I wrote on 2017-02-17:
> Please also fix the AC_REQUIRE([gl_UNKNOWN_WARNINGS_ARE_ERRORS]) invocation
> in gl_WARN_ADD and gl_CXX_WARN_ADD. Since this macro expands to different
> code after AC_LANG_PUSH([C++]) than after AC_LANG_PUSH([C]), it is wrong
> to just AC_REQUIRE it. Needs to be a bit more intelligent.
> 
> Look at how _AC_LANG_ABBREV and _AC_LANG_PREFIX can be used. Maybe libtool.m4,
> which also plays around with AC_LANG, gives you some hint about this.

To fix this, I prepared a test project (attached), and modified warnings.m4
and manywarnings.m4 until
  $ ./autogen.sh
  $ rm -f config.cache; ./configure -C
produced the config.status and config.cache that I expected.


2017-08-06  Bruno Haible  <br...@clisp.org>

        warnings, manywarnings: Add support for multiple languages, not just C.
        * warnings.m4 (gl_UNKNOWN_WARNINGS_ARE_ERRORS_IMPL): Renamed from
        gl_UNKNOWN_WARNINGS_ARE_ERRORS.
        (gl_UNKNOWN_WARNINGS_ARE_ERRORS(C)): New macro.
        (gl_UNKNOWN_WARNINGS_ARE_ERRORS(C++)): New macro.
        (gl_UNKNOWN_WARNINGS_ARE_ERRORS): Dispatch to
        gl_UNKNOWN_WARNINGS_ARE_ERRORS(_AC_LANG).
        (gl_WARN_ADD): Require the gl_UNKNOWN_WARNINGS_ARE_ERRORS specialization
        of the current language. If C++ is the current language, modify
        WARN_CXXFLAGS instead of WARN_CFLAGS.
        * manywarnings.m4 (gl_MANYWARN_ALL_GCC(C)): New macro, extracted from
        gl_MANYWARN_ALL_GCC.
        (gl_MANYWARN_ALL_GCC): Dispatch to gl_MANYWARN_ALL_GCC(_AC_LANG).

diff --git a/m4/manywarnings.m4 b/m4/manywarnings.m4
index 6a8939b..3124b1e 100644
--- a/m4/manywarnings.m4
+++ b/m4/manywarnings.m4
@@ -1,4 +1,4 @@
-# manywarnings.m4 serial 10
+# manywarnings.m4 serial 11
 dnl Copyright (C) 2008-2017 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -33,8 +33,16 @@ AC_DEFUN([gl_MANYWARN_COMPLEMENT],
 # Add all documented GCC warning parameters to variable VARIABLE.
 # Note that you need to test them using gl_WARN_ADD if you want to
 # make sure your gcc understands it.
+#
+# The effects of this macro depend on the current language (_AC_LANG).
 AC_DEFUN([gl_MANYWARN_ALL_GCC],
+[_AC_LANG_DISPATCH([$0], _AC_LANG, $@)])
+
+# Specialization for _AC_LANG = C. This macro can be AC_REQUIREd.
+AC_DEFUN([gl_MANYWARN_ALL_GCC(C)],
 [
+  AC_LANG_PUSH([C])
+
   dnl First, check for some issues that only occur when combining multiple
   dnl gcc warning categories.
   AC_REQUIRE([AC_PROG_CC])
@@ -303,4 +311,6 @@ AC_DEFUN([gl_MANYWARN_ALL_GCC],
   fi
 
   $1=$gl_manywarn_set
+
+  AC_LANG_POP([C])
 ])
diff --git a/m4/warnings.m4 b/m4/warnings.m4
index e697174..aa2735b 100644
--- a/m4/warnings.m4
+++ b/m4/warnings.m4
@@ -1,4 +1,4 @@
-# warnings.m4 serial 11
+# warnings.m4 serial 12
 dnl Copyright (C) 2008-2017 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -20,10 +20,12 @@ m4_ifdef([AS_VAR_APPEND],
 # -----------------------------------------------------------------
 # Check if the compiler supports OPTION when compiling PROGRAM.
 #
-# FIXME: gl_Warn must be used unquoted until we can assume Autoconf
-# 2.64 or newer.
+# The effects of this macro depend on the current language (_AC_LANG).
 AC_DEFUN([gl_COMPILER_OPTION_IF],
-[AS_VAR_PUSHDEF([gl_Warn], [gl_cv_warn_[]_AC_LANG_ABBREV[]_$1])dnl
+[
+dnl FIXME: gl_Warn must be used unquoted until we can assume Autoconf
+dnl 2.64 or newer.
+AS_VAR_PUSHDEF([gl_Warn], [gl_cv_warn_[]_AC_LANG_ABBREV[]_$1])dnl
 AS_VAR_PUSHDEF([gl_Flags], [_AC_LANG_PREFIX[]FLAGS])dnl
 AS_LITERAL_IF([$1],
   [m4_pushdef([gl_Positive], m4_bpatsubst([$1], [^-Wno-], [-W]))],
@@ -51,27 +53,50 @@ AS_VAR_POPDEF([gl_Warn])dnl
 # ------------------------------
 # Clang doesn't complain about unknown warning options unless one also
 # specifies -Wunknown-warning-option -Werror.  Detect this.
+#
+# The effects of this macro depend on the current language (_AC_LANG).
 AC_DEFUN([gl_UNKNOWN_WARNINGS_ARE_ERRORS],
+[_AC_LANG_DISPATCH([$0], _AC_LANG, $@)])
+
+# Specialization for _AC_LANG = C. This macro can be AC_REQUIREd.
+AC_DEFUN([gl_UNKNOWN_WARNINGS_ARE_ERRORS(C)],
+[
+  AC_LANG_PUSH([C])
+  gl_UNKNOWN_WARNINGS_ARE_ERRORS_IMPL
+  AC_LANG_POP([C])
+])
+
+# Specialization for _AC_LANG = C++. This macro can be AC_REQUIREd.
+AC_DEFUN([gl_UNKNOWN_WARNINGS_ARE_ERRORS(C++)],
+[
+  AC_LANG_PUSH([C++])
+  gl_UNKNOWN_WARNINGS_ARE_ERRORS_IMPL
+  AC_LANG_POP([C++])
+])
+
+AC_DEFUN([gl_UNKNOWN_WARNINGS_ARE_ERRORS_IMPL],
 [gl_COMPILER_OPTION_IF([-Werror -Wunknown-warning-option],
    [gl_unknown_warnings_are_errors='-Wunknown-warning-option -Werror'],
    [gl_unknown_warnings_are_errors=])])
 
-# gl_WARN_ADD(OPTION, [VARIABLE = WARN_CFLAGS],
+# gl_WARN_ADD(OPTION, [VARIABLE = WARN_CFLAGS/WARN_CXXFLAGS],
 #             [PROGRAM = AC_LANG_PROGRAM()])
-# ---------------------------------------------
-# Adds parameter to WARN_CFLAGS if the compiler supports it when
-# compiling PROGRAM.  For example, gl_WARN_ADD([-Wparentheses]).
+# -----------------------------------------------------------
+# Adds parameter to WARN_CFLAGS/WARN_CXXFLAGS if the compiler supports it
+# when compiling PROGRAM.  For example, gl_WARN_ADD([-Wparentheses]).
 #
 # If VARIABLE is a variable name, AC_SUBST it.
+#
+# The effects of this macro depend on the current language (_AC_LANG).
 AC_DEFUN([gl_WARN_ADD],
-[AC_REQUIRE([gl_UNKNOWN_WARNINGS_ARE_ERRORS])
+[AC_REQUIRE([gl_UNKNOWN_WARNINGS_ARE_ERRORS(]_AC_LANG[)])
 gl_COMPILER_OPTION_IF([$1],
-  [gl_AS_VAR_APPEND(m4_if([$2], [], [[WARN_CFLAGS]], [[$2]]), [" $1"])],
+  [gl_AS_VAR_APPEND(m4_if([$2], [], [[WARN_]_AC_LANG_PREFIX[FLAGS]], [[$2]]), 
[" $1"])],
   [],
   [$3])
 m4_ifval([$2],
          [AS_LITERAL_IF([$2], [AC_SUBST([$2])])],
-         [AC_SUBST([WARN_CFLAGS])])dnl
+         [AC_SUBST([WARN_]_AC_LANG_PREFIX[FLAGS])])dnl
 ])
 
 # Local Variables:

Attachment: hello-test.tar.gz
Description: application/compressed-tar

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