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CMake/Help/policy/CMP0155.rst
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Ben Boeckel 437280b127 cxxmodules: scan C++ sources for imports by default
Existing projects are not using C++ modules in their sources,
so introduce policy CMP0155 to enable scanning by default.
2023-10-02 10:17:31 -04:00

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CMP0155
-------
.. versionadded:: 3.28
C++ sources in targets with at least C++20 are scanned for imports.
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 :variable:`CMAKE_CXX_SCAN_FOR_MODULES` variable.
See the :manual:`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, and need to be scanned.
This policy was introduced in CMake version 3.28. Use the
:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
Unlike many policies, CMake version |release| does *not* warn
when this policy is not set and simply uses ``OLD`` behavior.
.. include:: DEPRECATED.txt