From 1610aa22f4cf626a3fe7b0a4dd306f3673d43870 Mon Sep 17 00:00:00 2001
From: Sharadh Rajaraman <r.sharadh@outlook.sg>
Date: Sat, 4 Jul 2026 22:41:17 +0100
Subject: [PATCH] Clang: Add support for `import std` when targeting the MSVC
 ABI

When Clang targets the MSVC ABI, discover the MSVC standard library's
import-std metadata discovery through the MSVC module logic.  Also
extend the latter to use `CMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES`
and honor `CMAKE_CXX_STDLIB_MODULES_JSON`.

Closes: #26761
Signed-off-by: Sharadh Rajaraman <r.sharadh@outlook.sg>
---
 .../ci/configure_windows_clang_ninja.cmake    |  2 +-
 Modules/Compiler/Clang-CXX-CXXImportStd.cmake |  7 ++++
 Modules/Compiler/MSVC-CXX-CXXImportStd.cmake  | 38 +++++++++----------
 3 files changed, 27 insertions(+), 20 deletions(-)

diff --git a/.gitlab/ci/configure_windows_clang_ninja.cmake b/.gitlab/ci/configure_windows_clang_ninja.cmake
index c92d9b6256..5c87bac892 100644
--- a/.gitlab/ci/configure_windows_clang_ninja.cmake
+++ b/.gitlab/ci/configure_windows_clang_ninja.cmake
@@ -1,4 +1,4 @@
-set(CMake_TEST_MODULE_COMPILATION "named,compile_commands,collation,partitions,internal_partitions,export_bmi,install_bmi,shared,bmionly,build_database" CACHE STRING "")
+set(CMake_TEST_MODULE_COMPILATION "named,compile_commands,collation,partitions,internal_partitions,export_bmi,install_bmi,shared,bmionly,build_database,import_std23" CACHE STRING "")
 if("$ENV{CMAKE_CI_BUILD_NAME}" MATCHES "(^|_)cl(_|$)")
   set(CMAKE_Fortran_COMPILER "flang" CACHE STRING "")
   set(CMAKE_Fortran_COMPILER_ID "LLVMFlang" CACHE STRING "")
diff --git a/Modules/Compiler/Clang-CXX-CXXImportStd.cmake b/Modules/Compiler/Clang-CXX-CXXImportStd.cmake
index 5d637b7a05..c8f94bfac0 100644
--- a/Modules/Compiler/Clang-CXX-CXXImportStd.cmake
+++ b/Modules/Compiler/Clang-CXX-CXXImportStd.cmake
@@ -1,3 +1,10 @@
+if(CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC" OR
+    CMAKE_CXX_STANDARD_LIBRARY STREQUAL "msvc" OR
+    CMAKE_CXX_COMPILER_TARGET MATCHES "(^|-)windows-msvc($|-)")
+  include(Compiler/MSVC-CXX-CXXImportStd)
+  return()
+endif()
+
 function (_cmake_cxx_find_modules_json)
   if (NOT CMAKE_CXX_STDLIB_MODULES_JSON)
     if (CMAKE_CXX_STANDARD_LIBRARY STREQUAL "libc++")
diff --git a/Modules/Compiler/MSVC-CXX-CXXImportStd.cmake b/Modules/Compiler/MSVC-CXX-CXXImportStd.cmake
index b7bd5254e5..d31c4c39a2 100644
--- a/Modules/Compiler/MSVC-CXX-CXXImportStd.cmake
+++ b/Modules/Compiler/MSVC-CXX-CXXImportStd.cmake
@@ -1,24 +1,24 @@
 function (_cmake_cxx_find_modules_json)
   if (CMAKE_CXX_STDLIB_MODULES_JSON)
-    return ()
-  endif ()
-
-  find_file(_msvc_modules_json_file
-    NAME modules.json
-    HINTS
-      "$ENV{VCToolsInstallDir}/modules"
-    PATHS
-      "$ENV{INCLUDE}"
-      "${CMAKE_CXX_COMPILER}/../../.."
-      "${CMAKE_CXX_COMPILER}/../.."    # msvc-wine layout
-    PATH_SUFFIXES
-      ../modules
-    NO_CACHE)
-
-  # Without this file, we do not have modules installed.
-  if (NOT EXISTS "${_msvc_modules_json_file}")
-    set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "Could not find `modules.json` resource" PARENT_SCOPE)
-    return ()
+    set(_msvc_modules_json_file "${CMAKE_CXX_STDLIB_MODULES_JSON}")
+  else ()
+    find_file(_msvc_modules_json_file
+      NAME modules.json
+      HINTS
+        "$ENV{VCToolsInstallDir}/modules"
+      PATHS
+        "$ENV{INCLUDE}"
+        ${CMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES}
+        "${CMAKE_CXX_COMPILER}/../../.."
+        "${CMAKE_CXX_COMPILER}/../.."    # msvc-wine layout
+      PATH_SUFFIXES
+        ../modules
+      NO_CACHE)
+    # Without this file, we do not have modules installed.
+    if (NOT EXISTS "${_msvc_modules_json_file}")
+      set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE "Could not find `modules.json` resource" PARENT_SCOPE)
+      return ()
+    endif ()
   endif ()
 
   file(READ "${_msvc_modules_json_file}" _msvc_modules_json)