8 files changed, 61 insertions, 4 deletions
diff --git a/Source/cmDyndepCollation.cxx b/Source/cmDyndepCollation.cxx
index 80e1357..b50718c 100644
--- a/Source/cmDyndepCollation.cxx
+++ b/Source/cmDyndepCollation.cxx
@@ -106,10 +106,11 @@ Json::Value CollationInformationCxxModules(
         auto lookup = sf_map.find(file);
         if (lookup == sf_map.end()) {
           gt->Makefile->IssueMessage(
-            MessageType::INTERNAL_ERROR,
-            cmStrCat("Target \"", tgt->GetName(), "\" has source file \"",
+            MessageType::FATAL_ERROR,
+            cmStrCat("Target \"", tgt->GetName(), "\" has source file\n  ",
                      file,
-                     R"(" which is not in any of its "FILE_SET BASE_DIRS".)"));
+                     "\nin a \"FILE_SET TYPE CXX_MODULES\" but it is not "
+                     "scheduled for compilation."));
           continue;
         }
 
diff --git a/Source/cmVisualStudio10TargetGenerator.cxx b/Source/cmVisualStudio10TargetGenerator.cxx
index b3699ae..95d0be2 100644
--- a/Source/cmVisualStudio10TargetGenerator.cxx
+++ b/Source/cmVisualStudio10TargetGenerator.cxx
@@ -2540,6 +2540,12 @@ void cmVisualStudio10TargetGenerator::WriteAllSources(Elem& e0)
         break;
     }
 
+    std::string config;
+    if (!this->Configurations.empty()) {
+      config = this->Configurations[si.Configs[0]];
+    }
+    auto const* fs =
+      this->GeneratorTarget->GetFileSetForSource(config, si.Source);
     if (tool) {
       // Compute set of configurations to exclude, if any.
       std::vector<size_t> const& include_configs = si.Configs;
@@ -2604,6 +2610,13 @@ void cmVisualStudio10TargetGenerator::WriteAllSources(Elem& e0)
       if (si.Kind == cmGeneratorTarget::SourceKindObjectSource ||
           si.Kind == cmGeneratorTarget::SourceKindUnityBatched) {
         this->OutputSourceSpecificFlags(e2, si.Source);
+      } else if (fs && fs->GetType() == "CXX_MODULES"_s) {
+        this->GeneratorTarget->Makefile->IssueMessage(
+          MessageType::FATAL_ERROR,
+          cmStrCat("Target \"", this->GeneratorTarget->GetName(),
+                   "\" has source file\n  ", si.Source->GetFullPath(),
+                   "\nin a \"FILE_SET TYPE CXX_MODULES\" but it is not "
+                   "scheduled for compilation."));
       }
       if (si.Source->GetPropertyAsBool("SKIP_PRECOMPILE_HEADERS")) {
         e2.Element("PrecompiledHeader", "NotUsing");
@@ -2613,6 +2626,13 @@ void cmVisualStudio10TargetGenerator::WriteAllSources(Elem& e0)
       }
 
       this->FinishWritingSource(e2, toolSettings);
+    } else if (fs && fs->GetType() == "CXX_MODULES"_s) {
+      this->GeneratorTarget->Makefile->IssueMessage(
+        MessageType::FATAL_ERROR,
+        cmStrCat("Target \"", this->GeneratorTarget->GetName(),
+                 "\" has source file\n  ", si.Source->GetFullPath(),
+                 "\nin a \"FILE_SET TYPE CXX_MODULES\" but it is not "
+                 "scheduled for compilation."));
     }
   }
 
diff --git a/Tests/RunCMake/CXXModules/NoCXX-stderr.txt b/Tests/RunCMake/CXXModules/NoCXX-stderr.txt
index aa7f406..7e8f384 100644
--- a/Tests/RunCMake/CXXModules/NoCXX-stderr.txt
+++ b/Tests/RunCMake/CXXModules/NoCXX-stderr.txt
@@ -15,6 +15,11 @@ CMake Error in CMakeLists.txt:
   been enabled
 |  The "nocxx" target contains C\+\+ module sources which are not supported by
   the generator
+|  Target "nocxx" has source file
+
+    .*/Tests/RunCMake/CXXModules/sources/module.cxx
+
+  in a "FILE_SET TYPE CXX_MODULES" but it is not scheduled for compilation.
 )
 )*
 CMake Generate step failed.  Build files cannot be regenerated correctly.
diff --git a/Tests/RunCMake/CXXModules/NotCompiledSourceModules-result.txt b/Tests/RunCMake/CXXModules/NotCompiledSourceModules-result.txt
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NotCompiledSourceModules-result.txt
@@ -0,0 +1 @@
+1
diff --git a/Tests/RunCMake/CXXModules/NotCompiledSourceModules-stderr.txt b/Tests/RunCMake/CXXModules/NotCompiledSourceModules-stderr.txt
new file mode 100644
index 0000000..52f4072
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NotCompiledSourceModules-stderr.txt
@@ -0,0 +1,16 @@
+CMake Warning \(dev\) at NotCompiledSourceModules.cmake:5 \(target_sources\):
+  CMake's C\+\+ module support is experimental.  It is meant only for
+  experimentation and feedback to CMake developers.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:6 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.
+
+(CMake Error in CMakeLists.txt:
+  Target "not-cxx-source" has source file
+
+    .*/Tests/RunCMake/CXXModules/sources/not-compiled.txt
+
+  in a "FILE_SET TYPE CXX_MODULES" but it is not scheduled for compilation.
+
+)+
+CMake Generate step failed.  Build files cannot be regenerated correctly.
diff --git a/Tests/RunCMake/CXXModules/NotCompiledSourceModules.cmake b/Tests/RunCMake/CXXModules/NotCompiledSourceModules.cmake
new file mode 100644
index 0000000..0bab635
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NotCompiledSourceModules.cmake
@@ -0,0 +1,13 @@
+enable_language(CXX)
+set(CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE "")
+
+add_library(not-cxx-source)
+target_sources(not-cxx-source
+  PRIVATE
+    sources/cxx-anchor.cxx
+  PUBLIC
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/not-compiled.txt)
+target_compile_features(not-cxx-source
+  PRIVATE
+    cxx_std_20)
diff --git a/Tests/RunCMake/CXXModules/RunCMakeTest.cmake b/Tests/RunCMake/CXXModules/RunCMakeTest.cmake
index d324ec9..89c198f 100644
--- a/Tests/RunCMake/CXXModules/RunCMakeTest.cmake
+++ b/Tests/RunCMake/CXXModules/RunCMakeTest.cmake
@@ -71,8 +71,9 @@ foreach (fileset_type IN LISTS fileset_types)
   endforeach ()
   run_cmake("FileSet${fileset_type}InterfaceImported")
 
-  # Test the error message when a non-C++ source file is found in the source
+  # Test the error messages when a non-C++ source file is found in the source
   # list.
+  run_cmake("NotCompiledSource${fileset_type}")
   run_cmake("NotCXXSource${fileset_type}")
 endforeach ()
 
diff --git a/Tests/RunCMake/CXXModules/sources/not-compiled.txt b/Tests/RunCMake/CXXModules/sources/not-compiled.txt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/sources/not-compiled.txt