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++ -*-
//===--------------------------- cwctype ----------------------------------===//
//
// 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_CWCTYPE
#define _LIBCPP_CWCTYPE
/*
cwctype synopsis
Macros:
WEOF
namespace std
{
Types:
wint_t
wctrans_t
wctype_t
int iswalnum(wint_t wc);
int iswalpha(wint_t wc);
int iswblank(wint_t wc); // C99
int iswcntrl(wint_t wc);
int iswdigit(wint_t wc);
int iswgraph(wint_t wc);
int iswlower(wint_t wc);
int iswprint(wint_t wc);
int iswpunct(wint_t wc);
int iswspace(wint_t wc);
int iswupper(wint_t wc);
int iswxdigit(wint_t wc);
int iswctype(wint_t wc, wctype_t desc);
wctype_t wctype(const char* property);
wint_t towlower(wint_t wc);
wint_t towupper(wint_t wc);
wint_t towctrans(wint_t wc, wctrans_t desc);
wctrans_t wctrans(const char* property);
} // std
*/
#include <__config>
#include <cctype>
#include <wctype.h>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif
_LIBCPP_BEGIN_NAMESPACE_STD
using ::wint_t _LIBCPP_USING_IF_EXISTS;
using ::wctrans_t _LIBCPP_USING_IF_EXISTS;
using ::wctype_t _LIBCPP_USING_IF_EXISTS;
using ::iswalnum _LIBCPP_USING_IF_EXISTS;
using ::iswalpha _LIBCPP_USING_IF_EXISTS;
using ::iswblank _LIBCPP_USING_IF_EXISTS;
using ::iswcntrl _LIBCPP_USING_IF_EXISTS;
using ::iswdigit _LIBCPP_USING_IF_EXISTS;
using ::iswgraph _LIBCPP_USING_IF_EXISTS;
using ::iswlower _LIBCPP_USING_IF_EXISTS;
using ::iswprint _LIBCPP_USING_IF_EXISTS;
using ::iswpunct _LIBCPP_USING_IF_EXISTS;
using ::iswspace _LIBCPP_USING_IF_EXISTS;
using ::iswupper _LIBCPP_USING_IF_EXISTS;
using ::iswxdigit _LIBCPP_USING_IF_EXISTS;
using ::iswctype _LIBCPP_USING_IF_EXISTS;
using ::wctype _LIBCPP_USING_IF_EXISTS;
using ::towlower _LIBCPP_USING_IF_EXISTS;
using ::towupper _LIBCPP_USING_IF_EXISTS;
using ::towctrans _LIBCPP_USING_IF_EXISTS;
using ::wctrans _LIBCPP_USING_IF_EXISTS;
_LIBCPP_END_NAMESPACE_STD
#endif // _LIBCPP_CWCTYPE