CMake/Modules/CheckCXXCompilerFlag.cmake

# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
# file Copyright.txt or https://cmake.org/licensing for details.

#[=======================================================================[.rst:
CheckCXXCompilerFlag
------------------------

Check whether the CXX compiler supports a given flag.

.. command:: check_cxx_compiler_flag

  .. code-block:: cmake

    check_cxx_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_cxx_source_compiles`` macro from the
:module:`CheckCXXSourceCompiles` 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_CXX_FLAGS <CMAKE_<LANG>_FLAGS>`, unknown flags
  in such variables may cause a false negative for this check.
#]=======================================================================]

include_guard(GLOBAL)
include(CheckCXXSourceCompiles)
include(CMakeCheckCompilerFlagCommonPatterns)

function(check_cxx_compiler_flag _flag _var)
  set(CMAKE_REQUIRED_DEFINITIONS "${_flag}")

  # Normalize locale during test compilation.
  set(_locale_vars LC_ALL LC_MESSAGES LANG)
  foreach(v IN LISTS _locale_vars)
    set(_locale_vars_saved_${v} "$ENV{${v}}")
    set(ENV{${v}} C)
  endforeach()
  check_compiler_flag_common_patterns(_common_patterns)
  check_cxx_source_compiles("int main() { return 0; }" ${_var}
    # Some compilers do not fail with a bad flag
    FAIL_REGEX "command line option .* is valid for .* but not for C\\\\+\\\\+" # GNU
    ${_common_patterns}
    )
  foreach(v IN LISTS _locale_vars)
    set(ENV{${v}} ${_locale_vars_saved_${v}})
  endforeach()
  set(${_var} "${${_var}}" PARENT_SCOPE)
endfunction()