VS: Add support for C++ module internal partitions in VS 17.6 and newer

VS 17.6 now implements `ScanSourceforModuleDependencies` using the same
`cl /scanDependencies` scanner that our Ninja generator uses.  It can
distinguish module internal partitions from module interface units based
on their content.  Switch from `CompileAsCppModule` to `CompileAsCpp`
for `CXX_MODULES` sources so that MSBuild can scan and classify them.

1 files changed, 8 insertions, 1 deletions

diff --git a/Source/cmVisualStudio10TargetGenerator.cxx b/Source/cmVisualStudio10TargetGenerator.cxx
index 7360bf5..6d62aff 100644
--- a/Source/cmVisualStudio10TargetGenerator.cxx
+++ b/Source/cmVisualStudio10TargetGenerator.cxx
@@ -2802,7 +2802,14 @@ void cmVisualStudio10TargetGenerator::OutputSourceSpecificFlags(
          fs->GetType() == "CXX_MODULE_HEADER_UNITS"_s)) {
       if (lang == "CXX"_s) {
         if (fs->GetType() == "CXX_MODULES"_s) {
-          compileAsPerConfig = "CompileAsCppModule";
+          if (shouldScanForModules &&
+              this->GlobalGenerator->IsScanDependenciesSupported()) {
+            // ScanSourceforModuleDependencies uses 'cl /scanDependencies' and
+            // can distinguish module interface units and internal partitions.
+            compileAsPerConfig = "CompileAsCpp";
+          } else {
+            compileAsPerConfig = "CompileAsCppModule";
+          }
         } else {
           compileAsPerConfig = "CompileAsHeaderUnit";
         }