cxxmodules: detect and message about non-compiled sources

Previously an internal error was raised which ended up causing an
internal exception to be thrown. This is a typo situation that should
fall into an explicit error.

Fixes: #25207

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.

Tests/RunCMake/CXXModules/NotCompiledSourceModules-result.txt

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

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.

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)

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 ()