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I have a set of C++ potential compiler flags stored in a variable and over it, I'm running the following test on CMake 3.14.5, to see which ones applies and which doesn't to a certain version of the compiler (I'm using GCC, CLANG and ICC to compile the same project):

foreach (FLAG IN LISTS CXX_COMPILER_FLAGS_TO_USE)
    # Check if the compiler supports the flag.
    string(REGEX REPLACE "[-=+]" "" FLAG_NO_SIGNS ${FLAG}) # <- The variable recieving the result of the test can't have those signs in its name
    check_cxx_compiler_flag(${FLAG} CXX_COMPILER_SUPPORTS_${FLAG_NO_SIGNS})
    if(CXX_COMPILER_SUPPORTS_${FLAG_NO_SIGNS})
        message(STATUS "Flag ${FLAG} accepted by C++ compiler")
        string(APPEND CMAKE_CXX_FLAGS " ${FLAG}")
    else()
        message(STATUS "Flag ${FLAG} not accepted by C++ compiler")
    endif()
endforeach()

For GCC 8 and the -Wabi flag, I get:

-- Performing Test CXX_COMPILER_SUPPORTS_Wabi
-- Performing Test CXX_COMPILER_SUPPORTS_Wabi - Failed
-- Flag -Wabi not accepted by C++ compiler

Now, if instead of pushing the flags inside CMAKE_CXX_FLAGS, I use add_compile_options(), the result of my test changes!!!:

foreach (FLAG IN LISTS CXX_COMPILER_FLAGS_TO_USE)
    # Check if the compiler supports the flag.
    string(REGEX REPLACE "[-=+]" "" FLAG_NO_SIGNS ${FLAG})
    check_cxx_compiler_flag(${FLAG} CXX_COMPILER_SUPPORTS_${FLAG_NO_SIGNS})
    if(CXX_COMPILER_SUPPORTS_${FLAG_NO_SIGNS})
        message(STATUS "Flag ${FLAG} accepted by C++ compiler")
        add_compile_options($<$<COMPILE_LANGUAGE:CXX>:${FLAG}>)  # <- ONLY THIS LINE CHANGED
    else()
        message(STATUS "Flag ${FLAG} not accepted by C++ compiler")
    endif()
endforeach()

The test for -Wabi now reports:

-- Performing Test CXX_COMPILER_SUPPORTS_Wabi
-- Performing Test CXX_COMPILER_SUPPORTS_Wabi - Success
-- Flag -Wabi accepted by C++ compiler  

In fact, the second case fails on compile time later:

cc1plus: error: -Wabi won't warn about anything [-Werror=abi]

Just out of curiosity, I combined both in the same test (which could sound redundant):

add_compile_options($<$<COMPILE_LANGUAGE:CXX>:${FLAG}>)
string(APPEND CMAKE_CXX_FLAGS " ${FLAG}")

and that works.

I wouldn't expect that replacing the usage of CMAKE_CXX_FLAGS by add_compile_options() would change the result of a test made using a third function.

Am I doing something I'm not supposed to? Or did I hit a real bug? I couldn't find anything similar in the knowledge base.

Thanks a lot for your help.