bpo-39689: Do not use native packing for format "?" with standard size

[skrah]

https://bugs.python.org/issue39689

Automerge-Triggered-By: @encukou

[skrah]

This implements the current documentation somewhat pedantically:

The '?' conversion code corresponds to the _Bool type defined by C99. If this type is not available, it is simulated using a char. In standard mode, it is always represented by one byte.

We always have C99 now, so native mode always uses _Bool. When compiling with
-fsanitize=bool, this should (and does) fail:

>>> from struct import *
>>> unpack("?", b"\x02")
/home/stefan/cpython/Modules/_struct.c:487:12: runtime error: load of value 2, which is not a valid value for type '_Bool'
(False,)

This, however, should pass (and does after this patch):

>>> unpack("<?", b"\x02")
(True,)

[encukou]

The docs currently say:

For the '?' format character, the return value is either True or False. When packing, the truth value of the argument object is used. Either 0 or 1 in the native or standard bool representation will be packed, and any non-zero value will be True when unpacking.

So, if we do want change that and expose UB to Python, the docs need to change. IMO, this will need a news entry as well.

The docs also currently talk about the characters @=<>! only affecting byte order, size, and alignment. Will that need to be expanded?

[bedevere-bot]

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

[miss-islington]

Thanks @skrah for the PR 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

[miss-islington]

Thanks @skrah for the PR 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[bedevere-bot]

GH-19154 is a backport of this pull request to the 3.7 branch.

[bedevere-bot]

GH-19155 is a backport of this pull request to the 3.8 branch.