AC_CHECK_HEADER/AC_CHECK_HEADERS: complete transition to compile tests.

* lib/autoconf/headers.m4 (AC_CHECK_HEADER): Use _AC_CHECK_HEADER_COMPILE
  by default.  Continue to use _AC_CHECK_HEADER_PREPROC if fourth arg is '-'.
  (_AC_CHECK_HEADER_PREPROC): Issue a deprecation warning.
  (_AC_CHECK_HEADER_MONGREL, _AC_CHECK_HEADER_MONGREL_BODY): Remove.

* tests/c.at, tests/semantics.at: Update uses of AC_CHECK_HEADER(S).
* doc/autoconf.texi, NEWS: Document change.

5 files changed, 122 insertions, 165 deletions

diff --git a/NEWS b/NEWS
index 7003c447..5d6c6579 100644
--- a/NEWS
+++ b/NEWS
@@ -17,6 +17,16 @@ GNU Autoconf NEWS - User visible changes.
    echo.  Macros AS_ECHO and AS_ECHO_N now expand unconditionally to
    'printf "%s\n"' and 'printf %s'.
 
+** AC_CHECK_HEADER and AC_CHECK_HEADERS now default to doing only a
+   compilation test.  This completes the transition from preprocessor-
+   based header tests begun in Autoconf 2.56.
+
+   The double test that was the default since Autoconf 2.64 is no
+   longer available.  You can still request a preprocessor-only test
+   by specifying '-' as the fourth argument to either macro, but this
+   is now deprecated.  If you really need that behavior use
+   AC_PREPROC_IFELSE.
+
 ** Macros
 
 - New macro AC_C__GENERIC.
diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 49320671..0c489a31 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -6264,26 +6264,21 @@ commands @var{action-if-found}, otherwise execute
 header file is available, consider using @code{AC_CHECK_HEADERS}
 instead.
 
-@var{includes} is decoded to determine the appropriate include
-directives.  If omitted or empty, @file{configure} will check for both header
-existence (with the preprocessor) and usability (with the compiler),
-using @code{AC_INCLUDES_DEFAULT} for the compile test.  If
-there is a discrepancy between the results, a warning is issued to the
-user, and the compiler results are favored (@pxref{Present But
-Cannot Be Compiled}).  In general, favoring the compiler results means
-that a header will be treated as not found even though the file exists,
-because you did not provide enough prerequisites.
-
-Providing a non-empty @var{includes} argument allows the code to provide
-any prerequisites prior to including the header under test; it is common
-to use the argument @code{AC_INCLUDES_DEFAULT} (@pxref{Default
-Includes}).  With an explicit fourth argument, no preprocessor test is
-needed.  As a special case, an @var{includes} of exactly @samp{-}
-triggers the older preprocessor check, which merely determines existence
-of the file in the preprocessor search path; this should only be used as
-a last resort (it is safer to determine the actual prerequisites and
-perform a compiler check, or else use @code{AC_PREPROC_IFELSE} to make
-it obvious that only a preprocessor check is desired).
+@var{includes} should be the appropriate @dfn{prerequisite} code, i.e.@:
+whatever might be required to appear above
+@samp{#include <@var{header-file}>} for it to compile without error.
+This can be anything, but will normally be additional @samp{#include}
+directives. If @var{includes} is omitted or empty, @file{configure} will
+use the contents of the macro @code{AC_INCLUDES_DEFAULT}.
+@xref{Default Includes}.
+
+This macro used to check only for the @emph{presence} of a header, not
+whether its contents were acceptable to the compiler.  Some older
+@command{configure} scripts rely on this behavior, so it is still
+available by specifying @samp{-} as @var{includes}.  This mechanism is
+deprecated as of Autoconf 2.70; situations where a preprocessor-only
+check is required should use @code{AC_PREPROC_IFELSE}.
+@xref{Running the Preprocessor}.
 
 This macro caches its result in the @code{ac_cv_header_@var{header-file}}
 variable, with characters not suitable for a variable name mapped to
@@ -6314,47 +6309,49 @@ variable, with characters not suitable for a variable name mapped to
 underscores.
 @end defmac
 
-Previous versions of Autoconf merely checked whether the header was
-accepted by the preprocessor.  This was changed because the old test was
-inappropriate for typical uses.  Headers are typically used to compile,
-not merely to preprocess, and the old behavior sometimes accepted
-headers that clashed at compile-time (@pxref{Present But Cannot Be
-Compiled}).  If you need to check whether a header is preprocessable,
-you can use @code{AC_PREPROC_IFELSE} (@pxref{Running the Preprocessor}).
+@defmac AC_CHECK_HEADERS_ONCE (@var{header-file}@dots{})
+@acindex{CHECK_HEADERS_ONCE}
+@cvindex HAVE_@var{header}
+For each given system header file @var{header-file} in the
+blank-separated argument list that exists, define
+@code{HAVE_@var{header-file}} (in all capitals).
 
-Actually requiring a header to compile improves the robustness of the
-test, but it also requires
-that you make sure that headers that must be included before the
-@var{header-file} be part of the @var{includes}, (@pxref{Default
-Includes}).  If looking for @file{bar.h}, which requires that
-@file{foo.h} be included before if it exists, we suggest the following
-scheme:
+If you do not need the full power of @code{AC_CHECK_HEADERS}, this
+variant generates smaller, faster @command{configure} files.  All
+headers passed to @code{AC_CHECK_HEADERS_ONCE} are checked for in one
+pass, early during the @command{configure} run.  The checks cannot be
+conditionalized, you cannot specify an @var{action-if-found} or
+@var{action-if-not-found}, and @code{AC_INCLUDES_DEFAULT} is always used
+for the prerequisites.
+@end defmac
+
+In previous versions of Autoconf, these macros merely checked whether
+the header was accepted by the preprocessor.  This was changed because
+the old test was inappropriate for typical uses.  Headers are typically
+used to compile, not merely to preprocess, and the old behavior
+sometimes accepted headers that clashed at compile-time
+(@pxref{Present But Cannot Be Compiled}).  If for some reason it is
+inappropriate to check whether a header is compilable, you should use
+@code{AC_PREPROC_IFELSE} (@pxref{Running the Preprocessor}) instead of
+these macros.
+
+Requiring each header to compile improves the robustness of the test,
+but it also requires you to make sure that the @var{includes} are
+correct.  Most system headers nowadays make sure to @code{#include}
+whatever they require, or else have their dependencies satisfied by
+@code{AC_INCLUDES_DEFAULT} (@pxref{Default Includes}), but
+@pxref{Header Portability}, for known exceptions.  In general, if you
+are looking for @file{bar.h}, which requires that @file{foo.h} be
+included first if it exists, you should do something like this:
 
-@verbatim
+@example
 AC_CHECK_HEADERS([foo.h])
 AC_CHECK_HEADERS([bar.h], [], [],
 [#ifdef HAVE_FOO_H
 # include <foo.h>
 #endif
 ])
-@end verbatim
-
-The following variant generates smaller, faster @command{configure}
-files if you do not need the full power of @code{AC_CHECK_HEADERS}.
-
-@defmac AC_CHECK_HEADERS_ONCE (@var{header-file}@dots{})
-@acindex{CHECK_HEADERS_ONCE}
-@cvindex HAVE_@var{header}
-For each given system header file @var{header-file} in the
-blank-separated argument list that exists, define
-@code{HAVE_@var{header-file}} (in all capitals).
-This is a once-only variant of @code{AC_CHECK_HEADERS}.  It generates the
-checking code at most once, so that @command{configure} is smaller and
-faster; but the checks cannot be conditionalized and are always done once,
-early during the @command{configure} run.  Thus, this macro is only safe
-for checking headers that do not have prerequisites beyond what
-@code{AC_INCLUDES_DEFAULT} provides.
-@end defmac
+@end example
 
 @node Declarations
 @section Declarations
@@ -25994,7 +25991,8 @@ elsewhere.
 The transition began with Autoconf 2.56.  As of Autoconf 2.64 both
 checks are performed, and @command{configure} complains loudly if the
 compiler and the preprocessor do not agree.  However, only the compiler
-result is considered.
+result is considered.  As of Autoconf 2.70, only the compiler check is
+performed.
 
 Consider the following example:
 
@@ -26044,7 +26042,7 @@ checking for pi.h... no
 @end smallexample
 
 @noindent
-The proper way the handle this case is using the fourth argument
+The proper way to handle this case is using the fourth argument
 (@pxref{Generic Headers}):
 
 @example
diff --git a/lib/autoconf/headers.m4 b/lib/autoconf/headers.m4
index 7f5dbce2..6afff7d7 100644
--- a/lib/autoconf/headers.m4
+++ b/lib/autoconf/headers.m4
@@ -46,99 +46,17 @@
 #		  [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND],
 #		  [INCLUDES])
 # ---------------------------------------------------------
-# We are slowly moving to checking headers with the compiler instead
-# of the preproc, so that we actually learn about the usability of a
-# header instead of its mere presence.  But since users are used to
-# the old semantics, they check for headers in random order and
-# without providing prerequisite headers.  This macro implements the
-# transition phase, and should be cleaned up latter to use compilation
-# only.
-#
-# If INCLUDES is empty, then check both via the compiler and preproc.
-# If the results are different, issue a warning, but keep the preproc
-# result.
-#
-# If INCLUDES is `-', keep only the old semantics.
-#
-# If INCLUDES is specified and different from `-', then use the new
-# semantics only.
+# This used to check for headers using the preprocessor only, but we
+# have now switched to running a full compilation, so that we learn
+# about the usability of a header instead of its mere presence.
+# The old behavior is still available by specifying `-' as the
+# INCLUDES, but this triggers a deprecation warning.
 #
 # The m4_indir allows for fewer expansions of $@.
 AC_DEFUN([AC_CHECK_HEADER],
-[m4_indir(m4_case([$4],
-		  [],  [[_AC_CHECK_HEADER_MONGREL]],
-		  [-], [[_AC_CHECK_HEADER_PREPROC]],
-		       [[_AC_CHECK_HEADER_COMPILE]]), $@)
-])# AC_CHECK_HEADER
-
-
-# _AC_CHECK_HEADER_MONGREL_BODY
-# -----------------------------
-# Shell function body for _AC_CHECK_HEADER_MONGREL
-m4_define([_AC_CHECK_HEADER_MONGREL_BODY],
-[  AS_LINENO_PUSH([$[]1])
-  AS_VAR_SET_IF([$[]3],
-		[AC_CACHE_CHECK([for $[]2], [$[]3], [])],
-		[# Is the header compilable?
-AC_MSG_CHECKING([$[]2 usability])
-AC_COMPILE_IFELSE([AC_LANG_SOURCE([$[]4
-@%:@include <$[]2>])],
-		  [ac_header_compiler=yes],
-		  [ac_header_compiler=no])
-AC_MSG_RESULT([$ac_header_compiler])
-
-# Is the header present?
-AC_MSG_CHECKING([$[]2 presence])
-AC_PREPROC_IFELSE([AC_LANG_SOURCE([@%:@include <$[]2>])],
-		  [ac_header_preproc=yes],
-		  [ac_header_preproc=no])
-AC_MSG_RESULT([$ac_header_preproc])
-
-# So?  What about this header?
-case $ac_header_compiler:$ac_header_preproc:$ac_[]_AC_LANG_ABBREV[]_preproc_warn_flag in #((
-  yes:no: )
-    AC_MSG_WARN([$[]2: accepted by the compiler, rejected by the preprocessor!])
-    AC_MSG_WARN([$[]2: proceeding with the compiler's result])
-    ;;
-  no:yes:* )
-    AC_MSG_WARN([$[]2: present but cannot be compiled])
-    AC_MSG_WARN([$[]2:     check for missing prerequisite headers?])
-    AC_MSG_WARN([$[]2: see the Autoconf documentation])
-    AC_MSG_WARN([$[]2:     section "Present But Cannot Be Compiled"])
-    AC_MSG_WARN([$[]2: proceeding with the compiler's result])
-m4_ifset([AC_PACKAGE_BUGREPORT],
-[m4_n([( AS_BOX([Report this to ]AC_PACKAGE_BUGREPORT)
-     ) | sed "s/^/$as_me: WARNING:     /" >&2])])dnl
-    ;;
-esac
-  AC_CACHE_CHECK([for $[]2], [$[]3],
-		 [AS_VAR_SET([$[]3], [$ac_header_compiler])])])
-  AS_LINENO_POP
-])#_AC_CHECK_HEADER_MONGREL_BODY
-
-# _AC_CHECK_HEADER_MONGREL(HEADER-FILE,
-#			   [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND],
-#			   [INCLUDES = DEFAULT-INCLUDES])
-# ------------------------------------------------------------------
-# Check using both the compiler and the preprocessor.  If they disagree,
-# warn, and the preproc wins.
-#
-# This is not based on _AC_CHECK_HEADER_COMPILE and _AC_CHECK_HEADER_PREPROC
-# because it obfuscate the code to try to factor everything, in particular
-# because of the cache variables, and the `checking ...' messages.
-AC_DEFUN([_AC_CHECK_HEADER_MONGREL],
-[AC_REQUIRE_SHELL_FN([ac_fn_]_AC_LANG_ABBREV[_check_header_mongrel],
-  [AS_FUNCTION_DESCRIBE([ac_fn_]_AC_LANG_ABBREV[_check_header_mongrel],
-    [LINENO HEADER VAR INCLUDES],
-    [Tests whether HEADER exists, giving a warning if it cannot be compiled
-     using the include files in INCLUDES and setting the cache variable VAR
-     accordingly.])],
-  [$0_BODY])]dnl
-[AS_VAR_PUSHDEF([ac_Header], [ac_cv_header_$1])]dnl
-[ac_fn_[]_AC_LANG_ABBREV[]_check_header_mongrel ]dnl
-["$LINENO" "$1" "ac_Header" "AS_ESCAPE([AC_INCLUDES_DEFAULT([$4])], [""])"
-AS_VAR_IF([ac_Header], [yes], [$2], [$3])
-AS_VAR_POPDEF([ac_Header])])# _AC_CHECK_HEADER_MONGREL
+[m4_indir(m4_if([$4], [-],
+                [[_AC_CHECK_HEADER_PREPROC]],
+                [[_AC_CHECK_HEADER_COMPILE]]), $@)])
 
 
 # _AC_CHECK_HEADER_COMPILE_BODY
@@ -154,6 +72,7 @@ m4_define([_AC_CHECK_HEADER_COMPILE_BODY],
   AS_LINENO_POP
 ])# _AC_CHECK_HEADER_COMPILE_BODY
 
+
 # _AC_CHECK_HEADER_COMPILE(HEADER-FILE,
 #		       [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND],
 #		       [INCLUDES = DEFAULT-INCLUDES])
@@ -172,6 +91,7 @@ AC_DEFUN([_AC_CHECK_HEADER_COMPILE],
 AS_VAR_IF([ac_Header], [yes], [$2], [$3])
 AS_VAR_POPDEF([ac_Header])])# _AC_CHECK_HEADER_COMPILE
 
+
 # _AC_CHECK_HEADER_PREPROC_BODY
 # -----------------------------
 # Shell function body for _AC_CHECK_HEADER_PREPROC.
@@ -185,22 +105,28 @@ m4_define([_AC_CHECK_HEADER_PREPROC_BODY],
 ])# _AC_CHECK_HEADER_PREPROC_BODY
 
 
-
 # _AC_CHECK_HEADER_PREPROC(HEADER-FILE,
 #		       [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])
 # --------------------------------------------------------------
 # Check the preprocessor accepts HEADER-FILE.
 AC_DEFUN([_AC_CHECK_HEADER_PREPROC],
+[AC_DIAGNOSE([obsolete], [Checking for headers with the preprocessor is
+deprecated. Specify prerequisite code to AC_CHECK_HEADER
+instead of using fourth argument `-'. (Many headers need
+no prerequisites. If you truly need to test whether
+something passes the preprocessor but not the compiler,
+use AC_PREPROC_IFELSE.)])]dnl
 [AC_REQUIRE_SHELL_FN([ac_fn_]_AC_LANG_ABBREV[_check_header_preproc],
   [AS_FUNCTION_DESCRIBE([ac_fn_]_AC_LANG_ABBREV[_check_header_preproc],
     [LINENO HEADER VAR],
-    [Tests whether HEADER is present, setting the cache variable VAR accordingly.])],
+    [Tests whether HEADER exists and can be preprocessed (in isolation),
+     setting the cache variable VAR accordingly.])],
   [$0_BODY])]dnl
 [AS_VAR_PUSHDEF([ac_Header], [ac_cv_header_$1])]dnl
 [ac_fn_[]_AC_LANG_ABBREV[]_check_header_preproc "$LINENO" "$1" "ac_Header"
 AS_VAR_IF([ac_Header], [yes], [$2], [$3])
-AS_VAR_POPDEF([ac_Header])dnl
-])# _AC_CHECK_HEADER_PREPROC
+AS_VAR_POPDEF([ac_Header])])# _AC_CHECK_HEADER_PREPROC
+
 
 # _AC_CHECK_HEADER_OLD(HEADER-FILE, [ACTION-IF-FOUND],
 #                      [ACTION-IF-NOT-FOUND])
@@ -222,6 +148,14 @@ AC_DEFUN([_AC_CHECK_HEADER_NEW],
 You should use AC_CHECK_HEADER with a fourth argument.])]dnl
 [_AC_CHECK_HEADER_COMPILE($@)])
 
+# _AC_CHECK_HEADER_MONGREL(HEADER-FILE,
+#			   [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])
+# ------------------------------------------------------------------
+# In case anyone used this undocumented macro.  Map to the _PREPROC
+# semantics to minimize the chance of breaking anything.
+AU_DEFUN([_AC_CHECK_HEADER_MONGREL],
+  [AC_CHECK_HEADER([$1], [$2], [$3], [-])])
+
 
 # _AH_CHECK_HEADER(HEADER-FILE)
 # -----------------------------
diff --git a/tests/c.at b/tests/c.at
index 2e8f1495..b1d46449 100644
--- a/tests/c.at
+++ b/tests/c.at
@@ -123,8 +123,12 @@ test "x$ac_c_preproc_warn_flag" = xyes &&
 CPP="./mycpp $CPP"
 
 # Exercise CPP.
-AC_CHECK_HEADERS(stdio.h autoconf_io.h, [], [], [-])]])
 
+AC_PREPROC_IFELSE([AC_LANG_SOURCE([[#include <stdio.h>]])],
+  [AC_DEFINE([HAVE_STDIO_H], [1], [Define if you have stdio.h])])
+AC_PREPROC_IFELSE([AC_LANG_SOURCE([[#include <autoconf_io.h>]])],
+  [AC_DEFINE([HAVE_AUTOCONF_IO_H], [1], [Define if you have autoconf_io.h])])
+]])
 AT_CHECK_DEFINES(
 [/* #undef HAVE_AUTOCONF_IO_H */
 #define HAVE_STDIO_H 1
@@ -139,14 +143,14 @@ AT_CLEANUP
 
 AT_SETUP([AC_PROG_CPP without warnings])
 
-# Ignore if /lib/cpp doesn't work
-AT_CHECK([[echo '#include <stdio.h>' | /lib/cpp || exit 77]],
+# Ignore if the cpp in $PATH doesn't work
+AT_CHECK([[echo '#include <stdio.h>' | cpp || exit 77]],
   [], [ignore], [ignore])
 
 # A cpp which exit status is meaningless.
 AT_DATA([mycpp],
 [[#! /bin/sh
-/lib/cpp "$@"
+cpp "$@"
 exit 0
 ]])
 
@@ -159,7 +163,11 @@ test "x$ac_c_preproc_warn_flag" != xyes &&
   AC_MSG_ERROR([failed to detect preprocessor warning option])
 
 # Exercise CPP.
-AC_CHECK_HEADERS(stdio.h autoconf_io.h, [], [], [-])]])
+AC_PREPROC_IFELSE([AC_LANG_SOURCE([[#include <stdio.h>]])],
+  [AC_DEFINE([HAVE_STDIO_H], [1], [Define if you have stdio.h])])
+AC_PREPROC_IFELSE([AC_LANG_SOURCE([[#include <autoconf_io.h>]])],
+  [AC_DEFINE([HAVE_AUTOCONF_IO_H], [1], [Define if you have autoconf_io.h])])
+]])
 
 AT_CHECK_DEFINES(
 [/* #undef HAVE_AUTOCONF_IO_H */
@@ -179,8 +187,8 @@ AT_CLEANUP
 
 AT_SETUP([AC_PROG_CPP via CC])
 
-# Ignore if /lib/cpp doesn't work
-AT_CHECK([[echo '#include <stdio.h>' | /lib/cpp || exit 77]],
+# Ignore if the cpp in $PATH doesn't work
+AT_CHECK([[echo '#include <stdio.h>' | cpp || exit 77]],
   [], [ignore], [ignore])
 
 AT_DATA([mycc],
@@ -204,7 +212,11 @@ test "$CPP" != "$CC -E" &&
   AC_MSG_ERROR([error messages on stderr cause the preprocessor selection to fail])
 
 # Exercise CPP.
-AC_CHECK_HEADERS(stdio.h autoconf_io.h, [], [], [-])]])
+AC_PREPROC_IFELSE([AC_LANG_SOURCE([[#include <stdio.h>]])],
+  [AC_DEFINE([HAVE_STDIO_H], [1], [Define if you have stdio.h])])
+AC_PREPROC_IFELSE([AC_LANG_SOURCE([[#include <autoconf_io.h>]])],
+  [AC_DEFINE([HAVE_AUTOCONF_IO_H], [1], [Define if you have autoconf_io.h])])
+]])
 
 AT_CHECK_DEFINES(
 [/* #undef HAVE_AUTOCONF_IO_H */
diff --git a/tests/semantics.at b/tests/semantics.at
index c0b78a13..f2b9ea5a 100644
--- a/tests/semantics.at
+++ b/tests/semantics.at
@@ -219,13 +219,7 @@ AT_DATA([autoconf_io.h],
 AT_CONFIGURE_AC([AC_CHECK_HEADERS(stdio.h autoconf_io.h)])
 AT_CHECK_AUTOCONF([-W obsolete])
 AT_CHECK_AUTOHEADER
-AT_CHECK_CONFIGURE([CPPFLAGS=-I.], [0], [ignore],
-[configure: WARNING: autoconf_io.h: present but cannot be compiled
-configure: WARNING: autoconf_io.h:     check for missing prerequisite headers?
-configure: WARNING: autoconf_io.h: see the Autoconf documentation
-configure: WARNING: autoconf_io.h:     section "Present But Cannot Be Compiled"
-configure: WARNING: autoconf_io.h: proceeding with the compiler's result
-])
+AT_CHECK_CONFIGURE([CPPFLAGS=-I.])
 AT_CHECK_ENV
 AT_CHECK_DEFINES(
 [/* #undef HAVE_AUTOCONF_IO_H */
@@ -251,7 +245,16 @@ AT_DATA([header2.h],
 
 AT_CONFIGURE_AC([AC_CHECK_HEADERS(header2.h, [], [], -)])
 
-AT_CHECK_AUTOCONF([-W obsolete])
+AT_CHECK_AUTOCONF([-W obsolete], [], [], [stderr])
+AT_CHECK([[grep '^configure\.ac' stderr]], [0],
+[configure.ac:4: warning: Checking for headers with the preprocessor is
+configure.ac:4: deprecated. Specify prerequisite code to AC_CHECK_HEADER
+configure.ac:4: instead of using fourth argument `-'. (Many headers need
+configure.ac:4: no prerequisites. If you truly need to test whether
+configure.ac:4: something passes the preprocessor but not the compiler,
+configure.ac:4: use AC_PREPROC_IFELSE.)
+configure.ac:4: the top level
+])
 AT_CHECK_AUTOHEADER
 AT_CHECK_CONFIGURE([CPPFLAGS=-I.])
 AT_CHECK_ENV