emacs/m4/00gnulib.m4

# 00gnulib.m4
# serial 9
dnl Copyright (C) 2009-2025 Free Software Foundation, Inc.
dnl This file is free software; the Free Software Foundation
dnl gives unlimited permission to copy and/or distribute it,
dnl with or without modifications, as long as this notice is preserved.
dnl This file is offered as-is, without any warranty.

dnl This file must be named something that sorts before all other
dnl gnulib-provided .m4 files.  It is needed until the clang fix has
dnl been included in Autoconf.

# The following definitions arrange to use a compiler option
# -Werror=implicit-function-declaration in AC_CHECK_DECL, when the
# compiler is clang.  Without it, clang implicitly declares "known"
# library functions in C mode, but not in C++ mode, which would cause
# Gnulib to omit a declaration and thus later produce an error in C++
# mode.  As of clang 9.0, these "known" functions are identified through
# LIBBUILTIN invocations in the LLVM source file
# llvm/tools/clang/include/clang/Basic/Builtins.def.
# It's not possible to AC_REQUIRE the extra tests from AC_CHECK_DECL,
# because AC_CHECK_DECL, like other Autoconf built-ins, is not supposed
# to AC_REQUIRE anything: some configure.ac files have their first
# AC_CHECK_DECL executed conditionally.  Therefore append the extra tests
# to AC_PROG_CC.
AC_DEFUN([gl_COMPILER_CLANG],
[
dnl AC_REQUIRE([AC_PROG_CC])
  AC_CACHE_CHECK([whether the compiler is clang],
    [gl_cv_compiler_clang],
    [dnl Use _AC_COMPILE_IFELSE instead of AC_EGREP_CPP, to avoid error
     dnl "circular dependency of AC_LANG_COMPILER(C)" if AC_PROG_CC has
     dnl not yet been invoked.
     _AC_COMPILE_IFELSE(
        [AC_LANG_PROGRAM([[
           #ifdef __clang__
           barfbarf
           #endif
           ]],[[]])
        ],
        [gl_cv_compiler_clang=no],
        [gl_cv_compiler_clang=yes])
    ])
])
AC_DEFUN([gl_COMPILER_PREPARE_CHECK_DECL],
[
dnl AC_REQUIRE([AC_PROG_CC])
dnl AC_REQUIRE([gl_COMPILER_CLANG])
  AC_CACHE_CHECK([for compiler option needed when checking for declarations],
    [gl_cv_compiler_check_decl_option],
    [if test $gl_cv_compiler_clang = yes; then
       dnl Test whether the compiler supports the option
       dnl '-Werror=implicit-function-declaration'.
       saved_ac_compile="$ac_compile"
       ac_compile="$ac_compile -Werror=implicit-function-declaration"
       dnl Use _AC_COMPILE_IFELSE instead of AC_COMPILE_IFELSE, to avoid a
       dnl warning "AC_COMPILE_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS".
       _AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]],[[]])],
         [gl_cv_compiler_check_decl_option='-Werror=implicit-function-declaration'],
         [gl_cv_compiler_check_decl_option=none])
       ac_compile="$saved_ac_compile"
     else
       gl_cv_compiler_check_decl_option=none
     fi
    ])
  if test "x$gl_cv_compiler_check_decl_option" != xnone; then
    ac_compile_for_check_decl="$ac_compile $gl_cv_compiler_check_decl_option"
  else
    ac_compile_for_check_decl="$ac_compile"
  fi
])
dnl Redefine _AC_CHECK_DECL_BODY so that it references ac_compile_for_check_decl
dnl instead of ac_compile.  If, for whatever reason, the override of AC_PROG_CC
dnl in zzgnulib.m4 is inactive, use the original ac_compile.
m4_define([_AC_CHECK_DECL_BODY],
[  ac_saved_ac_compile="$ac_compile"
  if test -n "$ac_compile_for_check_decl"; then
    ac_compile="$ac_compile_for_check_decl"
  fi]
m4_defn([_AC_CHECK_DECL_BODY])[  ac_compile="$ac_saved_ac_compile"
])

# gl_00GNULIB
# -----------
# Witness macro that this file has been included.  Needed to force
# Automake to include this file prior to all other gnulib .m4 files.
AC_DEFUN([gl_00GNULIB])