Languages: Add support for Objective-C++

Add entries in Modules and Modules/Platform to support
Objective-C++ compiler determination and identification.
Add Modules to check Objective-C++ compiler flags, source
compilations, program checks, etc...

Use OBJCXX as the designator of the language, eg:

project(foo OBJCXX)

Add various tests for Objective-C++ language features.  Add
tests to preserve C++ handling of .M and .mm files when
Objective-C++ is not a configured language.

Co-authored-by: Cristian Adam <cristian.adam@gmail.com>

1 files changed, 64 insertions, 0 deletions

diff --git a/Modules/CheckOBJCXXCompilerFlag.cmake b/Modules/CheckOBJCXXCompilerFlag.cmake
new file mode 100644
index 0000000..c32741b
--- /dev/null
+++ b/Modules/CheckOBJCXXCompilerFlag.cmake
@@ -0,0 +1,64 @@
+# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+# file Copyright.txt or https://cmake.org/licensing for details.
+
+#[=======================================================================[.rst:
+CheckOBJCXXCompilerFlag
+-----------------------
+
+Check whether the Objective-C++ compiler supports a given flag.
+
+.. command:: check_objcxx_compiler_flag
+
+  .. code-block:: cmake
+
+    check_objcxx_compiler_flag(<flag> <var>)
+
+  Check that the ``<flag>`` is accepted by the compiler without
+  a diagnostic.  Stores the result in an internal cache entry
+  named ``<var>``.
+
+This command temporarily sets the ``CMAKE_REQUIRED_DEFINITIONS`` variable
+and calls the ``check_objcxx_source_compiles`` macro from the
+:module:`CheckOBJCXXSourceCompiles` module.  See documentation of that
+module for a listing of variables that can otherwise modify the build.
+
+A positive result from this check indicates only that the compiler did not
+issue a diagnostic message when given the flag.  Whether the flag has any
+effect or even a specific one is beyond the scope of this module.
+
+.. note::
+  Since the :command:`try_compile` command forwards flags from variables
+  like :variable:`CMAKE_OBJCXX_FLAGS <CMAKE_<LANG>_FLAGS>`, unknown flags
+  in such variables may cause a false negative for this check.
+#]=======================================================================]
+
+include_guard(GLOBAL)
+include(CheckOBJCXXSourceCompiles)
+include(CMakeCheckCompilerFlagCommonPatterns)
+
+macro (CHECK_OBJCXX_COMPILER_FLAG _FLAG _RESULT)
+  set(SAFE_CMAKE_REQUIRED_DEFINITIONS "${CMAKE_REQUIRED_DEFINITIONS}")
+  set(CMAKE_REQUIRED_DEFINITIONS "${_FLAG}")
+
+  # Normalize locale during test compilation.
+  set(_CheckOBJCXXCompilerFlag_LOCALE_VARS LC_ALL LC_MESSAGES LANG)
+  foreach(v ${_CheckOBJCXXCompilerFlag_LOCALE_VARS})
+    set(_CheckOBJCXXCompilerFlag_SAVED_${v} "$ENV{${v}}")
+    set(ENV{${v}} OBJCXX)
+  endforeach()
+  CHECK_COMPILER_FLAG_COMMON_PATTERNS(_CheckOBJCXXCompilerFlag_COMMON_PATTERNS)
+  CHECK_OBJCXX_SOURCE_COMPILES("#ifndef __OBJC__\n#  error \"Not an Objective-C++ compiler\"\n#endif\nint main(void) { return 0; }" ${_RESULT}
+    # Some compilers do not fail with a bad flag
+    FAIL_REGEX "command line option .* is valid for .* but not for Objective-C\\\\+\\\\+" # GNU
+    FAIL_REGEX "argument unused during compilation: .*" # Clang
+    ${_CheckOBJCXXCompilerFlag_COMMON_PATTERNS}
+    )
+  foreach(v ${_CheckOBJCXXCompilerFlag_LOCALE_VARS})
+    set(ENV{${v}} ${_CheckOBJCXXCompilerFlag_SAVED_${v}})
+    unset(_CheckOBJCXXCompilerFlag_SAVED_${v})
+  endforeach()
+  unset(_CheckOBJCXXCompilerFlag_LOCALE_VARS)
+  unset(_CheckOBJCXXCompilerFlag_COMMON_PATTERNS)
+
+  set (CMAKE_REQUIRED_DEFINITIONS "${SAFE_CMAKE_REQUIRED_DEFINITIONS}")
+endmacro ()