fix #683: allow overriding C-API mapped builtin functions with (auto-)cpdef functions

Cython/Compiler/ParseTreeTransforms.py

@@ -1418,7 +1418,7 @@ class AlignFunctionDefinitions(CythonTransform):
 
     def visit_DefNode(self, node):
         pxd_def = self.scope.lookup(node.name)
-        if pxd_def:
+        if pxd_def and (not pxd_def.scope or not pxd_def.scope.is_builtin_scope):
             if not pxd_def.is_cfunction:
                 error(node.pos, "'%s' redeclared" % node.name)
                 if pxd_def.pos:

tests/run/auto_cpdef.py

+# cython: auto_cpdef=True
+# mode:run
+# tag: directive,auto_cpdef
+
+import cython
+
+def str(arg):
+    """
+    This is a bit evil - str gets mapped to a C-API function and is
+    being redefined here.
+
+    >>> print(str('TEST'))
+    STR
+    """
+    return 'STR'
+
+@cython.test_assert_path_exists('//SimpleCallNode[@function.type.is_cfunction = True]')
+@cython.test_fail_if_path_exists('//SimpleCallNode[@function.type.is_builtin_type = True]')
+def call_str(arg):
+    """
+    >>> print(call_str('TEST'))
+    STR
+    """
+    return str(arg)