Fix bug when calling cdef append.

Cython/Compiler/Optimize.py

@@ -2600,7 +2600,7 @@ class OptimizeBuiltinCalls(Visitor.NodeRefCleanupMixin,
         """Optimistic optimisation as X.append() is almost always
         referring to a list.
         """
-        if len(args) != 2 or node.result_is_used:
+        if function.type.is_cfunction or len(args) != 2 or node.result_is_used:
             return node
 
         return ExprNodes.PythonCapiCallNode(

tests/run/append.pyx

@@ -8,6 +8,16 @@ class B(list):
         for arg in args:
             list.append(self, arg)
 
+cdef class C:
+    """
+    >>> c = C(100)
+    appending 100
+    """
+    def __init__(self, value):
+        self.append(value)
+    cdef append(self, value):
+        print u"appending", value
+        return value
 
 def test_append(L):
     """