Louis Dionne a9c9183ca4 [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
2021-06-04 09:55:21 -04:00

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C++

// -*- C++ -*-
//===---------------------------- cctype ----------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#ifndef _LIBCPP_CCTYPE
#define _LIBCPP_CCTYPE
/*
cctype synopsis
namespace std
{
int isalnum(int c);
int isalpha(int c);
int isblank(int c); // C99
int iscntrl(int c);
int isdigit(int c);
int isgraph(int c);
int islower(int c);
int isprint(int c);
int ispunct(int c);
int isspace(int c);
int isupper(int c);
int isxdigit(int c);
int tolower(int c);
int toupper(int c);
} // std
*/
#include <__config>
#include <ctype.h>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif
_LIBCPP_BEGIN_NAMESPACE_STD
#ifdef isalnum
#undef isalnum
#endif
#ifdef isalpha
#undef isalpha
#endif
#ifdef isblank
#undef isblank
#endif
#ifdef iscntrl
#undef iscntrl
#endif
#ifdef isdigit
#undef isdigit
#endif
#ifdef isgraph
#undef isgraph
#endif
#ifdef islower
#undef islower
#endif
#ifdef isprint
#undef isprint
#endif
#ifdef ispunct
#undef ispunct
#endif
#ifdef isspace
#undef isspace
#endif
#ifdef isupper
#undef isupper
#endif
#ifdef isxdigit
#undef isxdigit
#endif
#ifdef tolower
#undef tolower
#endif
#ifdef toupper
#undef toupper
#endif
using ::isalnum _LIBCPP_USING_IF_EXISTS;
using ::isalpha _LIBCPP_USING_IF_EXISTS;
using ::isblank _LIBCPP_USING_IF_EXISTS;
using ::iscntrl _LIBCPP_USING_IF_EXISTS;
using ::isdigit _LIBCPP_USING_IF_EXISTS;
using ::isgraph _LIBCPP_USING_IF_EXISTS;
using ::islower _LIBCPP_USING_IF_EXISTS;
using ::isprint _LIBCPP_USING_IF_EXISTS;
using ::ispunct _LIBCPP_USING_IF_EXISTS;
using ::isspace _LIBCPP_USING_IF_EXISTS;
using ::isupper _LIBCPP_USING_IF_EXISTS;
using ::isxdigit _LIBCPP_USING_IF_EXISTS;
using ::tolower _LIBCPP_USING_IF_EXISTS;
using ::toupper _LIBCPP_USING_IF_EXISTS;
_LIBCPP_END_NAMESPACE_STD
#endif // _LIBCPP_CCTYPE