MSVC 5274 warning suppression for _Py_ALIGN_AS should be local #135183
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build
The build process and cross-build
interpreter-core
(Objects, Python, Grammar, and Parser dirs)
topic-C-API
type-bug
An unexpected behavior, bug, or error
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encukou
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Jun 5, 2025
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Bug report
In #133966 (comment) we added a
Python.h-wide suppression for MSVC warning 5274 “Application of _Alignas on a structured type in C”.I think it should be instead applied locally, after a bit of thought.
AFAICS, 5274 warns that structs compiled with MSVC versions older than 17.9 might end up with a different layout (ABI) than ones compiled with 17.9+. That can cause trouble for extensions.
In
PyASCIIObject,_Py_ALIGN_ASis applied to an anonymous struct. I think we can assume that no one is pullingstate's type out with something liketypeof(my_PyASCIIObject.state)– and if they do, getting an 4B-aligned struct is very unlikely to cause trouble in practice.But, that is the kind of analysis one should do when a 5274 warning pops up.
Linked PRs
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