--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2009-12-22  Bruno Haible  <bruno@clisp.org>
+
+	wchar: Diagnose broken combination of glibc and gcc versions and flags.
+	* m4/wchar.m4 (gl_WCHAR_H_INLINE_OK): New macro.
+	(gl_WCHAR_H): Invoke it.
+	* m4/btowc.m4 (gl_FUNC_BTOWC): Require it.
+	* doc/posix-headers/wchar.texi: Mention the interoperability problem.
+	Reported by Karl Berry <karl@freefriends.org>.
+
 2009-12-22  Eric Blake  <ebb9@byu.net>
 
 	math, unistd: avoid redundant includes

--- a/doc/posix-headers/wchar.texi
+++ b/doc/posix-headers/wchar.texi
@@ -25,4 +25,9 @@
 
 Portability problems not fixed by Gnulib:
 @itemize
+@item
+This header file leads to link errors and endless recursions or endless loops
+on some platforms:
+glibc version 2.5 or older, together with gcc version 4.3 or newer and the
+option @samp{-std=c99} or @samp{-std=gnu99}.
 @end itemize

--- a/m4/btowc.m4
+++ b/m4/btowc.m4
@@ -1,4 +1,4 @@
-# btowc.m4 serial 4
+# btowc.m4 serial 5
 dnl Copyright (C) 2008-2009 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -8,6 +8,11 @@
 [
   AC_REQUIRE([gl_WCHAR_H_DEFAULTS])
 
+  dnl Check whether <wchar.h> is usable at all, first. Otherwise the test
+  dnl program below may lead to an endless loop. See
+  dnl <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42440>.
+  AC_REQUIRE([gl_WCHAR_H_INLINE_OK])
+
   AC_CHECK_FUNCS_ONCE([btowc])
   if test $ac_cv_func_btowc = no; then
     HAVE_BTOWC=0

--- a/m4/wchar.m4
+++ b/m4/wchar.m4
@@ -7,11 +7,12 @@
 
 dnl Written by Eric Blake.
 
-# wchar.m4 serial 26
+# wchar.m4 serial 27
 
 AC_DEFUN([gl_WCHAR_H],
 [
   AC_REQUIRE([gl_WCHAR_H_DEFAULTS])
+  AC_REQUIRE([gl_WCHAR_H_INLINE_OK])
   AC_CACHE_CHECK([whether <wchar.h> is standalone],
     [gl_cv_header_wchar_h_standalone],
     [AC_COMPILE_IFELSE([[#include <wchar.h>
@@ -50,6 +51,54 @@
   gl_CHECK_NEXT_HEADERS([wchar.h])
 ])
 
+dnl Check whether <wchar.h> is usable at all.
+AC_DEFUN([gl_WCHAR_H_INLINE_OK],
+[
+  dnl Test whether <wchar.h> suffers due to the transition from '__inline' to
+  dnl 'gnu_inline'. See <http://sourceware.org/bugzilla/show_bug.cgi?id=4022>
+  dnl and <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42440>. In summary,
+  dnl glibc version 2.5 or older, together with gcc version 4.3 or newer and
+  dnl the option -std=c99 or -std=gnu99, leads to a broken <wchar.h>.
+  AC_CACHE_CHECK([whether <wchar.h> uses 'inline' correctly],
+    [gl_cv_header_wchar_h_correct_inline],
+    [gl_cv_header_wchar_h_correct_inline=yes
+     AC_LANG_CONFTEST([
+       AC_LANG_SOURCE([[#define wcstod renamed_wcstod
+#include <wchar.h>
+extern int zero (void);
+int main () { return zero(); }
+]])])
+     if AC_TRY_EVAL([ac_compile]); then
+       mv conftest.$ac_objext conftest1.$ac_objext
+       AC_LANG_CONFTEST([
+         AC_LANG_SOURCE([[#define wcstod renamed_wcstod
+#include <wchar.h>
+int zero (void) { return 0; }
+]])])
+       if AC_TRY_EVAL([ac_compile]); then
+         mv conftest.$ac_objext conftest2.$ac_objext
+         if $CC -o conftest$ac_exeext $CFLAGS $LDFLAGS conftest1.$ac_objext conftest2.$ac_objext $LIBS >&AS_MESSAGE_LOG_FD 2>&1; then
+           :
+         else
+           gl_cv_header_wchar_h_correct_inline=no
+         fi
+       fi
+     fi
+     rm -f conftest1.$ac_objext conftest2.$ac_objext conftest$ac_exeext
+    ])
+  if test $gl_cv_header_wchar_h_correct_inline = no; then
+    AC_MSG_ERROR([<wchar.h> cannot be used with this compiler ($CC $CFLAGS $CPPFLAGS).
+This is a known interoperability problem of glibc <= 2.5 with gcc >= 4.3 in
+C99 mode. You have four options:
+  - Add the flag -fgnu89-inline to CC and reconfigure, or
+  - Fix your include files, using parts of
+    <http://sourceware.org/git/?p=glibc.git;a=commitdiff;h=b037a293a48718af30d706c2e18c929d0e69a621>, or
+  - Use a gcc version older than 4.3, or
+  - Don't use the flags -std=c99 or -std=gnu99.
+Configuration aborted.])
+  fi
+])
+
 dnl Unconditionally enables the replacement of <wchar.h>.
 AC_DEFUN([gl_REPLACE_WCHAR_H],
 [