diff -r 582e8e71f635 a/Tools/clinic/clinic.py b/Tools/clinic/clinic.py
--- a/Tools/clinic/clinic.py
+++ b/Tools/clinic/clinic.py
@@ -3638,7 +3638,11 @@ class DSLParser:
         parameter_name = parameter.arg
         name, legacy, kwargs = self.parse_converter(parameter.annotation)
 
-        if not default:
+        if 'py_default' in kwargs and 'c_default' in kwargs:
+            # Specifying both a Python and C default will bypass further scrutiny
+            value = unknown
+            default = unspecified
+        elif not default:
             if self.parameter_state == self.ps_optional:
                 fail("Can't have a parameter without a default (" + repr(parameter_name) + ")\nafter a parameter with a default!")
             value = unspecified