CXX-2577 compile and test with c++20

[rcsanchez97]

I split my changes into separate commits so that it was clear where I needed a supporting change (newer CMake), where I was dealing with somewhat unrelated bugs (the output paths in case of failure and the incorrect CMake variables), the new tasks, and the syntax changes needed to make sure that the new tasks are successful.

I also want add an entirely new RHEL9 variant (that being the RHEL version with a new enough toolchain that supports c++20), but I need to investigate if the server is available for RHEL9 and I need to update the download script in DET. It seemed better to get these changes going for review as they currently are and then to either push additional commits on this PR or follow-up with a separate PR depending on what the RHEL9 variant ends up requiring.

Full Evergreen patch build: https://spruce.mongodb.com/version/6466d6f03e8e86357cad069e/tasks?sorts=STATUS%3AASC%3BBASE_STATUS%3ADESC

[rcsanchez97]

I was able to figure out a simple way to make the RHEL9 build work. I have pushed an additional commit with the new build variants. Here is a patch build with the new variants: https://spruce.mongodb.com/version/6469400b7742aea9a520062d/tasks?sorts=STATUS%3AASC%3BBASE_STATUS%3ADESC

[rcsanchez97]

Update patch build, to reflect the last few commits I made: https://spruce.mongodb.com/version/646bb408e3c331f46b5e294e/tasks?sorts=STATUS%3AASC%3BBASE_STATUS%3ADESC

[eramongodb]

Use of CMake versions newer than 3.15.0 with Boost polyfill enabled exposes a missing find_dependency call in the bsoncxx package config file, which is causing example project compilation failures on MacOS due to the imported Boost target being missing (library is known to link with target Boost::boost, but the target is not defined).

Suggest using a pattern similar to what is used for the bsoncxx_pkg_dep variable -> BSONCXX_PKG_DEP parameter -> expansion in config file to inject a find_dependency(Boost 1.56.0 REQUIRED) when BSONCXX_POLY_USE_BOOST is set.

See this patch and corresponding changes for reference:

• src/bsoncxx/CMakeLists.txt
• BsoncxxUtil.cmake
• bsoncxx-config.cmake.in

[eramongodb]

Alternatively, if the Boost dependency does not require link dependencies (which may be the case given only target_include_directories is used given CMake versions prior to 3.15.0), the changes proposed above to add find_dependency can be ignored, and instead the Boost library dependency can be hidden from package consumers via:

target_link_libraries(${TARGET} PRIVATE $<BUILD_INTERFACE:Boost::boost>)

However, I do not think this solution is recommended (even if it is the status quo) given the high potential for ODR violations due to the use of polyfills in the ABI (e.g. bsoncxx/mongocxx built with optional -> boost::optional, user builds with optional -> core::optional, user invokes bsoncxx::validate() which returns an optional -> <??? ODR violation>).

Another workaround (to minimize changes to the config) may be to ensure find_package(Boost 1.56.0 REQUIRED) is invoked before find_package(bsoncxx REQUIRED) in all example projects' CMake config files so that the Boost::boost target is known. find_package for Boost is currently only done on Windows (by requirement). This may be a bit tricky given it should only be done/required if bsoncxx was built with BSONCXX_POLY_USE_BOOST, which is info that is currently not being forwarded to example projects by build_example_projects.sh.

Among the options listed, I'm in favor of fixing the missing find_dependency in the package config file.

[vector-of-bool]

LGTM, other than the same comment from Ezra

[rcsanchez97]

Updated Evergreen patch build: https://spruce.mongodb.com/version/646f8ff52a60ed20864822fa/tasks?sorts=STATUS%3AASC%3BBASE_STATUS%3ADESC

This fixes the failure resulting from not being able to find Boost on macOS.