Fix use of libc++ <foo.h> headers from within an 'extern "C"' context in C++98.

Previously, this resulted in us declaring a template for static_assert emulation within the 'extern "C"' context, which is ill-formed.

llvm-svn: 250247

libcxx/include/__config

@@ -591,9 +591,11 @@ typedef unsigned int   char32_t;
 
 #ifdef _LIBCPP_HAS_NO_STATIC_ASSERT
 
+extern "C++" {
 template <bool> struct __static_assert_test;
 template <> struct __static_assert_test<true> {};
 template <unsigned> struct __static_assert_check {};
+}
 #define static_assert(__b, __m) \
     typedef __static_assert_check<sizeof(__static_assert_test<(__b)>)> \
     _LIBCPP_CONCAT(__t, __LINE__)

libcxx/test/std/depr/depr.c.headers/extern_c.pass.cpp

@@ -0,0 +1,43 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// Sometimes C++'s <foo.h> headers get included within extern "C" contexts. This
+// is ill-formed (no diagnostic required), per [using.headers]p3, but we permit
+// it as an extension.
+
+extern "C" {
+#include <assert.h>
+// complex.h is not supported in extern "C".
+#include <ctype.h>
+#include <errno.h>
+#include <fenv.h>
+#include <float.h>
+#include <inttypes.h>
+#include <iso646.h>
+#include <limits.h>
+#include <locale.h>
+#include <math.h>
+#include <setjmp.h>
+#include <signal.h>
+#include <stdalign.h>
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+// tgmath.h is not supported in extern "C".
+#include <time.h>
+// FIXME: #include <uchar.h>
+#include <wchar.h>
+#include <wctype.h>
+}
+
+int main() {}