Tests/RunCMake/CXXModules: add a test with circular imports

The build should fail. However, the error message is completely
generator-dependent as it is the build tool which (should) eventually
detect this case.

Tests/RunCMake/CXXModules/RunCMakeTest.cmake

@@ -120,7 +120,9 @@ function (run_cxx_module_test directory)
   if (RunCMake_CXXModules_INSTALL)
     run_cmake_command("examples/${test_name}-install" "${CMAKE_COMMAND}" --build . --target install --config Debug)
   endif ()
-  run_cmake_command("examples/${test_name}-test" "${CMAKE_CTEST_COMMAND}" -C Debug --output-on-failure)
+  if (NOT RunCMake_CXXModules_NO_TEST)
+    run_cmake_command("examples/${test_name}-test" "${CMAKE_CTEST_COMMAND}" -C Debug --output-on-failure)
+  endif ()
 endfunction ()
 
 string(REPLACE "," ";" CMake_TEST_MODULE_COMPILATION "${CMake_TEST_MODULE_COMPILATION}")
@@ -132,6 +134,9 @@ if ("named" IN_LIST CMake_TEST_MODULE_COMPILATION)
   run_cxx_module_test(generated)
   run_cxx_module_test(public-req-private)
   run_cxx_module_test(deep-chain)
+  set(RunCMake_CXXModules_NO_TEST 1)
+  run_cxx_module_test(circular)
+  unset(RunCMake_CXXModules_NO_TEST)
 endif ()
 
 # Tests which use named modules in shared libraries.

Tests/RunCMake/CXXModules/examples/circular-build-result.txt

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

Tests/RunCMake/CXXModules/examples/circular-build-stdout.txt

@@ -0,0 +1 @@
+(Ninja generators)?(build stopped: dependency cycle:)

Tests/RunCMake/CXXModules/examples/circular-stderr.txt

@@ -0,0 +1,9 @@
+CMake Warning \(dev\) at CMakeLists.txt:7 \(target_sources\):
+  CMake's C\+\+ module support is experimental.  It is meant only for
+  experimentation and feedback to CMake developers.
+This warning is for project developers.  Use -Wno-dev to suppress it.
+
+CMake Warning \(dev\):
+  C\+\+20 modules support via CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP is
+  experimental.  It is meant only for compiler developers to try.
+This warning is for project developers.  Use -Wno-dev to suppress it.

Tests/RunCMake/CXXModules/examples/circular/CMakeLists.txt

@@ -0,0 +1,15 @@
+cmake_minimum_required(VERSION 3.24)
+project(cxx_modules_circular CXX)
+
+include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")
+
+add_library(circular STATIC)
+target_sources(circular
+  PUBLIC
+    FILE_SET CXX_MODULES
+      BASE_DIRS
+        "${CMAKE_CURRENT_SOURCE_DIR}"
+      FILES
+        circular-a.cppm
+        circular-b.cppm)
+target_compile_features(circular PUBLIC cxx_std_20)

Tests/RunCMake/CXXModules/examples/circular/circular-a.cppm

@@ -0,0 +1,6 @@
+export module a;
+import b;
+
+export int a() {
+    return b();
+}

Tests/RunCMake/CXXModules/examples/circular/circular-b.cppm

@@ -0,0 +1,6 @@
+export module b;
+import a;
+
+export int b() {
+    return a();
+}