[CMake] [MSVC] Setting warning level on target feels like long-time bug

[Mateusz Loskot]

Hi,

I define a target for a library, and set warning level for MSVC compiler:

target_compile_options(mylib PRIVATE $<$<CXX_COMPILER_ID:MSVC>:-W4>)

Although this works well, the compiler throws this warning:

cl : Command line warning D9025: overriding '/W3' with '/W4'

I want to get rid of this warning, so I fix it this way

if(MSVC)
  string(REGEX REPLACE "/W3" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
  string(REGEX REPLACE "-W3" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
endif()

target_compile_options(mylib PRIVATE $<$<CXX_COMPILER_ID:MSVC>:-W4>)

This works like charm, but leaves me with sour feeling that
something is not right here.The REGEX REPLACE clean-up
has become such a habit, it's almost canonical thing I do in
all my CMake-based projects.

Shouldn't CMake drop the default when target_compile_options is called?
Is this behaviour by design, for MSVC?
Could anyone help me to understand if this is actually a bug
or am I misunderstanding anything?

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net