Objective-C: Do not treat Objective-C files as C++ files

When both C and C++ langauges are enabled, the Objective-C files
should be treated as C files and not as C++ files.

Modules/CMakeCXXCompiler.cmake.in

@@ -45,7 +45,7 @@ set(CMAKE_CXX_COMPILER_ID_RUN 1)
 set(CMAKE_CXX_SOURCE_FILE_EXTENSIONS C;M;c++;cc;cpp;cxx;m;mm;CPP)
 set(CMAKE_CXX_IGNORE_EXTENSIONS inl;h;hpp;HPP;H;o;O;obj;OBJ;def;DEF;rc;RC)
 
-foreach (lang OBJC OBJCXX)
+foreach (lang C OBJC OBJCXX)
   if (CMAKE_${lang}_COMPILER_ID_RUN)
     foreach(extension IN LISTS CMAKE_${lang}_SOURCE_FILE_EXTENSIONS)
       list(REMOVE_ITEM CMAKE_CXX_SOURCE_FILE_EXTENSIONS ${extension})

Tests/CMakeLists.txt

@@ -2353,6 +2353,18 @@ ${CMake_SOURCE_DIR}/Utilities/Release/push.bash --dir dev -- '${CMake_BUILD_NIGH
       )
       list(APPEND TEST_BUILD_DIRS "${CMake_BINARY_DIR}/Tests/Objective-C/c-file-extension-test")
 
+      add_test(Objective-C.cxx-file-extension-test ${CMAKE_CTEST_COMMAND}
+        --build-and-test
+        "${CMake_SOURCE_DIR}/Tests/Objective-C/cxx-file-extension-test"
+        "${CMake_BINARY_DIR}/Tests/Objective-C/cxx-file-extension-test"
+        --build-two-config
+        ${build_generator_args}
+        --build-project cxx-file-extension-test
+        --build-options ${build_options}
+        --test-command cxx-file-extension-test
+      )
+      list(APPEND TEST_BUILD_DIRS "${CMake_BINARY_DIR}/Tests/Objective-C/cxx-file-extension-test")
+
       add_test(Objective-C.objc-file-extension-test ${CMAKE_CTEST_COMMAND}
         --build-and-test
         "${CMake_SOURCE_DIR}/Tests/Objective-C/objc-file-extension-test"

Tests/Objective-C/cxx-file-extension-test/CMakeLists.txt

@@ -0,0 +1,8 @@
+cmake_minimum_required(VERSION 3.15)
+
+project(cxx-file-extension-test)
+
+string(APPEND CMAKE_CXX_FLAGS " -std=c++11")
+set(CMAKE_CXX_STANDARD 14)
+
+add_executable(cxx-file-extension-test main.m)

Tests/Objective-C/cxx-file-extension-test/main.m

@@ -0,0 +1,8 @@
+#ifndef __OBJC__
+#  error "Compiler cannot compile Objective-C"
+#endif
+
+int main()
+{
+  return 0;
+}