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.

1 files changed, 26 insertions, 0 deletions

diff --git a/Help/policy/CMP0155.rst b/Help/policy/CMP0155.rst
new file mode 100644
index 0000000..2dafadf
--- /dev/null
+++ b/Help/policy/CMP0155.rst
@@ -0,0 +1,26 @@
+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