MSVC warning flags are not in CMAKE_<LANG>_FLAGS by default.

When using MSVC-like compilers in CMake 3.14 and below, warning flags like /W3 are added to CMAKE_<LANG>_FLAGS by default. This is problematic for projects that want to choose a different warning level programmatically. In particular, it requires string editing of the CMAKE_<LANG>_FLAGS variables with knowledge of the CMake builtin defaults so they can be replaced.

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

This policy provides compatibility with projects that have not been updated to expect the lack of warning flags. The policy setting takes effect as of the first project() or enable_language() command that initializes CMAKE_<LANG>_FLAGS for a given lanuage <LANG>.


Once the policy has taken effect at the top of a project for a given language, that choice must be used throughout the tree for that language. In projects that have nested projects in subdirectories, be sure to convert everything together.

The OLD behavior for this policy is to place MSVC warning flags in the default CMAKE_<LANG>_FLAGS cache entries. The NEW behavior for this policy is to not place MSVC warning flags in the default cache entries.

This policy was introduced in CMake version 3.15. Use the cmake_policy() command to set it to OLD or NEW explicitly. Unlike many policies, CMake version 3.15.7 does not warn when this policy is not set and simply uses OLD behavior.


The OLD behavior of a policy is deprecated by definition and may be removed in a future version of CMake.