CMake project in Visual Studio gives flag override warnings (Command line warning D9025: overriding '/W4' with '/w')

Question

I have a CMake project, which I am building with Microsoft Visual Studio 2019. I am trying to fix and remove all warnings, but there is one type which I can't disable or fix.

All of them are of the type:

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

I tried fixing them, but I can't find out what's causing all of them.

My question is:

How can I disable the warnings using CMake? Or is there a surefire way to find the underlying cause of them and fix them?

Answer 1

This issue has been raised (here and here), and depending on your CMake version there are a couple solutions.

When building for MSVC with CMake, the compiler warning flags (like /W3) are added by default. In CMake 3.15, CMake introduced a fix for this, and the compiler warning flags are no longer automatically added so the warning should no longer appear. From the docs:

CMake 3.15 and above prefer to leave out warning flags from the value of CMAKE_<LANG>_FLAGS by default.

Along with this fix, CMake introduced policy CMP0092, which allows you to switch back to the OLD behavior (adding the warning flags by default) if necessary.

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If you are tied to a CMake version older than 3.15, you can manually manipulate the CMAKE_<LANG>_FLAGS variable to replace the warnings yourself using CMake's regular expressions. You can try something like this:

string(REGEX REPLACE "/W[3|4]" "/w" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")