Clang: Record Clang 16.0 flags for our experimental C++ modules support

LLVM/Clang 16.0 now contains official support for what CMake needs.

3 files changed, 44 insertions, 22 deletions

diff --git a/.gitlab/ci/cxx_modules_rules_clang.cmake b/.gitlab/ci/cxx_modules_rules_clang.cmake
index fcb2281..8088330 100644
--- a/.gitlab/ci/cxx_modules_rules_clang.cmake
+++ b/.gitlab/ci/cxx_modules_rules_clang.cmake
@@ -1,17 +1,6 @@
 set(CMake_TEST_CXXModules_UUID "a246741c-d067-4019-a8fb-3d16b0c9d1d3")
 
 set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)
-string(CONCAT CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE
-  "${CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS}"
-  " -format=p1689"
-  " --"
-  " <CMAKE_CXX_COMPILER> <DEFINES> <INCLUDES> <FLAGS>"
-  " -x c++ <SOURCE> -c -o <OBJECT>"
-  " -MT <DYNDEP_FILE>"
-  " -MD -MF <DEP_FILE>"
-  " > <DYNDEP_FILE>")
-set(CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FORMAT "clang")
-set(CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FLAG "@<MODULE_MAP_FILE>")
 
 # Default to C++ extensions being off. Clang's modules support have trouble
 # with extensions right now.
diff --git a/Help/dev/experimental.rst b/Help/dev/experimental.rst
index c97d542..c7581e8 100644
--- a/Help/dev/experimental.rst
+++ b/Help/dev/experimental.rst
@@ -57,17 +57,36 @@ dependency scanning.  This is similar to the Fortran modules support, but
 relies on external tools to scan C++20 translation units for module
 dependencies.  The approach is described by Kitware's `D1483r1`_ paper.
 
-The ``CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP`` variable can be set to ``1``
-in order to activate this undocumented experimental infrastructure.  This
-is **intended to make the functionality available to compiler writers** so
-they can use it to develop and test their dependency scanning tool.
-The ``CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE`` variable must also be set
-to tell CMake how to invoke the C++20 module dependency scanning tool.
-
-MSVC 19.34 (provided with Visual Studio 17.4) and above contains the support
-that CMake needs and has these variables already set up as required and only
-the UUID and the ``CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP`` variables need to be
-set.
+In order to activate CMake's experimental support for C++20 module
+dependencies, set the following variables:
+
+``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+  Set this to the UUID documented above.
+
+``CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP``
+  Set this to ``1`` in order to activate this undocumented experimental
+  infrastructure.  This is **intended to make the functionality available
+  to compiler writers** so they can use it to develop and test their
+  dependency scanning tool.
+
+Some compilers already have support for module dependency scanning:
+
+* MSVC 19.34 and newer (provided with Visual Studio 17.4 and newer)
+* LLVM/Clang 16.0 and newer
+
+For those, only the above variables need to be set by project code.
+For compilers with in-development support, additional variables must
+be set as follows.
+
+``CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE``
+  Set this to tell CMake how to invoke the C++20 module dependency
+  scanning tool.
+
+``CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FORMAT``
+  Set this for compilers that generate module maps.  See below.
+
+``CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FLAG``
+  Set this for compilers that generate module maps.  See below.
 
 For example, add code like the following to a test project:
 
diff --git a/Modules/Compiler/Clang-CXX.cmake b/Modules/Compiler/Clang-CXX.cmake
index 84b05d7..33154fd 100644
--- a/Modules/Compiler/Clang-CXX.cmake
+++ b/Modules/Compiler/Clang-CXX.cmake
@@ -29,3 +29,17 @@ if("x${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "xMSVC")
     set(CMAKE_CXX_DEPENDS_USE_COMPILER TRUE)
   endif()
 endif()
+
+if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0)
+  string(CONCAT CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE
+    "${CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS}"
+    " -format=p1689"
+    " --"
+    " <CMAKE_CXX_COMPILER> <DEFINES> <INCLUDES> <FLAGS>"
+    " -x c++ <SOURCE> -c -o <OBJECT>"
+    " -MT <DYNDEP_FILE>"
+    " -MD -MF <DEP_FILE>"
+    " > <DYNDEP_FILE>")
+  set(CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FORMAT "clang")
+  set(CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FLAG "@<MODULE_MAP_FILE>")
+endif ()