v25.0 regression: C2370 compile error from map_field.h when compiling with cl.exe and /permissive- option

[yukawa]

What version of protobuf and what language are you using?
Version: v25.0. Looks like a regression in 95d5723.
Language: C++

What operating system (Linux, Windows, ...) and version?
Windows 11 ver 22H2 (OS Build 22621.2428)

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

• Microsoft Visual Studio Community 2022 (64-bit) Version 17.7.0
  • Microsoft (R) C/C++ Optimizing Compiler Version 19.37.32822 for x64
  • cmake version 3.26.4-msvc4

What did you do?
Steps to reproduce the behavior:

"C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Auxiliary\Build\vcvarsall.bat" amd64_x86
git clone -b v25.0 https://github.com/protocolbuffers/protobuf.git --recursive
cd protobuf
cmake . -DCMAKE_CXX_STANDARD=20 -Dprotobuf_ABSL_PROVIDER=module
cmake --build . -j

What did you expect to see
Build succeeds.

What did you see instead?
Build fails with the following error.

printer.cc
C:\Users\builder\source\repos\protobuf\src\google/protobuf/map_field.h(682,66): error C2370: 'google::protobuf::internal::MapField<Derived,Key,T,kKeyFieldType_,kValueFieldType_>::kVTable': redefinition; different storage class (compiling source file C:\Users\builder\source\repos\protobuf\src\google\protobuf\struct.pb.cc) [C:\Users\builder\source\repos\protobuf\libprotobuf.vcxproj]
C:\Users\builder\source\repos\protobuf\src\google/protobuf/map_field.h(671,37): message : see declaration of 'google::protobuf::internal::MapField<Derived,Key,T,kKeyFieldType_,kValueFieldType_>::kVTable' (compiling source file C:\Users\builder\source\repos\protobuf\src\google\protobuf\struct.pb.cc) [C:\Users\builder\source\repos\protobuf\libprotobuf.vcxproj]
  strtod.cc

It's failing here.

protobuf/src/google/protobuf/map_field.h

Lines 671 to 683 in 6b5d8db

	static const MapFieldBase::VTable kVTable;
	friend class google::protobuf::Arena;
	friend class MapFieldBase;
	friend class MapFieldStateTest; // For testing, it needs raw access to impl_
	};
	template <typename Derived, typename Key, typename T,
	WireFormatLite::FieldType kKeyFieldType_,
	WireFormatLite::FieldType kValueFieldType_>
	constexpr MapFieldBase::VTable
	MapField<Derived, Key, T, kKeyFieldType_, kValueFieldType_>::kVTable =
	MapField::template MakeVTable<MapField>();

Anything else we should know about your project / environment

Seems that this is a known issue in cl.exe.

• C2370 is falsely emitted when a static const member is defined with the constexpr specifier outside a template class

The tricky thing is that /permissive- is enabled by default when /std:c++20 is specified. While developers can still work around this by manually setting /permissive only when building protobuf, it'd be really nice if we can avoid this error in protobuf in the first place.

[bebuch]

Same bug in 25.1

Can you please merge f78f9c5 into the 25.x line?

[ericroy]

Still not fixed in 25.2
The key to reproducing is that you must compile as c++20 (CMAKE_CXX_STANDARD=20)

[versat]

Still not fixed in 25.2 The key to reproducing is that you must compile as c++20 (CMAKE_CXX_STANDARD=20)

Looks like we have to wait until 26.0 is released. I hope there will not be any issues again :(