cmake does not treat Boost as a system library on github runner macos-15-intel

Code
1

In the repo GitHub - ralfkonrad/cmake_missing_warning I am describing a situation which I would assume to be a bug:

While the other GitHub runners suppress a Boost deprecation warning correctly due to

target_include_directories(cmake_missing_warning SYSTEM PUBLIC ${Boost_INCLUDE_DIRS})

the macos-15-intel runner does not because the corresponding -isystem [path/to/boost/include] flag is missing.

2

If you look at the compile line, you’ll see CMake isn’t using -I or -isystem. CMake has stripped the the directory from the compile line entirely.

This is the standard behavior for system directories already known to the compiler.