CMake incremental compilation through toolchain upgrade - cmake

Question

I am trying to find a way to enable incremental compilation with CMake through a toolchain upgrade. Here is the problematic scenario :
Branch main uses g++-9 (using CMAKE_CXX_COMPILER=g++-9)
A new branch uses g++-10 (using CMAKE_CXX_COMPILER=g++-10)
Commits are happening on both branches
Incremental builds on one branch work fine
Switching to the other branch and explicitly invoking CMake fails
My question is the following : I'm looking for the proper way to make the invocation of CMake succeed and rebuild all the project from scratch when a toolchain change happens.
Here is a script that will make it quick and easy to reproduce the problem. This script requires Docker. It will create folders Sources and Build at the location where it is executed to avoid littering your filesystem. It then creates Dockerfiles to build docker containers with both g++ and cmake. It then creates a dummy Hello World C++ CMake project. Finally, it creates a folder for build artifacts and then executes the build with g++-9 and then g++-10. The second build fails because CMake generates an error.
#!/bin/bash
set -e
mkdir -p Sources
mkdir -p Build
# Creates a script that will be executed inside the docker container to perform builds
cat << EOF > Sources/Compile.sh
cd /Build \
&& cmake /Sources \
&& make \
&& ./IncrementalBuild
EOF
# Creates a Dockerfile that will be used to have both gcc-9 and cmake
cat << EOF > Sources/Dockerfile-gcc9
FROM gcc:9
RUN apt-get update && apt-get install -y cmake
RUN ln -s /usr/local/bin/g++ /usr/local/bin/g++-9
ADD Compile.sh /Compile.sh
RUN chmod +x /Compile.sh
ENTRYPOINT /Compile.sh
EOF
# Creates a Dockerfile that will be used to have both gcc-10 and cmake
cat << EOF > Sources/Dockerfile-gcc10
FROM gcc:10
RUN apt-get update && apt-get install -y cmake
RUN ln -s /usr/local/bin/g++ /usr/local/bin/g++-10
ADD Compile.sh /Compile.sh
RUN chmod +x /Compile.sh
ENTRYPOINT /Compile.sh
EOF
# Creates a dummy C++ program that will be compiled
cat << EOF > Sources/main.cpp
#include <iostream>
int main()
{
std::cout << "Hello World!\n";
}
EOF
# Creates CMakeLists.txt that will be used to compile the dummy C++ program
cat << EOF > Sources/CMakeLists.txt
cmake_minimum_required(VERSION 3.9)
project(IncrementalBuild CXX)
add_executable(IncrementalBuild main.cpp)
set_target_properties(IncrementalBuild PROPERTIES CXX_STANDARD 17)
EOF
# Build the docker images with both Dockerfiles created earlier
docker build -t cmake-gcc:9 -f Sources/Dockerfile-gcc9 Sources
docker build -t cmake-gcc:10 -f Sources/Dockerfile-gcc10 Sources
# Run a build with g++-9
echo ""
echo "### Compiling with g++-9 and then running the result..."
docker run --rm --user $(id -u):$(id -g) -v $(pwd)/Sources:/Sources -v $(pwd)/Build:/Build -e CXX=g++-9 cmake-gcc:9
echo ""
# Run a build with g++-10
echo "### Compiling with g++-10 and then running the result..."
docker run --rm --user $(id -u):$(id -g) -v $(pwd)/Sources:/Sources -v $(pwd)/Build:/Build -e CXX=g++-10 cmake-gcc:10
echo ""
# Print success if we reach this point
echo "SUCCESS!"

Answer 1

I'm looking for the proper way to make the invocation of CMake succeed and rebuild all the project from scratch when a toolchain change happens.
The proper way is to use a fresh binary directory. Either remove the binary directory when changing and let it recreate or just use a separate different directory for each toolchain.
Use Build/gcc10 binary directory for gcc10 build and Build/gcc9 for gcc9 builds.
No need to cd Build and mkdir with nowadays cmake - use cmake -S. -BBuild. Also do not use make - prefer cmake --build Build to let you switch generator later.

Answer 2

"If you change the toolchain, you should start with a fresh build. There are too many things that assume the toolchain doesn’t change and while you may be able to find workarounds which appear to work, I recommend you always use a fresh build tree for a different toolchain. This same logic also applies if you update the existing toolchain in-place (e.g. you update to a newer version of GCC on Linux, a newer version of Xcode on macOS, etc.). CMake queries compiler capabilities and caches the results. If you change the toolchain in a way that CMake can’t catch, then you end up with stale cached capabilities being used for the new/updated toolchain. Please don’t do that." - Craig Scott
So essentially I don't think it's possible. You just need to blow away your build. The best thing you can do is alert users if CMake isn't doing it for you.
Perhaps reply on this also:
https://discourse.cmake.org/t/how-to-change-toolchain-without-breaking-developer-workflows/1166
Or start another discourse.