diff -r 92b3928bfde1 Lib/inspect.py
--- a/Lib/inspect.py	Sat Feb 01 21:22:26 2014 +0200
+++ b/Lib/inspect.py	Sat Feb 01 20:46:13 2014 +0100
@@ -2187,6 +2187,10 @@
 
     @classmethod
     def from_builtin(cls, func):
+        if _signature_is_functionlike(func):
+            # If we have a complete function signature, prefer analysing that.
+            return cls.from_function(func)
+
         if not _signature_is_builtin(func):
             raise TypeError("{!r} is not a Python builtin "
                             "function".format(func))