`opentelemetry::proto::common::v1::_KeyValue_default_instance_`: illegal initialization of `constinit` entity with a non-constant expression (C++20)

[maxgolov]

Building with latest version of Visual Studio 2019 - update 16.10.0 , C++20 - using -std:c++latest , generated protobufs fail with this error:

Something related to this code here:

// Our use of constinit does not yet work with GCC:
// https://github.com/protocolbuffers/protobuf/issues/8310
#if defined(__cpp_constinit) && !defined(__GNUC__)
#define PROTOBUF_CONSTINIT constinit
#elif defined(__has_cpp_attribute)
#if __has_cpp_attribute(clang::require_constant_initialization)
#define PROTOBUF_CONSTINIT [[clang::require_constant_initialization]]
#endif
#endif
#ifndef PROTOBUF_CONSTINIT
#define PROTOBUF_CONSTINIT
#endif

Since it used to work before, I wonder if it's related to 16.10.0 now defining __cpp_constinit. My speculation at this point that I'm using slightly older version of all dependencies (via vcpkg), and maybe missing this protobuf fix:

2021-04-06 version 3.16.0 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
...
  C++ 
  * Fix PROTOBUF_CONSTINIT macro redefinition (#8323)

And most likely the code still works well with Visual Studio 2019 update 16.9.x. Need to dig deeper to understand the best solution, most likely starting with pulling the latest protobuf. And checking if latest vs2019 compiler works well with latest protobuf. If not, possibly applying a patch to disable PROTOBUF_CONSTINIT how it's done for __GNUC__.

[maxgolov]

From latest Visual Studio 2019 release notes. That addition now breaks the slightly older protobuf we are using:

I am not sure if there is a fix in latest protobuf or not, i.e. if we maybe need to pick up the fix. Worst case: if there is no fix in mainline protobuf, then we need to apply our local vcpkg port file with a patch on top of protobuf port_def.inc header. To use it for Windows builds of SDK with Visual Studio 2019. That patch should disable constint the same way as it is presently done for __GNUC__ ... Then the code should compile again even with latest Visual Studio 2019 16.10.0 and above in C++20 mode.

[maxgolov]

protocolbuffers/protobuf#8688