Silent ctypes.struct layout change from native to Windows when bitfields are used

Bug report

Bug description:

Python 3.14 implements a special field _layout_ in ctypes.Structure that allows the programmer to explicitly choose the layout rules for bitfields used by either GCC/Clang or MSVC. The default layout is appropiate for the OS platform (MSVC on Windows, GCC otherwise) which is expected behaviour. It is also possible to use field _pack_ to change the layout to match the compiler. This is currently only implemented for the MSVC layout, not for GCC.

The problem is that using _pack_ on non-Windows systems silently switches the layout rules from GCC to MSVC, which is an unexpected layout by default on non-Windows systems. I propose that this case be handled as an error (NotImplementedError) rather than the silent layout switch. The error message may contain a hint that if the MSVC layout is indeed what is intended, one need to select it explicitly by setting field _layout_.

From the Zen of Python:

Explicit is better than implicit.
Readability counts.
Special cases aren't special enough to break the rules.
Errors should never pass silently.

Tested on 3.14a6

CPython versions tested on:

3.14

Operating systems tested on:

macOS

Linked PRs

• gh-133205

The issue identifies ctypes.Structure and its pack and layout fields as the entry point. Trace the non-Windows bitfield layout handling and its ctypes tests, then verify that unsupported pack behavior raises NotImplementedError while an explicitly selected MSVC layout remains usable.