How to set warning level in CMake?

In modern CMake, the following works well:

if(MSVC)
  target_compile_options(${TARGET_NAME} PRIVATE /W4 /WX)
else()
  target_compile_options(${TARGET_NAME} PRIVATE -Wall -Wextra -Wpedantic -Werror)
endif()

My colleague suggested an alternative version:

target_compile_options(${TARGET_NAME} PRIVATE
  $<$<CXX_COMPILER_ID:MSVC>:/W4 /WX>
  $<$<NOT:$<CXX_COMPILER_ID:MSVC>>:-Wall -Wextra -Wpedantic -Werror>
)

Replace ${TARGET_NAME} with the actual target name. -Werror is optional, it turns all warnings into errors.

Or use add_compile_options(...) if you want to apply it to all targets as suggested by @aldo in the comments.

Also, be sure to understand the difference between PRIVATE and PUBLIC (public options will be inherited by targets that depend on the given target).

As @davidfong notes in the comments, since CMake v3.24, there is the CMAKE_COMPILE_WARNING_AS_ERROR variable that switches on treating compile warings as errors. In case it is set inside CMakeLists.txt, the user can still turn it off with the --compile-no-warning-as-error cmake flag. In case you want to add warning-as-error manually, add /WX in Windows and -Werror elsewhere to target_compile_options.