VS: Avoid unsupported ScanSourceForModuleDependencies under ClangCl

As of commit ed48feeae8 (clang-cl: Add support for C++ modules,
2026-03-27, v4.4.0-rc1~428^2) CMake tries to support modules for
clang-cl, which can mostly be accomplished, except for the ClangCl VS
toolset, where the /scanDependencies flag is not yet supported.

Amend commit 3022f0363f (VS: set ScanSourceForModuleDependencies at
vcxproj level, 2024-04-27, v3.28.5~3^2) to only emit this setting when
scanning is supported by the toolchain.

Fixes: #27957
Issue: #27977

Help/manual/cmake-cxxmodules.7.rst

@@ -77,7 +77,9 @@ modules includes:
 
   .. versionadded:: 4.4
 
-    ``clang-cl`` version 19.1 and newer
+    ``clang-cl`` version 19.1 and newer, except with the
+    :ref:`Visual Studio Generators` (i.e., the ``ClangCl``
+    :variable:`toolset <CMAKE_GENERATOR_TOOLSET>`).
 
 * GCC 14 and newer
 

Help/release/4.4.rst

@@ -111,7 +111,9 @@ Languages
 Compilers
 ---------
 
-* C++ 20 named modules are now supported with ``clang-cl``.
+* C++ 20 named modules are now supported with ``clang-cl``, except with the
+  :ref:`Visual Studio Generators` (i.e., the ``ClangCl``
+  :variable:`toolset <CMAKE_GENERATOR_TOOLSET>`).
   See :manual:`cmake-cxxmodules(7)`.
 
 * The ``ibm-flang`` compiler is now supported with

Source/cmGlobalVisualStudio10Generator.cxx

@@ -742,6 +742,16 @@ std::string const& cmGlobalVisualStudio10Generator::GetPlatformToolsetString()
   return empty;
 }
 
+bool cmGlobalVisualStudio10Generator::IsClangClToolset() const
+{
+  std::string const& toolset = this->GetPlatformToolsetString();
+  static cmsys::RegularExpression llvmToolset(
+    "^[Ll][Ll][Vv][Mm](_v[0-9]+(_xp)?)?$");
+  static cmsys::RegularExpression clangClToolset(
+    "^[Cc][Ll][Aa][Nn][Gg]([Cc][Ll]$|_[0-9])");
+  return llvmToolset.find(toolset) || clangClToolset.find(toolset);
+}
+
 std::string const&
 cmGlobalVisualStudio10Generator::GetPlatformToolsetVersionProps() const
 {

Source/cmGlobalVisualStudio10Generator.h

@@ -71,6 +71,9 @@ public:
   char const* GetPlatformToolset() const;
   std::string const& GetPlatformToolsetString() const;
 
+  /** Return true when using a Visual Studio clang-cl toolset.  */
+  bool IsClangClToolset() const;
+
   /** The toolset version props file, if any.  */
   std::string const& GetPlatformToolsetVersionProps() const;
 

Source/cmGlobalVisualStudioVersionedGenerator.cxx

@@ -777,6 +777,12 @@ bool cmGlobalVisualStudioVersionedGenerator::IsUtf8EncodingSupported() const
 bool cmGlobalVisualStudioVersionedGenerator::IsScanDependenciesSupported()
   const
 {
+  if (this->IsClangClToolset()) {
+    // FIXME(#27977): The ClangCL toolset does not support
+    // ScanSourceForModuleDependencies yet.
+    return false;
+  }
+
   // Supported from Visual Studio 17.6 Preview 7.
   if (this->Version > cmGlobalVisualStudioGenerator::VSVersion::VS17) {
     return true;

Source/cmVisualStudio10TargetGenerator.cxx

@@ -293,13 +293,15 @@ cmVisualStudio10TargetGenerator::cmVisualStudio10TargetGenerator(
   this->Android = gg->TargetsAndroid();
   this->WindowsKernelMode = gg->TargetsWindowsKernelModeDriver();
   auto scanProp = target->GetProperty("CXX_SCAN_FOR_MODULES");
+  bool const scanDepsSupported = gg->IsScanDependenciesSupported();
   for (auto const& config : this->Configurations) {
     if (scanProp.IsSet()) {
-      this->ScanSourceForModuleDependencies[config] = scanProp.IsOn();
-    } else {
       this->ScanSourceForModuleDependencies[config] =
+        scanDepsSupported && scanProp.IsOn();
+    } else {
+      this->ScanSourceForModuleDependencies[config] = scanDepsSupported &&
         target->NeedCxxDyndep(config) ==
-        cmGeneratorTarget::CxxModuleSupport::Enabled;
+          cmGeneratorTarget::CxxModuleSupport::Enabled;
     }
   }
   for (unsigned int& version : this->NsightTegraVersion) {
@@ -3012,8 +3014,9 @@ void cmVisualStudio10TargetGenerator::OutputSourceSpecificFlags(
       if (compileAsPerConfig) {
         clOptions.AddFlag("CompileAs", compileAsPerConfig);
       }
-      if (shouldScanForModules !=
-          this->ScanSourceForModuleDependencies[config]) {
+      if (gg->IsScanDependenciesSupported() &&
+          shouldScanForModules !=
+            this->ScanSourceForModuleDependencies[config]) {
         clOptions.AddFlag("ScanSourceForModuleDependencies",
                           shouldScanForModules ? "true" : "false");
       }