# CMP0155

New in version 3.28.

C++ sources in targets with at least C++20 are scanned for imports when supported.

CMake 3.27 and below assume that C++ sources do not `import` modules. CMake 3.28 and above prefer to assume that C++ sources in targets using C++20 or higher might `import` modules, and must be scanned before compiling, unless explicitly disabled. This policy provides compatibility for projects that use C++20 or higher, without modules, that have not been updated to turn off scanning, e.g., via the `CMAKE_CXX_SCAN_FOR_MODULES` variable. See the `cmake-cxxmodules(7)` manual for more details on C++ module support.

The `OLD` behavior for this policy is to assume that C++ 20 and newer sources do not import modules. The `NEW` behavior for this policy is to assume that C++ 20 and newer files may import modules if the compiler understands how to scan for their dependencies, and need to be scanned.

This policy was introduced in CMake version 3.28. Use the `cmake_policy()` command to set it to `OLD` or `NEW` explicitly. Unlike many policies, CMake version 3.28.4 does not warn when this policy is not set and simply uses `OLD` behavior.

Note

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