackingTransform, SkipDeclarations):
                         return self._reject_decorated_property(node, decorator_node)
                     return self._add_to_property(properties, node, handler_name, decorator_node)
 
+        # we clear node.decorators, so we need to set the
+        # is_staticmethod/is_classmethod attributes now
+        for decorator in node.decorators:
+            func = decorator.decorator
+            if func.is_name:
+                node.is_classmethod |= func.name == 'classmethod'
+                node.is_staticmethod |= func.name == 'staticmethod'
+
         # transform normal decorators
-        return self.chain_decorators(node, node.decorators, node.name)
+        decs = node.decorators
+        node.decorators = None
+        return self.chain_decorators(node, decs, node.name)
 
     @staticmethod
     def _reject_decorated_property(node, decorator_node):

tests/run/cyfunction.pyx

@@ -304,3 +304,47 @@ def test_annotations(a: "test", b: "other" = 2, c: 123 = 4) -> "ret":
     def inner(x: "banana", y: b()) -> c():
         return x,y
     return inner
+
+
+def add_one(func):
+    "Decorator to add 1 to the last argument of the function call"
+    def inner(*args):
+        args = args[:-1] + (args[-1] + 1,)
+        return func(*args)
+    return inner
+
+@add_one
+def test_decorated(x):
+    """
+    >>> test_decorated(0)
+    1
+    """
+    return x
+
+@add_one
+@add_one
+def test_decorated2(x):
+    """
+    >>> test_decorated2(0)
+    2
+    """
+    return x
+
+
+cdef class TestDecoratedMethods:
+    @add_one
+    def test(self, x):
+        """
+        >>> TestDecoratedMethods().test(0)
+        1
+        """
+        return x
+
+    @add_one
+    @add_one
+    def test2(self, x):
+        """
+        >>> TestDecoratedMethods().test2(0)
+        2
+        """
+        return x