This popped out at me while looking for something else. It's probably not much of an actual problem, since the wrong datatype is larger than the correct one, but it's worth fixing.

The problem is in overlapped_RegisterWaitWithQueue, at overlapped.c:297 (in _overlapped):

    if (!RegisterWaitForSingleObject(
            &NewWaitObject, Object, (WAITORTIMERCALLBACK)PostToQueueCallback,
            pdata, Milliseconds,
            WT_EXECUTEINWAITTHREAD | WT_EXECUTEONLYONCE))


...it stood out to me immediately while I was paging past, since function pointer casts of this sort are almost always wrong, and I've seen it too many times.

WAITORTIMERCALLBACK is a typedef of WAITORTIMERCALLBACKFUNC:

typedef VOID (NTAPI * WAITORTIMERCALLBACKFUNC) (PVOID, BOOLEAN );

...and PostToQueueCallback is defined as:

static VOID CALLBACK
PostToQueueCallback(PVOID lpParameter, BOOL TimerOrWaitFired)

...where BOOL is an int, and BOOLEAN is an unsigned char. I guess there could be some kind of issue down the line if the generated code tries to read the BOOLEAN/int from the register/memory location where there's actually an unsigned char, but after about an hour of debugging, I can't see it. The documentation also states explicitly that this should be either TRUE or FALSE. Also, it doesn't matter what we actually pass to PostQueuedCompletionStatus: https://devblogs.microsoft.com/oldnewthing/20070525-00/?p=26693

By changing the (incorrect) BOOL declaration to BOOLEAN, then we don't need the cast.

New changeset af4eda46d1538b1da700a86588bdb94b0a4d1ff2 by Zackery Spytz in branch 'master':
bpo-40150: Fix mismatched argument in RegisterWaitForSingleObject() call (GH-19686)
https://github.com/python/cpython/commit/af4eda46d1538b1da700a86588bdb94b0a4d1ff2

New changeset f8055fb0f1054fce6a21047da39b92ae32416b80 by Miss Islington (bot) in branch '3.8':
bpo-40150: Fix mismatched argument in RegisterWaitForSingleObject() call (GH-19686)
https://github.com/python/cpython/commit/f8055fb0f1054fce6a21047da39b92ae32416b80

New changeset 4a02da4f95cfff679e38a13ca0f44d660c5669b5 by Miss Islington (bot) in branch '3.9':
bpo-40150: Fix mismatched argument in RegisterWaitForSingleObject() call (GH-19686)
https://github.com/python/cpython/commit/4a02da4f95cfff679e38a13ca0f44d660c5669b5