Warnings with Protobuf headers and MSVC

[coryan]

What version of protobuf and what language are you using?

Version: 22.x but I think the problem is also in main
Language: C++

What operating system (Linux, Windows, ...) and version?

Windows

What runtime / compiler are you using (e.g., python version or gcc version)

MSVC 2019 or MSVC 2022

What did you do?

Compiled some generated protos with /WX /W4, this is analogous to -Wall -Werror with GCC or Clang.

What did you expect to see

A successful compilation.

What did you see instead?

A compilation warning. As I have enabled "warnings as errors" it breaks the build.

# Configuration: f149506ea002c5503763e19bb7f79693d083f873032cd55c19002b510a750b73
# Execution platform: @local_config_platform//:host
bazel-out/x64_windows-opt/bin/external/com_google_protobuf/src/google/protobuf/_virtual_includes/arena_align\google/protobuf/arena_align.h(102): error C2220: the following warning is treated as an error
bazel-out/x64_windows-opt/bin/external/com_google_protobuf/src/google/protobuf/_virtual_includes/arena_align\google/protobuf/arena_align.h(102): warning C4146: unary minus operator applied to unsigned type, result still unsigned
bazel-out/x64_windows-opt/bin/external/com_google_protobuf/src/google/protobuf/_virtual_includes/arena_align\google/protobuf/arena_align.h(145): warning C4146: unary minus operator applied to unsigned type, result still unsigned
bazel-out/x64_windows-opt/bin/external/com_google_protobuf/src/google/protobuf/_virtual_includes/arena_align\google/protobuf/arena_align.h(159): warning C4146: unary minus operator applied to unsigned type, result still unsigned
bazel-out/x64_windows-opt/bin/external/com_google_protobuf/src/google/protobuf/_virtual_includes/protobuf_lite\google/protobuf/arena.h(300): note: see reference to function template instantiation 'T *google::protobuf::internal::ArenaAlign::Ceil<void>(T *) const' being compiled
        with
        [
            T=void
        ]
INFO: From Action external/com_google_googleapis/google/cloud/certificatemanager/v1/certificate_issuance_config.grpc.pb.h:

Anything else we should know about your project / environment

I usually try to workaround these problems by disabling the warning when including external headers, but I have not been very successful this time. If at all possible consider patching the headers.

You can repro the problem using godbolt:

https://godbolt.org/z/ces8Y7EPb

Changing the alignment to an int works:

https://godbolt.org/z/e837Kz6fc

Adding some casts also works:

https://godbolt.org/z/9M44v7bc6

Note that the same problem appears in 3 places in the code:

https://github.com/protocolbuffers/protobuf/blob/0ae328929ee2b6378d8cb3bb2c26700922098910/src/google/protobuf/arena_align.h#LL101C1-L103C4

protobuf/src/google/protobuf/arena_align.h

Line 145 in 0ae3289

constexpr size_t Ceil(size_t n) const { return (n + align - 1) & -align; }

and

protobuf/src/google/protobuf/arena_align.h

Line 159 in 0ae3289

return reinterpret_cast<T*>((intptr + align - 1) & -align);

That is the "fixes" are more like "proof-of-concepts" or "sketches".