Incompatible MSVC runtime linking between Abseil dependency and Protobuf

[ollijs]

What version of protobuf and what language are you using?
Version: v30.0-rc1
Language: C++

Windows 11

What runtime / compiler are you using (e.g., python version or gcc version)
Visual Studio 2022 / MSVC 19.43.34808 / Cmake 3.31.1

What did you do?
Steps to reproduce the behavior:

1. Cloned protobuf from repo (tag v30.0-rc1)
2. Opened protobuf in Visual Studio as folder (i.e. using VS cmake integration)
3. Set in CMakePresets.json: "CMAKE_CXX_STANDARD": "17", "protobuf_MSVC_STATIC_RUNTIME": false
4. Built protobuf (and installed). The Abseil dependency is automatically downloaded and built

What did you expect to see
Abseil and consecutively, protobuf built with dynamically linked MSVC runtime as per protobuf_MSVC_STATIC_RUNTIME flag

What did you see instead?
Abseil is built with static MSVC runtime linking, Protobuf is build with dynamic MSVC linking. Build fails

The problem is caused by protobuf/CMakeLists.txt lines 202-212, specifically 206: set(ABSL_MSVC_STATIC_RUNTIME ON)

if (protobuf_BUILD_SHARED_LIBS)
  set(protobuf_SHARED_OR_STATIC "SHARED")
else (protobuf_BUILD_SHARED_LIBS)
  set(protobuf_SHARED_OR_STATIC "STATIC")
  set(ABSL_MSVC_STATIC_RUNTIME ON)
  if (protobuf_MSVC_STATIC_RUNTIME)
      set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreaded$<$<CONFIG:Debug>:Debug>)
  else()
      set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreaded$<$<CONFIG:Debug>:Debug>DLL)
  endif()
endif (protobuf_BUILD_SHARED_LIBS)

The logic sets ABSL to use static runtime linking when protobuf is configured to produce a static lib. Those are two different things though, and which kind of a lib is produced shouldn't affect how the MSVC runtime is linked to them.

A simple fix for me to move forward was to fix set ABSL_MSVC_STATIC_RUNTIME to OFF instead since I don't care about static runtime linking. A proper fix would probably be to set ABSL_MSVC_STATIC_RUNTIME only based on protobuf_MSVC_STATIC_RUNTIME, regardless of whether the output is dynamic or static.