regression : [protobuf] generated code fails to compile with MSVC after upgrade to 3.25.1 with C2370 error

[davidhunter22]

This is as much an information message to save other people trying to understand this issue.

The following PR was just merged #35781
This upgrades protobuf to version 3.25.1

This causes a regression for MSVC
The code in the protobuf package can't be compiled with the MSVC compiler.
The compiler issue is known and is described here https://developercommunity.visualstudio.com/t/c2370-is-falsely-emitted-when-a-static-const-membe/898167

There are a number of issues on the protobuf/grpc github related to this
protocolbuffers/protobuf#14602
grpc/grpc#35297

The suggested work around seem to be to add /permissive to your build which is sad. Even sadder the bug is in code in the generated header file so /permissive has to be specified in any including code.

Note the problematic code is only generated if you use a map field in your protobuf definition i.e. map<string,int>

[zlojvavan]

suggested workaround lead to an ICE for me, see this

[davidhunter22]

After some discussions I was informed that code that compiles in strict mode i.e /permissive- can fail to compile in lax mode i.e. /permissive. This means that using /permissive may not be a work around.
Note a "fix" for this has been implemented in Protobuf 26.0. See protocolbuffers/protobuf@f78f9c5 and related issue protocolbuffers/protobuf#14602
At present I don't know of a work around that always works for protobuf 25.0 and MSVC

[davidhunter22]

I just realized the problem is not in the code generated by the protoc compiler but in the map_field.h file in the package.

So a patch for this would be to change line 681 of map_field.h from
constexpr MapFieldBase::VTable
to
const MapFieldBase::VTable
at least for MSVC

Note this is exactly the change made by the patch protocolbuffers/protobuf@f78f9c5 in protobuf 2.6.0

[FangCunWuChang]

I hit this. It breaks my proto file generation.