[libc++] Use the using_if_exists attribute when provided
As discussed on cfe-dev [1], use the using_if_exists Clang attribute when
the compiler supports it. This makes it easier to port libc++ on top of
new platforms that don't fully support the C Standard library.
Previously, libc++ would fail to build when trying to import a missing
declaration in a <cXXXX> header. With the attribute, the declaration will
simply not be imported into namespace std, and hence it won't be available
for libc++ to use. In many cases, the declarations were *not* actually
required for libc++ to work (they were only surfaced for users to use
them as std::XXXX), so not importing them into namespace std is acceptable.
The same thing could be achieved by conscious usage of `#ifdef` along
with platform detection, however that quickly creates a maintenance
problem as libc++ is ported to new platforms. Furthermore, this problem
is exacerbated when mixed with vendor internal-only platforms, which can
lead to difficulties maintaining a downstream fork of the library.
For the time being, we only use the using_if_exists attribute when it
is supported. At some point in the future, we will start removing #ifdef
paths that are unnecessary when the attribute is supported, and folks
who need those #ifdef paths will be required to use a compiler that
supports the attribute.
[1]: http://lists.llvm.org/pipermail/cfe-dev/2020-June/066038.html
Differential Revision: https://reviews.llvm.org/D90257
NOKEYCHECK=True
GitOrigin-RevId: a9c9183ca42629fa83cdda297d1d30c7bc1d7c91
diff --git a/include/cstring b/include/cstring
index 6781415..34449ab 100644
--- a/include/cstring
+++ b/include/cstring
@@ -65,31 +65,31 @@
_LIBCPP_BEGIN_NAMESPACE_STD
-using ::size_t;
-using ::memcpy;
-using ::memmove;
-using ::strcpy;
-using ::strncpy;
-using ::strcat;
-using ::strncat;
-using ::memcmp;
-using ::strcmp;
-using ::strncmp;
-using ::strcoll;
-using ::strxfrm;
-using ::memchr;
-using ::strchr;
-using ::strcspn;
-using ::strpbrk;
-using ::strrchr;
-using ::strspn;
-using ::strstr;
+using ::size_t _LIBCPP_USING_IF_EXISTS;
+using ::memcpy _LIBCPP_USING_IF_EXISTS;
+using ::memmove _LIBCPP_USING_IF_EXISTS;
+using ::strcpy _LIBCPP_USING_IF_EXISTS;
+using ::strncpy _LIBCPP_USING_IF_EXISTS;
+using ::strcat _LIBCPP_USING_IF_EXISTS;
+using ::strncat _LIBCPP_USING_IF_EXISTS;
+using ::memcmp _LIBCPP_USING_IF_EXISTS;
+using ::strcmp _LIBCPP_USING_IF_EXISTS;
+using ::strncmp _LIBCPP_USING_IF_EXISTS;
+using ::strcoll _LIBCPP_USING_IF_EXISTS;
+using ::strxfrm _LIBCPP_USING_IF_EXISTS;
+using ::memchr _LIBCPP_USING_IF_EXISTS;
+using ::strchr _LIBCPP_USING_IF_EXISTS;
+using ::strcspn _LIBCPP_USING_IF_EXISTS;
+using ::strpbrk _LIBCPP_USING_IF_EXISTS;
+using ::strrchr _LIBCPP_USING_IF_EXISTS;
+using ::strspn _LIBCPP_USING_IF_EXISTS;
+using ::strstr _LIBCPP_USING_IF_EXISTS;
#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
-using ::strtok;
+using ::strtok _LIBCPP_USING_IF_EXISTS;
#endif
-using ::memset;
-using ::strerror;
-using ::strlen;
+using ::memset _LIBCPP_USING_IF_EXISTS;
+using ::strerror _LIBCPP_USING_IF_EXISTS;
+using ::strlen _LIBCPP_USING_IF_EXISTS;
_LIBCPP_END_NAMESPACE_STD