I found that the parser fails to handle correctly the (incorrect) case
where the single-star (*), used for delimiting keyword-only arguments,
is immediately followed by a **keywords parameter:

>>> def f(*, **kw):
...   pass
... 
python: Python/ast.c:652: handle_keywordonly_args: Assertion `kwonlyargs
!= ((void *)0)' failed.
[1]    7872 abort (core dumped)  ./python

Fixed in r59432

I've altered the assert(). It now checks if either kwonlyargs and
kwdefault or both not NULL or the next node is a DOUBLESTAR.

Err... I think it should raise a SyntaxError in this case.
See my attached patch.

Why do you want to forbid 

def f(*, **kw)

? It's useful and it also works in release builds of Python 3.0a2. It
only breaks in debug builds because the assert() gets triggered.

Amaury is right. "def f(*, **kw): pass" should raise a SyntaxError. The
keyword-only delimiter is useless since the **kw parameter already only
accepts keywords.

> Why do you want to forbid
>   def f(*, **kw)

Well, TOOWTDI and the like...
and the first time I saw it, it seemed that any number of parameters is
allowed!

Ah, you and Amaury are right! But I don't like Amaury's error message:

    SyntaxError: no name for vararg

It doesn't explain what's wrong. How about

    SyntaxError: keyword only arguments require at least one keyword

Kirk McDonald has an even better error message for us:

SyntaxError: Cannot specify keyword only arguments without named arguments

Here is another error message from Thomas Wouters 'named arguments must
follow bare *'

Right.
We should also replace the other occurence of "no name for vararg".
Here is another patch, against the current revision (59434).

I'm fine with your patch. Can you commit it please?

Committed revision 59443.