From ed48feeae8e257d4ff0b9b3033d02b015f7011b2 Mon Sep 17 00:00:00 2001
From: Brad King <brad.king@kitware.com>
Date: Fri, 27 Mar 2026 11:51:18 -0400
Subject: [PATCH] clang-cl: Add support for C++ modules

Since commit 4a4986d28a (cxxmodules: support command templates for BMI
compilation, 2026-03-25, v4.3.1~2^2~1) we can express module interface
unit compilation to either object files or BMI-only in a way that works
with `clang-scan-deps` and `clang-cl` together.

Fixes: #25731
---
 .gitlab/ci/configure_windows_clang_ninja.cmake |  4 +---
 Help/manual/cmake-cxxmodules.7.rst             |  6 ++++++
 Help/release/dev/clang-cl-cxxmodules.rst       |  5 +++++
 Modules/Compiler/Clang-CXX.cmake               | 17 ++++++++++++-----
 4 files changed, 24 insertions(+), 8 deletions(-)
 create mode 100644 Help/release/dev/clang-cl-cxxmodules.rst

diff --git a/.gitlab/ci/configure_windows_clang_ninja.cmake b/.gitlab/ci/configure_windows_clang_ninja.cmake
index 214c754891..e8a017148e 100644
--- a/.gitlab/ci/configure_windows_clang_ninja.cmake
+++ b/.gitlab/ci/configure_windows_clang_ninja.cmake
@@ -1,4 +1,2 @@
-if("$ENV{CMAKE_CI_BUILD_NAME}" MATCHES "(^|_)gnu(_|$)")
-  set(CMake_TEST_MODULE_COMPILATION "named,compile_commands,collation,partitions,internal_partitions,export_bmi,install_bmi,shared,bmionly,build_database" CACHE STRING "")
-endif()
+set(CMake_TEST_MODULE_COMPILATION "named,compile_commands,collation,partitions,internal_partitions,export_bmi,install_bmi,shared,bmionly,build_database" CACHE STRING "")
 include("${CMAKE_CURRENT_LIST_DIR}/configure_windows_clang_common.cmake")
diff --git a/Help/manual/cmake-cxxmodules.7.rst b/Help/manual/cmake-cxxmodules.7.rst
index 9dc14d2353..396438645f 100644
--- a/Help/manual/cmake-cxxmodules.7.rst
+++ b/Help/manual/cmake-cxxmodules.7.rst
@@ -69,7 +69,13 @@ The list of compilers for which CMake supports scanning sources for C++
 modules includes:
 
 * MSVC toolset 14.34 and newer (provided with Visual Studio 17.4 and newer)
+
 * LLVM/Clang 16.0 and newer
+
+  .. versionadded:: 4.4
+
+    ``clang-cl`` version 19.1 and newer
+
 * GCC 14 and newer
 
 ``import std`` Support
diff --git a/Help/release/dev/clang-cl-cxxmodules.rst b/Help/release/dev/clang-cl-cxxmodules.rst
new file mode 100644
index 0000000000..7a3a23fc09
--- /dev/null
+++ b/Help/release/dev/clang-cl-cxxmodules.rst
@@ -0,0 +1,5 @@
+clang-cl-cxxmodules
+-------------------
+
+* C++ 20 named modules are now supported with ``clang-cl``.
+  See :manual:`cmake-cxxmodules(7)`.
diff --git a/Modules/Compiler/Clang-CXX.cmake b/Modules/Compiler/Clang-CXX.cmake
index 45e06a4e21..7b3bd4557f 100644
--- a/Modules/Compiler/Clang-CXX.cmake
+++ b/Modules/Compiler/Clang-CXX.cmake
@@ -20,7 +20,8 @@ if("x${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "xMSVC")
   set(CMAKE_CXX_INCLUDE_WHAT_YOU_USE_DRIVER_MODE "cl")
 endif()
 
-if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0 AND CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "GNU")
+if((CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0 AND CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "GNU") OR
+   (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 19.1 AND CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC"))
   if (CMAKE_CXX_COMPILER_CLANG_RESOURCE_DIR)
     set(_clang_scan_deps_resource_dir
       " -resource-dir \"${CMAKE_CXX_COMPILER_CLANG_RESOURCE_DIR}\"")
@@ -34,15 +35,20 @@ if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0 AND CMAKE_CXX_COMPILER_
   else ()
     set(_clang_scan_deps_mv "mv")
   endif ()
+  if(CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC")
+    set(_clang "-clang:")
+  else()
+    set(_clang "")
+  endif()
   string(CONCAT CMAKE_CXX_SCANDEP_SOURCE
     "\"${CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS}\""
     " -format=p1689"
     " --"
     " <CMAKE_CXX_COMPILER> <DEFINES> <INCLUDES> <FLAGS>"
-    " -x c++ <SOURCE> -c -o <OBJECT>"
+    " -x c++ <SOURCE> -c ${_clang}-o ${_clang}<OBJECT>"
     "${_clang_scan_deps_resource_dir}"
-    " -MT <DYNDEP_FILE>"
-    " -MD -MF <DEP_FILE>"
+    " ${_clang}-MT ${_clang}<DYNDEP_FILE>"
+    " ${_clang}-MD ${_clang}-MF ${_clang}<DEP_FILE>"
     # Write to a temporary file. If the scan fails, we do not want to update
     # the actual output file as `ninja` (at least) assumes that failed
     # commands either delete or leave output files alone. See Issue#25419.
@@ -56,5 +62,6 @@ if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0 AND CMAKE_CXX_COMPILER_
   set(CMAKE_CXX_MODULE_MAP_FORMAT "clang")
   set(CMAKE_CXX_MODULE_MAP_FLAG "@<MODULE_MAP_FILE>")
   set(CMAKE_CXX_COMPILE_BMI
-    "<CMAKE_CXX_COMPILER> <DEFINES> <INCLUDES> <FLAGS> -o <OBJECT> --precompile <SOURCE>")
+    "<CMAKE_CXX_COMPILER> <DEFINES> <INCLUDES> <FLAGS> ${_clang}-o ${_clang}<OBJECT> --precompile <SOURCE>")
+  unset(_clang)
 endif()