I noticed that passing keyword arguments to types.GenericAlias's __new__ causes the interpreter to hard crash and exit due to an assertion failure:

import types
types.GenericAlias(bad=float)


Result:
Assertion failed: PyTuple_CheckExact(kwnames), file ....\cpython\Python\getargs.c, line 2767

A simple fix is to just use _PyArg_NoKeywords instead of _PyArg_NoKwnames. 

Looking through the rest of the C code, it seems that apart from GenericAlias, only vectorcalls for various builtins use _PyArg_NoKwnames. However, they don't seem to be affected by the bug.

Oh, this looks like I copied the wrong idiom. Good catch!

New changeset 804d6893b801e8f30318afc38c20d4d0e6161db3 by kj in branch 'master':
bpo-42576: Raise TypeError when passing in keyword arguments to GenericAlias (GH-23656)
https://github.com/python/cpython/commit/804d6893b801e8f30318afc38c20d4d0e6161db3

New changeset d5c029b1a9b47737efb8966f804d28b99a2de239 by Miss Islington (bot) in branch '3.9':
bpo-42576: Raise TypeError when passing in keyword arguments to GenericAlias (GH-23656)
https://github.com/python/cpython/commit/d5c029b1a9b47737efb8966f804d28b99a2de239

Thanks for the quick fix!

Thanks for taking the time to review this! I forgot to specify this only affects debug builds in the news entry.  I'm opening a PR to edit that because I don't want people to mistake it for an actual interpreter-crashing bug in the release build (that would probably cause some amount of unneeded worry).

New changeset 6a7fb9d31bce8590e30c44458d1fc1da4539743d by kj in branch 'master':
bpo-42576: Clarify only debug builds are affected in news (GH-23663)
https://github.com/python/cpython/commit/6a7fb9d31bce8590e30c44458d1fc1da4539743d

New changeset 8502d46e3649bedf6b58a30b0a4b6b726e32b84b by Miss Islington (bot) in branch '3.9':
[3.9] bpo-42576: Clarify only debug builds are affected in news (GH-23663) (GH-23666)
https://github.com/python/cpython/commit/8502d46e3649bedf6b58a30b0a4b6b726e32b84b