cmTarget: add support for C++ module fileset types

C++ modules have two variants which are of importance to CMake:

  - `CXX_MODULES`: interface modules (those using `export module M;`,
    `export module M:part;`, or `module M:internal_part;`)
  - `CXX_MODULE_HEADER_UNITS`: importable header units

Creating C++ modules or partitions are *not* supported in any other
source listing. This is because the source files must be installed (so
their scope matters), but not part of usage requirements (what it means
for a module source to be injected into a consumer is not clear at this
moment). Due to the way `FILE_SET` works with scopes, they are a perfect
fit as long as `INTERFACE` is not allowed (which it is not).

1 files changed, 12 insertions, 0 deletions

diff --git a/Source/cmGlobalXCodeGenerator.cxx b/Source/cmGlobalXCodeGenerator.cxx
index e02f30e..5738799 100644
--- a/Source/cmGlobalXCodeGenerator.cxx
+++ b/Source/cmGlobalXCodeGenerator.cxx
@@ -1376,6 +1376,18 @@ bool cmGlobalXCodeGenerator::CreateXCodeTarget(
     return true;
   }
 
+  for (std::string const& configName : this->CurrentConfigurationTypes) {
+    gtgt->CheckCxxModuleStatus(configName);
+  }
+
+  if (gtgt->HaveCxx20ModuleSources()) {
+    gtgt->Makefile->IssueMessage(
+      MessageType::FATAL_ERROR,
+      cmStrCat("The \"", gtgt->GetName(),
+               "\" target contains C++ module sources which are not "
+               "supported by the generator"));
+  }
+
   auto& gtgt_visited = this->CommandsVisited[gtgt];
   auto& deps = this->GetTargetDirectDepends(gtgt);
   for (auto& d : deps) {