[PATCH] AC_C__GENERIC: New macro.

[Paul Eggert]

I needed this for an Emacs feature and it seemed like something
Autoconf should have and which won't break any existing usage, so
I took the liberty of pushing it.

* NEWS, doc/autoconf.texi (C Compiler): Document it.
* lib/autoconf/c.m4 (AC_C__GENERIC): Implement it.
---
 NEWS              |  2 ++
 doc/autoconf.texi |  7 +++++++
 lib/autoconf/c.m4 | 27 +++++++++++++++++++++++++++
 3 files changed, 36 insertions(+)

diff --git a/NEWS b/NEWS
index 185ca26..a4cd3a1 100644
--- a/NEWS
+++ b/NEWS
@@ -10,6 +10,8 @@ GNU Autoconf NEWS - User visible changes.
 
 ** Macros
 
+- New macro AC_C__GENERIC.
+
 - AC_CONFIG_MACRO_DIRS
   New macro, used to declare multiple directories when looking for
   local M4 macros.  This macro overcomes some of the shortfalls in the
diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index aa948a9..bcf8f86 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -7434,6 +7434,13 @@ This macro is obsolescent, as current C compilers 
support @code{const}.
 New programs need not use this macro.
 @end defmac
 
address@hidden AC_C__GENERIC
address@hidden
address@hidden _Generic
+If the C compiler supports C11-style generic selection using the
address@hidden keyword, define @code{HAVE_C__GENERIC}.
address@hidden defmac
+
 @defmac AC_C_RESTRICT
 @acindex{C_RESTRICT}
 @cvindex restrict
diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
index e17df1e..edc663b 100644
--- a/lib/autoconf/c.m4
+++ b/lib/autoconf/c.m4
@@ -1690,6 +1690,33 @@ dnl default behavior for universal builds.
 ])# AC_C_BIGENDIAN
 
 
+# AC_C__GENERIC
+# -------------
+# Define HAVE_C__GENERIC if _Generic works, a la C11.
+AN_IDENTIFIER([_Generic], [AC_C__GENERIC])
+AC_DEFUN([AC_C__GENERIC],
+[AC_CACHE_CHECK([for _Generic], ac_cv_c__Generic,
+[AC_COMPILE_IFELSE(
+   [AC_LANG_SOURCE(
+      [[int
+        main (int argc, char **argv)
+        {
+          int a = _Generic (argc, int: argc = 1);
+          int *b = &_Generic (argc, default: argc);
+          char ***c = _Generic (argv, int: argc, default: argv ? &argv : 0);
+          _Generic (1 ? 0 : b, int: a, default: b) = &argc;
+          _Generic (a = 1, default: a) = 3;
+          return a + !b + !c;
+        }
+      ]])],
+   [ac_cv_c__Generic=yes],
+   [ac_cv_c__Generic=no])])
+if test $ac_cv_c__Generic = yes; then
+  AC_DEFINE([HAVE_C__GENERIC], 1,
+           [Define to 1 if C11-style _Generic works.])
+fi
+])# AC_C__GENERIC
+
 # AC_C_INLINE
 # -----------
 # Do nothing if the compiler accepts the inline keyword.
--
1.7.11.7