CMP0091

MSVC runtime library flags are selected by an abstraction.

Compilers targeting the MSVC ABI have flags to select the MSVC runtime library. Runtime library selection typically varies with build configuration because there is a separate runtime library for Debug builds.

In CMake 3.14 and below, MSVC runtime library selection flags are added to the default CMAKE_<LANG>_FLAGS_<CONFIG> cache entries by CMake automatically. This allows users to edit their cache entries to adjust the flags. However, the presence of such default flags is problematic for projects that want to choose a different runtime library programmatically. In particular, it requires string editing of the CMAKE_<LANG>_FLAGS_<CONFIG> variables with knowledge of the CMake builtin defaults so they can be replaced.

CMake 3.15 and above prefer to leave the MSVC runtime library selection flags out of the default CMAKE_<LANG>_FLAGS_<CONFIG> values and instead offer a first-class abstraction. The CMAKE_MSVC_RUNTIME_LIBRARY variable and MSVC_RUNTIME_LIBRARY target property may be set to select the MSVC runtime library. If they are not set then CMake uses the default value MultiThreaded$<$<CONFIG:Debug>:Debug>DLL which is equivalent to the original flags.

This policy provides compatibility with projects that have not been updated to be aware of the abstraction. The policy setting takes effect as of the first project() or enable_language() command that enables a language whose compiler targets the MSVC ABI.

The OLD behavior for this policy is to place MSVC runtime library flags in the default CMAKE_<LANG>_FLAGS_<CONFIG> cache entries and ignore the CMAKE_MSVC_RUNTIME_LIBRARY abstraction. The NEW behavior for this policy is to not place MSVC runtime library flags in the default cache entries and use the abstraction instead.

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 3.19.0-rc3 does not warn when this policy is not set and simply uses OLD behavior.