From 7c3ca57b9102b475daa890a90d8f161add68ad4a Mon Sep 17 00:00:00 2001
From: Joerg Bornemann <joerg.bornemann@qt.io>
Date: Fri, 18 Sep 2026 11:35:02 +0200
Subject: [PATCH] MSVC,Clang: Set CMAKE_CXX_COMPILER_PREDEFINES_COMMAND for the
 MSVC ABI

The variable was only set by the GNU-like compiler modules, so AUTOMOC
never generated a moc_predefs.h for compilers targeting the MSVC ABI
and moc parsed headers without the compiler's predefined macros.

MSVC dumps its predefined macros with '/PD', which needs the conforming
preprocessor. Require VS 2019 16.8, because 16.7 and older ignore the
flag with warning D9002 and emit no macros at all. clang-cl takes '-dM'
through '-Xclang'. Both also echo the preprocessed source, so feed them
an empty translation unit. clang with the GNU driver is excluded from
'Compiler/GNU' as well when it simulates MSVC, so give it the usual
'-dM -E' command there.

Fixes: #28093
---
 Modules/CMakeCXXCompilerPredefines.cpp    |  3 +++
 Modules/Compiler/Clang-CXX.cmake          | 25 +++++++++++++++++++++++
 Modules/Compiler/MSVC.cmake               | 17 +++++++++++++++
 Tests/RunCMake/Autogen_1/MocPredefs.cmake |  8 ++++++++
 4 files changed, 53 insertions(+)
 create mode 100644 Modules/CMakeCXXCompilerPredefines.cpp

diff --git a/Modules/CMakeCXXCompilerPredefines.cpp b/Modules/CMakeCXXCompilerPredefines.cpp
new file mode 100644
index 0000000000..418a8d1e32
--- /dev/null
+++ b/Modules/CMakeCXXCompilerPredefines.cpp
@@ -0,0 +1,3 @@
+// An empty translation unit for CMAKE_<LANG>_COMPILER_PREDEFINES_COMMAND.
+// MSVC-like compilers echo the preprocessed source along with the macro
+// definitions, so the input must contribute no output of its own.
diff --git a/Modules/Compiler/Clang-CXX.cmake b/Modules/Compiler/Clang-CXX.cmake
index 5a8bc3fda3..d0c3228c79 100644
--- a/Modules/Compiler/Clang-CXX.cmake
+++ b/Modules/Compiler/Clang-CXX.cmake
@@ -20,6 +20,31 @@ if("x${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "xMSVC")
   set(CMAKE_CXX_INCLUDE_WHAT_YOU_USE_DRIVER_MODE "cl")
 endif()
 
+# 'Compiler/GNU' is not included when clang simulates MSVC, so the predefines
+# command is not set up there.
+if(NOT DEFINED CMAKE_CXX_COMPILER_PREDEFINES_COMMAND)
+  set(CMAKE_CXX_COMPILER_PREDEFINES_COMMAND "${CMAKE_CXX_COMPILER}")
+  if(CMAKE_CXX_COMPILER_ARG1)
+    separate_arguments(_COMPILER_ARGS NATIVE_COMMAND "${CMAKE_CXX_COMPILER_ARG1}")
+    list(APPEND CMAKE_CXX_COMPILER_PREDEFINES_COMMAND ${_COMPILER_ARGS})
+    unset(_COMPILER_ARGS)
+  endif()
+  if("x${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "xMSVC")
+    # '-dM' is a cc1 option in this driver mode, and '-EP' also echoes the
+    # preprocessed source, hence the empty translation unit.
+    list(APPEND CMAKE_CXX_COMPILER_PREDEFINES_COMMAND
+      "-nologo" "-w" "-Xclang" "-dM" "-EP"
+      "${CMAKE_ROOT}/Modules/CMakeCXXCompilerPredefines.cpp")
+  else()
+    list(APPEND CMAKE_CXX_COMPILER_PREDEFINES_COMMAND
+      "-w" "-dM" "-E" "${CMAKE_ROOT}/Modules/CMakeCXXCompilerABI.cpp")
+  endif()
+  if(CMAKE_CXX_COMPILER_TARGET)
+    list(APPEND CMAKE_CXX_COMPILER_PREDEFINES_COMMAND
+      "--target=${CMAKE_CXX_COMPILER_TARGET}")
+  endif()
+endif()
+
 if((CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0 AND CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "GNU") OR
    (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 19.1 AND CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC"))
   if (CMAKE_CXX_COMPILER_CLANG_RESOURCE_DIR)
diff --git a/Modules/Compiler/MSVC.cmake b/Modules/Compiler/MSVC.cmake
index 40517b93e2..35c67e4170 100644
--- a/Modules/Compiler/MSVC.cmake
+++ b/Modules/Compiler/MSVC.cmake
@@ -23,5 +23,22 @@ macro(__compiler_msvc lang)
     set(CMAKE_INCLUDE_SYSTEM_FLAG_${lang}_WARNING "-external:W0 ")
   endif ()
 
+  # '/PD' dumps the predefined macros, but it needs the conforming
+  # preprocessor and one of '/E', '/EP' or '/P'.  VS 2019 16.7 and older
+  # ignore it with warning D9002, so require 16.8.  '/EP' also echoes the
+  # preprocessed source, hence the empty translation unit.
+  if("${lang}" STREQUAL "CXX" AND
+      CMAKE_${lang}_COMPILER_VERSION VERSION_GREATER_EQUAL 19.28)
+    set(CMAKE_${lang}_COMPILER_PREDEFINES_COMMAND "${CMAKE_${lang}_COMPILER}")
+    if(CMAKE_${lang}_COMPILER_ARG1)
+      separate_arguments(_COMPILER_ARGS NATIVE_COMMAND "${CMAKE_${lang}_COMPILER_ARG1}")
+      list(APPEND CMAKE_${lang}_COMPILER_PREDEFINES_COMMAND ${_COMPILER_ARGS})
+      unset(_COMPILER_ARGS)
+    endif()
+    list(APPEND CMAKE_${lang}_COMPILER_PREDEFINES_COMMAND
+      "-nologo" "-w" "-Zc:preprocessor" "-PD" "-EP"
+      "${CMAKE_ROOT}/Modules/CMakeCXXCompilerPredefines.cpp")
+  endif()
+
   set(CMAKE_${lang}_LINK_MODE LINKER)
 endmacro()
diff --git a/Tests/RunCMake/Autogen_1/MocPredefs.cmake b/Tests/RunCMake/Autogen_1/MocPredefs.cmake
index 6ea192a273..cc71119f8e 100644
--- a/Tests/RunCMake/Autogen_1/MocPredefs.cmake
+++ b/Tests/RunCMake/Autogen_1/MocPredefs.cmake
@@ -11,6 +11,14 @@ set(CMAKE_CXX_EXTENSIONS OFF)
 add_library(MocPredefs MocPredefs.cxx)
 
 if(NOT DEFINED CMAKE_CXX_COMPILER_PREDEFINES_COMMAND)
+  # Do not silently skip on toolchains that are expected to provide it.
+  if((CMAKE_CXX_COMPILER_ID STREQUAL "MSVC" AND MSVC_VERSION GREATER_EQUAL 1928)
+      OR (CMAKE_CXX_COMPILER_ID MATCHES "Clang"
+          AND CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC"))
+    message(FATAL_ERROR
+      "CMAKE_CXX_COMPILER_PREDEFINES_COMMAND is not defined for "
+      "${CMAKE_CXX_COMPILER_ID} ${CMAKE_CXX_COMPILER_VERSION}.")
+  endif()
   return()
 endif()