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, 34 insertions, 0 deletions

diff --git a/Source/cmExportInstallFileGenerator.cxx b/Source/cmExportInstallFileGenerator.cxx
index adccdfe..3a06769 100644
--- a/Source/cmExportInstallFileGenerator.cxx
+++ b/Source/cmExportInstallFileGenerator.cxx
@@ -7,6 +7,9 @@
 #include <sstream>
 #include <utility>
 
+#include <cm/string_view>
+#include <cmext/string_view>
+
 #include "cmExportSet.h"
 #include "cmFileSet.h"
 #include "cmGeneratedFileStream.h"
@@ -18,6 +21,7 @@
 #include "cmInstallTargetGenerator.h"
 #include "cmLocalGenerator.h"
 #include "cmMakefile.h"
+#include "cmMessageType.h"
 #include "cmOutputConverter.h"
 #include "cmPolicies.h"
 #include "cmStateTypes.h"
@@ -562,6 +566,21 @@ std::string cmExportInstallFileGenerator::GetFileSetDirectories(
                            cge->Evaluate(gte->LocalGenerator, config, gte),
                            cmOutputConverter::WrapQuotes::NoWrap));
 
+    auto const& type = fileSet->GetType();
+    // C++ modules do not support interface file sets which are dependent upon
+    // the configuration.
+    if (cge->GetHadContextSensitiveCondition() &&
+        (type == "CXX_MODULES"_s || type == "CXX_MODULE_HEADER_UNITS"_s)) {
+      auto* mf = this->IEGen->GetLocalGenerator()->GetMakefile();
+      std::ostringstream e;
+      e << "The \"" << gte->GetName() << "\" target's interface file set \""
+        << fileSet->GetName() << "\" of type \"" << type
+        << "\" contains context-sensitive base file entries which is not "
+           "supported.";
+      mf->IssueMessage(MessageType::FATAL_ERROR, e.str());
+      return std::string{};
+    }
+
     if (cge->GetHadContextSensitiveCondition() && configs.size() != 1) {
       resultVector.push_back(
         cmStrCat("\"$<$<CONFIG:", config, ">:", dest, ">\""));
@@ -610,6 +629,21 @@ std::string cmExportInstallFileGenerator::GetFileSetFiles(
       std::any_of(fileEntries.begin(), fileEntries.end(),
                   EntryIsContextSensitive);
 
+    auto const& type = fileSet->GetType();
+    // C++ modules do not support interface file sets which are dependent upon
+    // the configuration.
+    if (contextSensitive &&
+        (type == "CXX_MODULES"_s || type == "CXX_MODULE_HEADER_UNITS"_s)) {
+      auto* mf = this->IEGen->GetLocalGenerator()->GetMakefile();
+      std::ostringstream e;
+      e << "The \"" << gte->GetName() << "\" target's interface file set \""
+        << fileSet->GetName() << "\" of type \"" << type
+        << "\" contains context-sensitive base file entries which is not "
+           "supported.";
+      mf->IssueMessage(MessageType::FATAL_ERROR, e.str());
+      return std::string{};
+    }
+
     for (auto const& it : files) {
       auto prefix = it.first.empty() ? "" : cmStrCat(it.first, '/');
       for (auto const& filename : it.second) {