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cxxmodules: Fix CMP0155 NEW behavior when C++ compile features are not known

Brad King requested to merge brad.king/cmake:cxxmodules-compiler-inspection into master

With CMP0155 NEW behavior, we scan C++ sources in targets using C++ 20, i.e., in which the cxx_std_20 feature is available. However, our check for C++ feature availability may incorrectly succeed in two cases:

  • MSVC versions from VS 2013 do not model C++ standard levels, so we assume all features are available as a heuristic to let projects at least try compiling with them.

  • During ABI detection the CMAKE_CXX20_COMPILE_FEATURES variable is not populated so we assume all features are available, knowing that our ABI detection project does not need them.

For purposes of detecting targets using C++ 20, we do not want to assume cxx_std_20 is available, so verify that we really know it is.

For reference, this bug was exposed by commit 8bd2c8d1.

Backport: release
Topic-rename: cxxmodules-verify-c++20

Edited by Brad King

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