cmake/Modules/WriteCompilerDetectionHeader.cmake

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

#.rst:
# WriteCompilerDetectionHeader
# ----------------------------
#
# This module provides the function write_compiler_detection_header().
#
# The ``WRITE_COMPILER_DETECTION_HEADER`` function can be used to generate
# a file suitable for preprocessor inclusion which contains macros to be
# used in source code::
#
#    write_compiler_detection_header(
#              FILE <file>
#              PREFIX <prefix>
#              [OUTPUT_FILES_VAR <output_files_var> OUTPUT_DIR <output_dir>]
#              COMPILERS <compiler> [...]
#              FEATURES <feature> [...]
#              [BARE_FEATURES <feature> [...]]
#              [VERSION <version>]
#              [PROLOG <prolog>]
#              [EPILOG <epilog>]
#              [ALLOW_UNKNOWN_COMPILERS]
#              [ALLOW_UNKNOWN_COMPILER_VERSIONS]
#    )
#
# The ``write_compiler_detection_header`` function generates the
# file ``<file>`` with macros which all have the prefix ``<prefix>``.
#
# By default, all content is written directly to the ``<file>``.  The
# ``OUTPUT_FILES_VAR`` may be specified to cause the compiler-specific
# content to be written to separate files.  The separate files are then
# available in the ``<output_files_var>`` and may be consumed by the caller
# for installation for example.  The ``OUTPUT_DIR`` specifies a relative
# path from the main ``<file>`` to the compiler-specific files. For example:
#
# .. code-block:: cmake
#
#    write_compiler_detection_header(
#      FILE climbingstats_compiler_detection.h
#      PREFIX ClimbingStats
#      OUTPUT_FILES_VAR support_files
#      OUTPUT_DIR compilers
#      COMPILERS GNU Clang MSVC Intel
#      FEATURES cxx_variadic_templates
#    )
#    install(FILES
#      ${CMAKE_CURRENT_BINARY_DIR}/climbingstats_compiler_detection.h
#      DESTINATION include
#    )
#    install(FILES
#      ${support_files}
#      DESTINATION include/compilers
#    )
#
#
# ``VERSION`` may be used to specify the API version to be generated.
# Future versions of CMake may introduce alternative APIs.  A given
# API is selected by any ``<version>`` value greater than or equal
# to the version of CMake that introduced the given API and less
# than the version of CMake that introduced its succeeding API.
# The value of the :variable:`CMAKE_MINIMUM_REQUIRED_VERSION`
# variable is used if no explicit version is specified.
# (As of CMake version |release| there is only one API version.)
#
# ``PROLOG`` may be specified as text content to write at the start of the
# header. ``EPILOG`` may be specified as text content to write at the end
# of the header
#
# At least one ``<compiler>`` and one ``<feature>`` must be listed.  Compilers
# which are known to CMake, but not specified are detected and a preprocessor
# ``#error`` is generated for them.  A preprocessor macro matching
# ``<PREFIX>_COMPILER_IS_<compiler>`` is generated for each compiler
# known to CMake to contain the value ``0`` or ``1``.
#
# Possible compiler identifiers are documented with the
# :variable:`CMAKE_<LANG>_COMPILER_ID` variable.
# Available features in this version of CMake are listed in the
# :prop_gbl:`CMAKE_C_KNOWN_FEATURES` and
# :prop_gbl:`CMAKE_CXX_KNOWN_FEATURES` global properties.
# The ``{c,cxx}_std_*`` meta-features are ignored if requested.
#
# See the :manual:`cmake-compile-features(7)` manual for information on
# compile features.
#
# ``BARE_FEATURES`` will define the compatibility macros with the name used in
# newer versions of the language standard, so the code can use the new feature
# name unconditionally.
#
# ``ALLOW_UNKNOWN_COMPILERS`` and ``ALLOW_UNKNOWN_COMPILER_VERSIONS`` cause
# the module to generate conditions that treat unknown compilers as simply
# lacking all features.  Without these options the default behavior is to
# generate a ``#error`` for unknown compilers and versions.
#
# Feature Test Macros
# ===================
#
# For each compiler, a preprocessor macro is generated matching
# ``<PREFIX>_COMPILER_IS_<compiler>`` which has the content either ``0``
# or ``1``, depending on the compiler in use. Preprocessor macros for
# compiler version components are generated matching
# ``<PREFIX>_COMPILER_VERSION_MAJOR`` ``<PREFIX>_COMPILER_VERSION_MINOR``
# and ``<PREFIX>_COMPILER_VERSION_PATCH`` containing decimal values
# for the corresponding compiler version components, if defined.
#
# A preprocessor test is generated based on the compiler version
# denoting whether each feature is enabled.  A preprocessor macro
# matching ``<PREFIX>_COMPILER_<FEATURE>``, where ``<FEATURE>`` is the
# upper-case ``<feature>`` name, is generated to contain the value
# ``0`` or ``1`` depending on whether the compiler in use supports the
# feature:
#
# .. code-block:: cmake
#
#    write_compiler_detection_header(
#      FILE climbingstats_compiler_detection.h
#      PREFIX ClimbingStats
#      COMPILERS GNU Clang AppleClang MSVC Intel
#      FEATURES cxx_variadic_templates
#    )
#
# .. code-block:: c++
#
#    #if ClimbingStats_COMPILER_CXX_VARIADIC_TEMPLATES
#    template<typename... T>
#    void someInterface(T t...) { /* ... */ }
#    #else
#    // Compatibility versions
#    template<typename T1>
#    void someInterface(T1 t1) { /* ... */ }
#    template<typename T1, typename T2>
#    void someInterface(T1 t1, T2 t2) { /* ... */ }
#    template<typename T1, typename T2, typename T3>
#    void someInterface(T1 t1, T2 t2, T3 t3) { /* ... */ }
#    #endif
#
# Symbol Macros
# =============
#
# Some additional symbol-defines are created for particular features for
# use as symbols which may be conditionally defined empty:
#
# .. code-block:: c++
#
#    class MyClass ClimbingStats_FINAL
#    {
#        ClimbingStats_CONSTEXPR int someInterface() { return 42; }
#    };
#
# The ``ClimbingStats_FINAL`` macro will expand to ``final`` if the
# compiler (and its flags) support the ``cxx_final`` feature, and the
# ``ClimbingStats_CONSTEXPR`` macro will expand to ``constexpr``
# if ``cxx_constexpr`` is supported.
#
# If ``BARE_FEATURES cxx_final`` was given as argument the ``final`` keyword
# will be defined for old compilers, too.
#
# The following features generate corresponding symbol defines and if they
# are available as ``BARE_FEATURES``:
#
# ========================== =================================== ================= ======
#         Feature                          Define                      Symbol       bare
# ========================== =================================== ================= ======
# ``c_restrict``              ``<PREFIX>_RESTRICT``               ``restrict``      yes
# ``cxx_constexpr``           ``<PREFIX>_CONSTEXPR``              ``constexpr``     yes
# ``cxx_deleted_functions``   ``<PREFIX>_DELETED_FUNCTION``       ``= delete``
# ``cxx_extern_templates``    ``<PREFIX>_EXTERN_TEMPLATE``        ``extern``
# ``cxx_final``               ``<PREFIX>_FINAL``                  ``final``         yes
# ``cxx_noexcept``            ``<PREFIX>_NOEXCEPT``               ``noexcept``      yes
# ``cxx_noexcept``            ``<PREFIX>_NOEXCEPT_EXPR(X)``       ``noexcept(X)``
# ``cxx_override``            ``<PREFIX>_OVERRIDE``               ``override``      yes
# ========================== =================================== ================= ======
#
# Compatibility Implementation Macros
# ===================================
#
# Some features are suitable for wrapping in a macro with a backward
# compatibility implementation if the compiler does not support the feature.
#
# When the ``cxx_static_assert`` feature is not provided by the compiler,
# a compatibility implementation is available via the
# ``<PREFIX>_STATIC_ASSERT(COND)`` and
# ``<PREFIX>_STATIC_ASSERT_MSG(COND, MSG)`` function-like macros. The macros
# expand to ``static_assert`` where that compiler feature is available, and
# to a compatibility implementation otherwise. In the first form, the
# condition is stringified in the message field of ``static_assert``.  In
# the second form, the message ``MSG`` is passed to the message field of
# ``static_assert``, or ignored if using the backward compatibility
# implementation.
#
# The ``cxx_attribute_deprecated`` feature provides a macro definition
# ``<PREFIX>_DEPRECATED``, which expands to either the standard
# ``[[deprecated]]`` attribute or a compiler-specific decorator such
# as ``__attribute__((__deprecated__))`` used by GNU compilers.
#
# The ``cxx_alignas`` feature provides a macro definition
# ``<PREFIX>_ALIGNAS`` which expands to either the standard ``alignas``
# decorator or a compiler-specific decorator such as
# ``__attribute__ ((__aligned__))`` used by GNU compilers.
#
# The ``cxx_alignof`` feature provides a macro definition
# ``<PREFIX>_ALIGNOF`` which expands to either the standard ``alignof``
# decorator or a compiler-specific decorator such as ``__alignof__``
# used by GNU compilers.
#
# ============================= ================================ ===================== ======
#           Feature                          Define                     Symbol          bare
# ============================= ================================ ===================== ======
# ``cxx_alignas``                ``<PREFIX>_ALIGNAS``             ``alignas``
# ``cxx_alignof``                ``<PREFIX>_ALIGNOF``             ``alignof``
# ``cxx_nullptr``                ``<PREFIX>_NULLPTR``             ``nullptr``           yes
# ``cxx_static_assert``          ``<PREFIX>_STATIC_ASSERT``       ``static_assert``
# ``cxx_static_assert``          ``<PREFIX>_STATIC_ASSERT_MSG``   ``static_assert``
# ``cxx_attribute_deprecated``   ``<PREFIX>_DEPRECATED``          ``[[deprecated]]``
# ``cxx_attribute_deprecated``   ``<PREFIX>_DEPRECATED_MSG``      ``[[deprecated]]``
# ``cxx_thread_local``           ``<PREFIX>_THREAD_LOCAL``        ``thread_local``
# ============================= ================================ ===================== ======
#
# A use-case which arises with such deprecation macros is the deprecation
# of an entire library.  In that case, all public API in the library may
# be decorated with the ``<PREFIX>_DEPRECATED`` macro.  This results in
# very noisy build output when building the library itself, so the macro
# may be may be defined to empty in that case when building the deprecated
# library:
#
# .. code-block:: cmake
#
#   add_library(compat_support ${srcs})
#   target_compile_definitions(compat_support
#     PRIVATE
#       CompatSupport_DEPRECATED=
#   )

include(${CMAKE_CURRENT_LIST_DIR}/CMakeCompilerIdDetection.cmake)

function(_load_compiler_variables CompilerId lang)
  include("${CMAKE_ROOT}/Modules/Compiler/${CompilerId}-${lang}-FeatureTests.cmake" OPTIONAL)
  set(_cmake_oldestSupported_${CompilerId} ${_cmake_oldestSupported} PARENT_SCOPE)
  foreach(feature ${ARGN})
    set(_cmake_feature_test_${CompilerId}_${feature} ${_cmake_feature_test_${feature}} PARENT_SCOPE)
  endforeach()
  include("${CMAKE_ROOT}/Modules/Compiler/${CompilerId}-${lang}-DetermineCompiler.cmake" OPTIONAL
      RESULT_VARIABLE determinedCompiler)
  if (NOT determinedCompiler)
    include("${CMAKE_ROOT}/Modules/Compiler/${CompilerId}-DetermineCompiler.cmake" OPTIONAL)
  endif()
  set(_compiler_id_version_compute_${CompilerId} ${_compiler_id_version_compute} PARENT_SCOPE)
endfunction()

macro(_simpledefine FEATURE_NAME FEATURE_TESTNAME FEATURE_STRING FEATURE_DEFAULT_STRING)
  if (feature STREQUAL "${FEATURE_NAME}")
        set(def_value "${prefix_arg}_${FEATURE_TESTNAME}")
        string(APPEND file_content "
#  if defined(${def_name}) && ${def_name}
#    define ${def_value} ${FEATURE_STRING}
#  else
#    define ${def_value} ${FEATURE_DEFAULT_STRING}
#  endif
\n")
  endif()
endmacro()

macro(_simplebaredefine FEATURE_NAME FEATURE_STRING FEATURE_DEFAULT_STRING)
  if (feature STREQUAL "${FEATURE_NAME}")
        string(APPEND file_content "
#  if !(defined(${def_name}) && ${def_name})
#    define ${FEATURE_STRING} ${FEATURE_DEFAULT_STRING}
#  endif
\n")
  endif()
endmacro()

function(_check_feature_lists C_FEATURE_VAR CXX_FEATURE_VAR)
  foreach(feature ${ARGN})
    if (feature MATCHES "^c_std_")
      # ignored
    elseif (feature MATCHES "^cxx_std_")
      # ignored
    elseif (feature MATCHES "^cxx_")
      list(APPEND _langs CXX)
      list(APPEND ${CXX_FEATURE_VAR} ${feature})
    elseif (feature MATCHES "^c_")
      list(APPEND _langs C)
      list(APPEND ${C_FEATURE_VAR} ${feature})
    else()
      message(FATAL_ERROR "Unsupported feature ${feature}.")
    endif()
  endforeach()
  set(${C_FEATURE_VAR} ${${C_FEATURE_VAR}} PARENT_SCOPE)
  set(${CXX_FEATURE_VAR} ${${CXX_FEATURE_VAR}} PARENT_SCOPE)
  set(_langs ${_langs} PARENT_SCOPE)
endfunction()

function(write_compiler_detection_header
    file_keyword file_arg
    prefix_keyword prefix_arg
    )
  if (NOT "x${file_keyword}" STREQUAL "xFILE")
    message(FATAL_ERROR "write_compiler_detection_header: FILE parameter missing.")
  endif()
  if (NOT "x${prefix_keyword}" STREQUAL "xPREFIX")
    message(FATAL_ERROR "write_compiler_detection_header: PREFIX parameter missing.")
  endif()
  set(options ALLOW_UNKNOWN_COMPILERS ALLOW_UNKNOWN_COMPILER_VERSIONS)
  set(oneValueArgs VERSION EPILOG PROLOG OUTPUT_FILES_VAR OUTPUT_DIR)
  set(multiValueArgs COMPILERS FEATURES BARE_FEATURES)
  cmake_parse_arguments(_WCD "${options}" "${oneValueArgs}" "${multiValueArgs}" ${ARGN})

  if (NOT _WCD_COMPILERS)
    message(FATAL_ERROR "Invalid arguments.  write_compiler_detection_header requires at least one compiler.")
  endif()
  if (NOT _WCD_FEATURES AND NOT _WCD_BARE_FEATURES)
    message(FATAL_ERROR "Invalid arguments.  write_compiler_detection_header requires at least one feature.")
  endif()

  if(_WCD_UNPARSED_ARGUMENTS)
    message(FATAL_ERROR "Unparsed arguments: ${_WCD_UNPARSED_ARGUMENTS}")
  endif()

  if (prefix_arg STREQUAL "")
    message(FATAL_ERROR "A prefix must be specified")
  endif()
  string(MAKE_C_IDENTIFIER ${prefix_arg} cleaned_prefix)
  if (NOT prefix_arg STREQUAL cleaned_prefix)
    message(FATAL_ERROR "The prefix must be a valid C identifier.")
  endif()

  if(NOT _WCD_VERSION)
    set(_WCD_VERSION ${CMAKE_MINIMUM_REQUIRED_VERSION})
  endif()
  set(_min_version 3.1.0) # Version which introduced this function
  if (_WCD_VERSION VERSION_LESS _min_version)
    set(err "VERSION compatibility for write_compiler_detection_header is set to ${_WCD_VERSION}, which is too low.")
    string(APPEND err "  It must be set to at least ${_min_version}.  ")
    string(APPEND err "  Either set the VERSION parameter to the write_compiler_detection_header function, or update")
    string(APPEND err " your minimum required CMake version with the cmake_minimum_required command.")
    message(FATAL_ERROR "${err}")
  endif()

  if(_WCD_OUTPUT_FILES_VAR)
    if(NOT _WCD_OUTPUT_DIR)
      message(FATAL_ERROR "If OUTPUT_FILES_VAR is specified, then OUTPUT_DIR must also be specified.")
    endif()
  endif()
  if(_WCD_OUTPUT_DIR)
    if(NOT _WCD_OUTPUT_FILES_VAR)
      message(FATAL_ERROR "If OUTPUT_DIR is specified, then OUTPUT_FILES_VAR must also be specified.")
    endif()
    get_filename_component(main_file_dir ${file_arg} DIRECTORY)
    if (NOT IS_ABSOLUTE ${main_file_dir})
      set(main_file_dir "${CMAKE_CURRENT_BINARY_DIR}/${main_file_dir}")
    endif()
    if (NOT IS_ABSOLUTE ${_WCD_OUTPUT_DIR})
      set(_WCD_OUTPUT_DIR "${CMAKE_CURRENT_BINARY_DIR}/${_WCD_OUTPUT_DIR}")
    endif()
    get_filename_component(out_file_dir ${_WCD_OUTPUT_DIR} ABSOLUTE)
    string(FIND ${out_file_dir} ${main_file_dir} idx)
    if (NOT idx EQUAL 0)
      message(FATAL_ERROR "The compiler-specific output directory must be within the same directory as the main file.")
    endif()

    if (main_file_dir STREQUAL out_file_dir)
      unset(_WCD_OUTPUT_DIR)
    else()
      string(REPLACE "${main_file_dir}/" "" _WCD_OUTPUT_DIR "${out_file_dir}/")
    endif()
  endif()

  set(compilers
    GNU
    Clang
    AppleClang
    MSVC
    SunPro
    Intel
  )

  set(_hex_compilers ADSP Borland Embarcadero SunPro)

  foreach(_comp ${_WCD_COMPILERS})
    list(FIND compilers ${_comp} idx)
    if (idx EQUAL -1)
      message(FATAL_ERROR "Unsupported compiler ${_comp}.")
    endif()
    if (NOT _need_hex_conversion)
      list(FIND _hex_compilers ${_comp} idx)
      if (NOT idx EQUAL -1)
        set(_need_hex_conversion TRUE)
      endif()
    endif()
  endforeach()

  set(file_content "
// This is a generated file. Do not edit!

#ifndef ${prefix_arg}_COMPILER_DETECTION_H
#define ${prefix_arg}_COMPILER_DETECTION_H
")

  if (_WCD_PROLOG)
    string(APPEND file_content "\n${_WCD_PROLOG}\n")
  endif()

  if (_need_hex_conversion)
    string(APPEND file_content "
#define ${prefix_arg}_DEC(X) (X)
#define ${prefix_arg}_HEX(X) ( \\
    ((X)>>28 & 0xF) * 10000000 + \\
    ((X)>>24 & 0xF) *  1000000 + \\
    ((X)>>20 & 0xF) *   100000 + \\
    ((X)>>16 & 0xF) *    10000 + \\
    ((X)>>12 & 0xF) *     1000 + \\
    ((X)>>8  & 0xF) *      100 + \\
    ((X)>>4  & 0xF) *       10 + \\
    ((X)     & 0xF) \\
    )\n")
  endif()

  _check_feature_lists(C_features CXX_features ${_WCD_FEATURES})
  _check_feature_lists(C_bare_features CXX_bare_features ${_WCD_BARE_FEATURES})
  list(REMOVE_DUPLICATES _langs)

  if(_WCD_OUTPUT_FILES_VAR)
    get_filename_component(main_file_name ${file_arg} NAME)
    set(compiler_file_content_
"#ifndef ${prefix_arg}_COMPILER_DETECTION_H
#  error This file may only be included from ${main_file_name}
#endif\n")
  endif()

  foreach(_lang ${_langs})
    set(target_compilers)
    foreach(compiler ${_WCD_COMPILERS})
      _load_compiler_variables(${compiler} ${_lang} ${${_lang}_features})
      if(_cmake_oldestSupported_${compiler})
        list(APPEND target_compilers ${compiler})
      endif()
    endforeach()

    get_property(known_features GLOBAL PROPERTY CMAKE_${_lang}_KNOWN_FEATURES)
    foreach(feature ${${_lang}_features})
      list(FIND known_features ${feature} idx)
      if (idx EQUAL -1)
        message(FATAL_ERROR "Unsupported feature ${feature}.")
      endif()
    endforeach()

    if(_lang STREQUAL CXX)
      string(APPEND file_content "\n#ifdef __cplusplus\n")
    else()
      string(APPEND file_content "\n#ifndef __cplusplus\n")
    endif()

    compiler_id_detection(ID_CONTENT ${_lang} PREFIX ${prefix_arg}_
      ID_DEFINE
    )

    string(APPEND file_content "${ID_CONTENT}\n")

    set(pp_if "if")
    foreach(compiler ${target_compilers})
      string(APPEND file_content "\n#  ${pp_if} ${prefix_arg}_COMPILER_IS_${compiler}\n")

      if(_WCD_OUTPUT_FILES_VAR)
        set(compile_file_name "${_WCD_OUTPUT_DIR}${prefix_arg}_COMPILER_INFO_${compiler}_${_lang}.h")
        string(APPEND file_content "\n#    include \"${compile_file_name}\"\n")
      endif()

      if(_WCD_OUTPUT_FILES_VAR)
        set(compiler_file_content compiler_file_content_${compiler}_${_lang})
      else()
        set(compiler_file_content file_content)
      endif()

      if(NOT _WCD_ALLOW_UNKNOWN_COMPILER_VERSIONS)
        string(APPEND ${compiler_file_content} "
#    if !(${_cmake_oldestSupported_${compiler}})
#      error Unsupported compiler version
#    endif\n")
      endif()

      set(PREFIX ${prefix_arg}_)
      if (_need_hex_conversion)
        set(MACRO_DEC ${prefix_arg}_DEC)
        set(MACRO_HEX ${prefix_arg}_HEX)
      else()
        set(MACRO_DEC)
        set(MACRO_HEX)
      endif()
      string(CONFIGURE "${_compiler_id_version_compute_${compiler}}" VERSION_BLOCK @ONLY)
      string(APPEND ${compiler_file_content} "${VERSION_BLOCK}\n")
      set(PREFIX)
      set(MACRO_DEC)
      set(MACRO_HEX)

      set(pp_if "elif")
      foreach(feature ${${_lang}_features})
        string(TOUPPER ${feature} feature_upper)
        set(feature_PP "COMPILER_${feature_upper}")
        set(_define_item "\n#    define ${prefix_arg}_${feature_PP} 0\n")
        if (_cmake_feature_test_${compiler}_${feature} STREQUAL "1")
          set(_define_item "\n#    define ${prefix_arg}_${feature_PP} 1\n")
        elseif (_cmake_feature_test_${compiler}_${feature})
          set(_define_item "\n#      define ${prefix_arg}_${feature_PP} 0\n")
          set(_define_item "\n#    if ${_cmake_feature_test_${compiler}_${feature}}\n#      define ${prefix_arg}_${feature_PP} 1\n#    else${_define_item}#    endif\n")
        endif()
        string(APPEND ${compiler_file_content} "${_define_item}")
      endforeach()
    endforeach()
    if(pp_if STREQUAL "elif")
      if(_WCD_ALLOW_UNKNOWN_COMPILERS)
        string(APPEND file_content "
#  endif\n")
      else()
        string(APPEND file_content "
#  else
#    error Unsupported compiler
#  endif\n")
      endif()
    endif()
    foreach(feature ${${_lang}_features})
      string(TOUPPER ${feature} feature_upper)
      set(feature_PP "COMPILER_${feature_upper}")
      set(def_name ${prefix_arg}_${feature_PP})
      _simpledefine(c_restrict RESTRICT restrict "")
      _simpledefine(cxx_constexpr CONSTEXPR constexpr "")
      _simpledefine(cxx_final FINAL final "")
      _simpledefine(cxx_override OVERRIDE override "")
      if (feature STREQUAL cxx_static_assert)
        set(def_value "${prefix_arg}_STATIC_ASSERT(X)")
        set(def_value_msg "${prefix_arg}_STATIC_ASSERT_MSG(X, MSG)")
        set(def_fallback "enum { ${prefix_arg}_STATIC_ASSERT_JOIN(${prefix_arg}StaticAssertEnum, __LINE__) = sizeof(${prefix_arg}StaticAssert<X>) }")
        string(APPEND file_content "#  if defined(${def_name}) && ${def_name}
#    define ${def_value} static_assert(X, #X)
#    define ${def_value_msg} static_assert(X, MSG)
#  else
#    define ${prefix_arg}_STATIC_ASSERT_JOIN(X, Y) ${prefix_arg}_STATIC_ASSERT_JOIN_IMPL(X, Y)
#    define ${prefix_arg}_STATIC_ASSERT_JOIN_IMPL(X, Y) X##Y
template<bool> struct ${prefix_arg}StaticAssert;
template<> struct ${prefix_arg}StaticAssert<true>{};
#    define ${def_value} ${def_fallback}
#    define ${def_value_msg} ${def_fallback}
#  endif
\n")
      endif()
      if (feature STREQUAL cxx_alignas)
        set(def_value "${prefix_arg}_ALIGNAS(X)")
        string(APPEND file_content "
#  if defined(${def_name}) && ${def_name}
#    define ${def_value} alignas(X)
#  elif ${prefix_arg}_COMPILER_IS_GNU || ${prefix_arg}_COMPILER_IS_Clang || ${prefix_arg}_COMPILER_IS_AppleClang
#    define ${def_value} __attribute__ ((__aligned__(X)))
#  elif ${prefix_arg}_COMPILER_IS_MSVC
#    define ${def_value} __declspec(align(X))
#  else
#    define ${def_value}
#  endif
\n")
      endif()
      if (feature STREQUAL cxx_alignof)
        set(def_value "${prefix_arg}_ALIGNOF(X)")
        string(APPEND file_content "
#  if defined(${def_name}) && ${def_name}
#    define ${def_value} alignof(X)
#  elif ${prefix_arg}_COMPILER_IS_GNU || ${prefix_arg}_COMPILER_IS_Clang || ${prefix_arg}_COMPILER_IS_AppleClang
#    define ${def_value} __alignof__(X)
#  elif ${prefix_arg}_COMPILER_IS_MSVC
#    define ${def_value} __alignof(X)
#  endif
\n")
      endif()
      _simpledefine(cxx_deleted_functions DELETED_FUNCTION "= delete" "")
      _simpledefine(cxx_extern_templates EXTERN_TEMPLATE extern "")
      if (feature STREQUAL cxx_noexcept)
        set(def_value "${prefix_arg}_NOEXCEPT")
        string(APPEND file_content "
#  if defined(${def_name}) && ${def_name}
#    define ${def_value} noexcept
#    define ${def_value}_EXPR(X) noexcept(X)
#  else
#    define ${def_value}
#    define ${def_value}_EXPR(X)
#  endif
\n")
      endif()
      if (feature STREQUAL cxx_nullptr)
        set(def_value "${prefix_arg}_NULLPTR")
        string(APPEND file_content "
#  if defined(${def_name}) && ${def_name}
#    define ${def_value} nullptr
#  elif ${prefix_arg}_COMPILER_IS_GNU
#    define ${def_value} __null
#  else
#    define ${def_value} 0
#  endif
\n")
      endif()
      if (feature STREQUAL cxx_thread_local)
        set(def_value "${prefix_arg}_THREAD_LOCAL")
        string(APPEND file_content "
#  if defined(${def_name}) && ${def_name}
#    define ${def_value} thread_local
#  elif ${prefix_arg}_COMPILER_IS_GNU || ${prefix_arg}_COMPILER_IS_Clang || ${prefix_arg}_COMPILER_IS_AppleClang
#    define ${def_value} __thread
#  elif ${prefix_arg}_COMPILER_IS_MSVC
#    define ${def_value} __declspec(thread)
#  else
// ${def_value} not defined for this configuration.
#  endif
\n")
      endif()
      if (feature STREQUAL cxx_attribute_deprecated)
        set(def_name ${prefix_arg}_${feature_PP})
        set(def_value "${prefix_arg}_DEPRECATED")
        string(APPEND file_content "
#  ifndef ${def_value}
#    if defined(${def_name}) && ${def_name}
#      define ${def_value} [[deprecated]]
#      define ${def_value}_MSG(MSG) [[deprecated(MSG)]]
#    elif ${prefix_arg}_COMPILER_IS_GNU || ${prefix_arg}_COMPILER_IS_Clang
#      define ${def_value} __attribute__((__deprecated__))
#      define ${def_value}_MSG(MSG) __attribute__((__deprecated__(MSG)))
#    elif ${prefix_arg}_COMPILER_IS_MSVC
#      define ${def_value} __declspec(deprecated)
#      define ${def_value}_MSG(MSG) __declspec(deprecated(MSG))
#    else
#      define ${def_value}
#      define ${def_value}_MSG(MSG)
#    endif
#  endif
\n")
      endif()
    endforeach()

    foreach(feature ${${_lang}_bare_features})
      string(TOUPPER ${feature} feature_upper)
      set(feature_PP "COMPILER_${feature_upper}")
      set(def_name ${prefix_arg}_${feature_PP})
      _simplebaredefine(c_restrict restrict "")
      _simplebaredefine(cxx_constexpr constexpr "")
      _simplebaredefine(cxx_final final "")
      _simplebaredefine(cxx_override override "")
      if (feature STREQUAL cxx_nullptr)
        set(def_value "nullptr")
        string(APPEND file_content "
#  if !(defined(${def_name}) && ${def_name})
#    if ${prefix_arg}_COMPILER_IS_GNU
#      define ${def_value} __null
#    else
#      define ${def_value} 0
#    endif
#  endif
\n")
      endif()
      _simplebaredefine(cxx_noexcept noexcept "")
    endforeach()

    string(APPEND file_content "#endif\n")

  endforeach()

  if(_WCD_OUTPUT_FILES_VAR)
    foreach(compiler ${_WCD_COMPILERS})
      foreach(_lang ${_langs})
        if(compiler_file_content_${compiler}_${_lang})
          set(CMAKE_CONFIGURABLE_FILE_CONTENT "${compiler_file_content_}")
          string(APPEND CMAKE_CONFIGURABLE_FILE_CONTENT "${compiler_file_content_${compiler}_${_lang}}")

          set(compile_file_name "${_WCD_OUTPUT_DIR}${prefix_arg}_COMPILER_INFO_${compiler}_${_lang}.h")
          set(full_path "${main_file_dir}/${compile_file_name}")
          list(APPEND ${_WCD_OUTPUT_FILES_VAR} ${full_path})
          configure_file("${CMAKE_ROOT}/Modules/CMakeConfigurableFile.in"
            "${full_path}"
            @ONLY
          )
        endif()
      endforeach()
    endforeach()
    set(${_WCD_OUTPUT_FILES_VAR} ${${_WCD_OUTPUT_FILES_VAR}} PARENT_SCOPE)
  endif()

  if (_WCD_EPILOG)
    string(APPEND file_content "\n${_WCD_EPILOG}\n")
  endif()
  string(APPEND file_content "\n#endif")

  set(CMAKE_CONFIGURABLE_FILE_CONTENT ${file_content})
  configure_file("${CMAKE_ROOT}/Modules/CMakeConfigurableFile.in"
    "${file_arg}"
    @ONLY
  )
endfunction()