Thanks Meador Inge for the bug report and thanks Sean Gillespie for the fix! It just took 9 years to fix this corner case ;-)

Copy of the comment on the PR:
https://github.com/python/cpython/pull/19914#pullrequestreview-419331432

I tried to rewrite _ctypes_callproc() to use PyMem_Malloc() instead of alloca(), but it's quite complicated. There are 3 arrays with a length of argcount items: args, avalues, atypes. Moreover, resbuf is also allocated with alloca(). When using PyMem_Malloc(), error handling is much more complicated.

I also tried to restrict the overall usage of stack memory to 4096 bytes (size of one page on x86), but users would be surprised by CTYPES_MAX_ARGCOUNT value.

I would say that raising an exception is better than crashing for a lot of arguments. If someone is blocked by this new limitation, in that case we can revisit the PyMem_Malloc() idea.