c++modules: create synthetic targets only when needed

When the target has only private c++ modules, do not create a synthetic
target.

Source/cmGeneratorTarget.cxx

@@ -5392,7 +5392,7 @@ bool cmGeneratorTarget::DiscoverSyntheticTargets(
 
   for (auto const& entry : impl.Libraries) {
     auto const* gt = entry.Target;
-    if (!gt || !gt->HaveCxx20ModuleSources()) {
+    if (!gt || !gt->HaveInterfaceCxx20ModuleSources()) {
       continue;
     }
 
@@ -5882,6 +5882,10 @@ bool cmGeneratorTarget::HaveFortranSources() const
   return have_direct || have_via_target_objects;
 }
 
+bool cmGeneratorTarget::HaveInterfaceCxx20ModuleSources() const
+{
+  return !this->GetInterfaceFileSets(cm::FileSetMetadata::CXX_MODULES).empty();
+}
 bool cmGeneratorTarget::HaveCxx20ModuleSources() const
 {
   return !this->GetFileSets(cm::FileSetMetadata::CXX_MODULES).empty() ||

Source/cmGeneratorTarget.h

@@ -1546,6 +1546,7 @@ public:
    * This will inspect the target itself to see if C++20 module
    * support is expected to work based on its sources.
    */
+  bool HaveInterfaceCxx20ModuleSources() const;
   bool HaveCxx20ModuleSources() const;
 
   enum class Cxx20SupportLevel

Tests/RunCMake/CXXModulesCompile/RunCMakeTest.cmake

@@ -174,6 +174,9 @@ if ("named" IN_LIST CMake_TEST_MODULE_COMPILATION)
   run_cxx_module_test(non-trivial-collation-order-randomized)
   run_cxx_module_test(duplicate)
   set(RunCMake_CXXModules_NO_TEST 1)
+  if ("collation" IN_LIST CMake_TEST_MODULE_COMPILATION)
+    run_cxx_module_test(private-cxx-modules)
+  endif ()
   run_cxx_module_test(imp-from-object)
   run_cxx_module_test(circular)
   run_cxx_module_test(try-compile)

Tests/RunCMake/CXXModulesCompile/private-cxx-modules-build-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/CXXModulesCompile/private-cxx-modules-build-stdout.txt

@@ -0,0 +1 @@
+CMake Error: Unable to use module 'importable' as it is 'PRIVATE' and therefore not accessible outside of its owning target\.

Tests/RunCMake/CXXModulesCompile/private-cxx-modules/CMakeLists.txt

@@ -0,0 +1,31 @@
+cmake_minimum_required(VERSION 3.24...3.28)
+project(cxx_modules_library CXX)
+
+include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")
+
+include(GenerateExportHeader)
+
+add_library(library)
+generate_export_header(library)
+target_sources(library
+  PUBLIC
+    FILE_SET HEADERS
+      BASE_DIRS
+        "${CMAKE_CURRENT_BINARY_DIR}"
+      FILES
+        "${CMAKE_CURRENT_BINARY_DIR}/library_export.h"
+  PRIVATE
+    FILE_SET CXX_MODULES
+      BASE_DIRS
+        "${CMAKE_CURRENT_SOURCE_DIR}"
+      FILES
+        importable.ixx
+  PRIVATE
+    importable.cxx)
+target_compile_features(library PUBLIC cxx_std_20)
+
+add_executable(exe)
+target_link_libraries(exe PRIVATE library)
+target_sources(exe
+  PRIVATE
+    main.cxx)

Tests/RunCMake/CXXModulesCompile/private-cxx-modules/importable.cxx

@@ -0,0 +1,6 @@
+module importable;
+
+int from_import()
+{
+  return 0;
+}

Tests/RunCMake/CXXModulesCompile/private-cxx-modules/importable.ixx

@@ -0,0 +1,5 @@
+export module importable;
+
+#include "library_export.h"
+
+export LIBRARY_EXPORT int from_import();

Tests/RunCMake/CXXModulesCompile/private-cxx-modules/main.cxx

@@ -0,0 +1,6 @@
+import importable;
+
+int main(int argc, char* argv[])
+{
+  return from_import();
+}