Re: Messed up gl_COMPILER_PREPARE_CHECK_DECL

[Bruno Haible]

Hi Paul,

> Thanks for the diagnosis. I came up with a simpler patch to Emacs, and 
> installed it into Emacs master (attached).
> 
> This patch doesn't solve the general problem, just this particular case. 

Yes. Among the hundreds of packages that use gnulib, it's likely that
some other package has the same issue.

> I don't have any specific suggestion 
> to work around this problem in Gnulib, though.

OK, here's my take at it:
- Restore the assumption that Autoconf built-in macros like AC_CHECK_DECL
  don't AC_REQUIRE anything.
- Perform the clang tests at the end of AC_PROG_CC instead.

Since I was testing this with the Emacs git, I noticed that the override
of AC_PROG_CC was not working, because Emacs redefines AC_PROG_CC.
As a mitigation:
  1) I moved the AC_PROG_CC override from 00gnulib.m4 to ~~gnulib.m4; this
     file sorts after the other *.m4 files in the same directory.
  2) Since I don't see a way to make things work if a package is redefining
     AC_PROG_CC in a file that sorts after m4/~~gnulib.m4: Use ac_compile
     if ac_compile_for_check_decl is not defined.


2020-01-18  Bruno Haible  <address@hidden>

        Fix major regression from 2020-01-10.
        Reported by Paul Eggert in
        <https://lists.gnu.org/archive/html/bug-gnulib/2020-01/msg00079.html>.
        * m4/00gnulib.m4 (gl_COMPILER_CLANG, gl_COMPILER_PREPARE_CHECK_DECL):
        Don't AC_REQUIRE anything.
        (gl_COMPILER_PREPARE_CHECK_DECL): Define through AC_DEFUN, not
        AC_DEFUN_ONCE. Use _AC_COMPILE_IFELSE, not AC_COMPILE_IFELSE.
        (_AC_CHECK_DECL_BODY): If ac_compile_for_check_decl has not been set,
        use ac_compile instead.
        (AC_CHECK_DECL): Remove override.
        * m4/~~gnulib.m4: New file.
        * gnulib-tool (func_get_filelist): Add also ~~gnulib.m4.
        * pygnulib/GLModuleSystem.py (getFiles): Likewise.

diff --git a/m4/00gnulib.m4 b/m4/00gnulib.m4
index 58bc4ef..36304b9 100644
--- a/m4/00gnulib.m4
+++ b/m4/00gnulib.m4
@@ -1,4 +1,4 @@
-# 00gnulib.m4 serial 5
+# 00gnulib.m4 serial 6
 dnl Copyright (C) 2009-2020 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -48,9 +48,14 @@ m4_version_prereq([2.63.263], [],
 # 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],
 [
-  AC_REQUIRE([AC_PROG_CC])
+dnl AC_REQUIRE([AC_PROG_CC])
   AC_CACHE_CHECK([whether the compiler is clang],
     [gl_cv_compiler_clang],
     [AC_EGREP_CPP([barfbarf],[
@@ -62,10 +67,10 @@ barfbarf
        [gl_cv_compiler_clang=no])
     ])
 ])
-AC_DEFUN_ONCE([gl_COMPILER_PREPARE_CHECK_DECL],
+AC_DEFUN([gl_COMPILER_PREPARE_CHECK_DECL],
 [
-  AC_REQUIRE([AC_PROG_CC])
-  AC_REQUIRE([gl_COMPILER_CLANG])
+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 $gl_cv_compiler_clang = yes; then
@@ -73,7 +78,9 @@ AC_DEFUN_ONCE([gl_COMPILER_PREPARE_CHECK_DECL],
        dnl '-Werror=implicit-function-declaration'.
        save_ac_compile="$ac_compile"
        ac_compile="$ac_compile -Werror=implicit-function-declaration"
-       AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]],[[]])],
+       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="$save_ac_compile"
@@ -88,18 +95,15 @@ AC_DEFUN_ONCE([gl_COMPILER_PREPARE_CHECK_DECL],
   fi
 ])
 dnl Redefine _AC_CHECK_DECL_BODY so that it references 
ac_compile_for_check_decl
-dnl instead of ac_compile.
+dnl instead of ac_compile.  If, for whatever reason, the override of AC_PROG_CC
+dnl in ~~gnulib.m4 is inactive, use the original ac_compile.
 m4_define([_AC_CHECK_DECL_BODY],
 [  ac_save_ac_compile="$ac_compile"
-  ac_compile="$ac_compile_for_check_decl"]
+  if test -n "$ac_compile_for_check_decl"; then
+    ac_compile="$ac_compile_for_check_decl"
+  fi]
 m4_defn([_AC_CHECK_DECL_BODY])[  ac_compile="$ac_save_ac_compile"
 ])
-  ])
-dnl Redefine AC_CHECK_DECL so that it starts with an invocation of
-dnl gl_COMPILER_PREPARE_CHECK_DECL.
-m4_define([AC_CHECK_DECL],
-  [gl_COMPILER_PREPARE_CHECK_DECL dnl
-]m4_defn([AC_CHECK_DECL]))
 
 # gl_00GNULIB
 # -----------
diff --git a/m4/~~gnulib.m4 b/m4/~~gnulib.m4
new file mode 100644
index 0000000..bb5e910
--- /dev/null
+++ b/m4/~~gnulib.m4
@@ -0,0 +1,17 @@
+# ~~gnulib.m4 serial 1
+dnl Copyright (C) 2020 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 must be named something that sorts after all other
+dnl package- or gnulib-provided .m4 files - at least for those packages
+dnl that redefine AC_PROG_CC.
+
+dnl Redefine AC_PROG_CC so that it ends with invocations of gl_COMPILER_CLANG
+dnl and gl_COMPILER_PREPARE_CHECK_DECL.
+m4_define([AC_PROG_CC],
+  m4_defn([AC_PROG_CC])[
+gl_COMPILER_CLANG
+gl_COMPILER_PREPARE_CHECK_DECL
+])
diff --git a/gnulib-tool b/gnulib-tool
index 1bf1be3..ba51606 100755
--- a/gnulib-tool
+++ b/gnulib-tool
@@ -2294,6 +2294,7 @@ func_get_filelist ()
     fi
   fi
   echo m4/00gnulib.m4
+  echo 'm4/~~gnulib.m4'
   echo m4/gnulib-common.m4
 }
 
diff --git a/pygnulib/GLModuleSystem.py b/pygnulib/GLModuleSystem.py
index 9560bc2..72a6c9f 100644
--- a/pygnulib/GLModuleSystem.py
+++ b/pygnulib/GLModuleSystem.py
@@ -544,6 +544,7 @@ Include:|Link:|License:|Maintainer:)'
                     parts += [line]
                 result = [part.strip() for part in parts if part.strip()]
             result += [joinpath('m4', '00gnulib.m4')]
+            result += [joinpath('m4', '~~gnulib.m4')]
             result += [joinpath('m4', 'gnulib-common.m4')]
             self.cache['files'] = list(result)
         return(list(self.cache['files']))