Tests/CXXModules; add tests for `CXX_MODULE_STD` behavior

Tests/RunCMake/CXXModules/Inspect.cmake

@@ -15,11 +15,17 @@ if (CMAKE_CXX_FLAGS MATCHES "-std=")
   set(forced_cxx_standard 1)
 endif ()
 
+set(have_cxx23_import_std 0)
+if (TARGET "__CMAKE::CXX23")
+  set(have_cxx23_import_std 1)
+endif ()
+
 # Forward information about the C++ compile features.
 string(APPEND info "\
 set(CMAKE_CXX_COMPILE_FEATURES \"${CMAKE_CXX_COMPILE_FEATURES}\")
 set(CMAKE_MAKE_PROGRAM \"${CMAKE_MAKE_PROGRAM}\")
 set(forced_cxx_standard \"${forced_cxx_standard}\")
+set(have_cxx23_import_std \"${have_cxx23_import_std}\")
 set(CMAKE_CXX_COMPILER_VERSION \"${CMAKE_CXX_COMPILER_VERSION}\")
 set(CMAKE_CXX_OUTPUT_EXTENSION \"${CMAKE_CXX_OUTPUT_EXTENSION}\")
 set(CXXModules_default_build_type \"${CMAKE_BUILD_TYPE}\")

Tests/RunCMake/CXXModules/NoCXX23TargetNotRequired.cmake

@@ -0,0 +1,11 @@
+enable_language(CXX)
+set(CMAKE_CXX_SCANDEP_SOURCE "echo")
+
+set(CMAKE_CXX_MODULE_STD 0)
+
+add_library(nocxx23target)
+target_sources(nocxx23target
+  PRIVATE
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/module.cxx)
+target_compile_features(nocxx23target PRIVATE cxx_std_23)

Tests/RunCMake/CXXModules/NoCXX23TargetRequired-result.txt

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

Tests/RunCMake/CXXModules/NoCXX23TargetRequired-stderr.txt

@@ -0,0 +1,4 @@
+CMake Error in CMakeLists.txt:
+  The "CXX_MODULE_STD" property on the target "nocxx23target" requires that
+  the "__CMAKE::CXX23" target exist, but it was not provided by the
+  toolchain.

Tests/RunCMake/CXXModules/NoCXX23TargetRequired.cmake

@@ -0,0 +1,11 @@
+enable_language(CXX)
+set(CMAKE_CXX_SCANDEP_SOURCE "echo")
+
+set(CMAKE_CXX_MODULE_STD 1)
+
+add_library(nocxx23target)
+target_sources(nocxx23target
+  PRIVATE
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/module.cxx)
+target_compile_features(nocxx23target PRIVATE cxx_std_23)

Tests/RunCMake/CXXModules/NoCXX23TargetUnset.cmake

@@ -0,0 +1,13 @@
+enable_language(CXX)
+set(CMAKE_CXX_SCANDEP_SOURCE "echo")
+
+# TODO(cxxmodules): Add instances of this test which test the policy
+# of the property's unset behavior.
+# set(CMAKE_CXX_MODULE_STD …)
+
+add_library(nocxx23target)
+target_sources(nocxx23target
+  PRIVATE
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/module.cxx)
+target_compile_features(nocxx23target PRIVATE cxx_std_23)

Tests/RunCMake/CXXModules/RunCMakeTest.cmake

@@ -90,6 +90,13 @@ if ("cxx_std_23" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
   run_cmake(CXXImportStdInvalidGenex)
 endif ()
 
+if ("cxx_std_23" IN_LIST CMAKE_CXX_COMPILE_FEATURES AND
+    NOT have_cxx23_import_std)
+  run_cmake(NoCXX23TargetUnset)
+  run_cmake(NoCXX23TargetNotRequired)
+  run_cmake(NoCXX23TargetRequired)
+endif ()
+
 run_cmake(InstallBMI)
 run_cmake(InstallBMIGenericArgs)
 run_cmake(InstallBMIIgnore)