[libc++] Use the using_if_exists attribute when provided

[ldionne]

Summary

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

[ldionne]

Note that once this is in, we can also start dropping existing #ifs in the headers to workaround various declarations missing in the underlying C stdlib, as long as we require a sufficiently recent Clang.

[lebedev.ri]

In D90257#2357188, @ldionne wrote:

Note that once this is in, we can also start dropping existing #ifs in the headers to workaround various declarations missing in the underlying C stdlib, as long as we require a sufficiently recent Clang.

What about other compilers?

[ldionne]

In D90257#2357223, @lebedev.ri wrote:

In D90257#2357188, @ldionne wrote:

Note that once this is in, we can also start dropping existing #ifs in the headers to workaround various declarations missing in the underlying C stdlib, as long as we require a sufficiently recent Clang.

What about other compilers?

We're not there yet -- I was just pointing to a possible future cleanup.

For other compilers: it seems reasonable to mandate that the attribute is supported on the compiler you're using if you're trying to port libc++ to a platform that doesn't provide a full standard library. It's a balance between ease-of-porting and ease-of-maintaining.

[ldionne]

Rebase onto main since the attribute has landed.

[ldionne]

After a quick grep through the sources, it seems like the following macros are the ones that currently gate using declarations:

_LIBCPP_C_HAS_NO_GETS
_LIBCPP_HAS_ALIGNED_ALLOC
_LIBCPP_HAS_NO_FGETPOS_FSETPOS
_LIBCPP_HAS_NO_GLOBAL_FILESYSTEM_NAMESPACE
_LIBCPP_HAS_NO_LONG_LONG
_LIBCPP_HAS_NO_STDIN
_LIBCPP_HAS_NO_STDOUT
_LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
_LIBCPP_HAS_QUICK_EXIT
_LIBCPP_HAS_TIMESPEC_GET
_LIBCPP_WINDOWS_STORE_APP

If you see this review and are currently depending on one of these macros, please be aware that we will eventually stop guarding those declarations based on those macros (and in some cases that means a macro will become unused and we'll remove it). I'll do my best to communicate with vendors before that happens, however this is an early heads up.

@jasonliu @lebedev.ri @mstorsjo @danalbert @phosek for awareness.

[ldionne]

Rebase onto main

[ldionne]

Rebase and add a few usings that had been forgotten.

libcxx/include/__config

 
 #if __has_feature(cxx_explicit_conversions) || defined(__IBMCPP__) || \
     (!defined(_LIBCPP_CXX03_LANG) && defined(__GNUC__)) // All supported GCC versions
 #  define _LIBCPP_EXPLICIT explicit
 #else
 #  define _LIBCPP_EXPLICIT
 #endif
 
+#if __has_attribute(using_if_exists)
+# define _LIBCPP_USING_IF_EXISTS __attribute__((using_if_exists))
+#else
+# define _LIBCPP_USING_IF_EXISTS
+#endif
+
 #ifdef _LIBCPP_HAS_NO_STRONG_ENUMS
 #  define _LIBCPP_DECLARE_STRONG_ENUM(x) struct _LIBCPP_TYPE_VIS x { enum __lx
 #  define _LIBCPP_DECLARE_STRONG_ENUM_EPILOG(x) \
      __lx __v_; \
      _LIBCPP_INLINE_VISIBILITY x(__lx __v) : __v_(__v) {} \
      _LIBCPP_INLINE_VISIBILITY explicit x(int __v) : __v_(static_cast<__lx>(__v)) {} \
      _LIBCPP_INLINE_VISIBILITY operator int() const {return __v_;} \
      };

libcxx/include/cctype

 #undef tolower
 #endif
 
 #ifdef toupper
 #undef toupper
 #endif
 
 
-using ::isalnum;
-using ::isalpha;
-using ::isblank;
-using ::iscntrl;
-using ::isdigit;
-using ::isgraph;
-using ::islower;
-using ::isprint;
-using ::ispunct;
-using ::isspace;
-using ::isupper;
-using ::isxdigit;
-using ::tolower;
-using ::toupper;
+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

libcxx/include/cfenv

 #include <fenv.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::fenv_t;
-using ::fexcept_t;
+using ::fenv_t _LIBCPP_USING_IF_EXISTS;
+using ::fexcept_t _LIBCPP_USING_IF_EXISTS;
 
-using ::feclearexcept;
-using ::fegetexceptflag;
-using ::feraiseexcept;
-using ::fesetexceptflag;
-using ::fetestexcept;
-using ::fegetround;
-using ::fesetround;
-using ::fegetenv;
-using ::feholdexcept;
-using ::fesetenv;
-using ::feupdateenv;
+using ::feclearexcept _LIBCPP_USING_IF_EXISTS;
+using ::fegetexceptflag _LIBCPP_USING_IF_EXISTS;
+using ::feraiseexcept _LIBCPP_USING_IF_EXISTS;
+using ::fesetexceptflag _LIBCPP_USING_IF_EXISTS;
+using ::fetestexcept _LIBCPP_USING_IF_EXISTS;
+using ::fegetround _LIBCPP_USING_IF_EXISTS;
+using ::fesetround _LIBCPP_USING_IF_EXISTS;
+using ::fegetenv _LIBCPP_USING_IF_EXISTS;
+using ::feholdexcept _LIBCPP_USING_IF_EXISTS;
+using ::fesetenv _LIBCPP_USING_IF_EXISTS;
+using ::feupdateenv _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CFENV

libcxx/include/cinttypes

 #include <inttypes.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using::imaxdiv_t;
-using::imaxabs;
-using::imaxdiv;
-using::strtoimax;
-using::strtoumax;
-using::wcstoimax;
-using::wcstoumax;
+using ::imaxdiv_t _LIBCPP_USING_IF_EXISTS;
+using ::imaxabs _LIBCPP_USING_IF_EXISTS;
+using ::imaxdiv _LIBCPP_USING_IF_EXISTS;
+using ::strtoimax _LIBCPP_USING_IF_EXISTS;
+using ::strtoumax _LIBCPP_USING_IF_EXISTS;
+using ::wcstoimax _LIBCPP_USING_IF_EXISTS;
+using ::wcstoumax _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CINTTYPES

libcxx/include/clocale

 #include <locale.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::lconv;
+using ::lconv _LIBCPP_USING_IF_EXISTS;
 #ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
-using ::setlocale;
+using ::setlocale _LIBCPP_USING_IF_EXISTS;
 #endif
-using ::localeconv;
+using ::localeconv _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CLOCALE

libcxx/include/cmath

 #pragma GCC system_header
 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::signbit;
-using ::fpclassify;
-using ::isfinite;
-using ::isinf;
-using ::isnan;
-using ::isnormal;
-using ::isgreater;
-using ::isgreaterequal;
-using ::isless;
-using ::islessequal;
-using ::islessgreater;
-using ::isunordered;
-using ::isunordered;
-
-using ::float_t;
-using ::double_t;
-
-using ::abs;
-
-using ::acos;
-using ::acosf;
-using ::asin;
-using ::asinf;
-using ::atan;
-using ::atanf;
-using ::atan2;
-using ::atan2f;
-using ::ceil;
-using ::ceilf;
-using ::cos;
-using ::cosf;
-using ::cosh;
-using ::coshf;
-
-using ::exp;
-using ::expf;
-
-using ::fabs;
-using ::fabsf;
-using ::floor;
-using ::floorf;
-
-using ::fmod;
-using ::fmodf;
-
-using ::frexp;
-using ::frexpf;
-using ::ldexp;
-using ::ldexpf;
-
-using ::log;
-using ::logf;
-
-using ::log10;
-using ::log10f;
-using ::modf;
-using ::modff;
-
-using ::pow;
-using ::powf;
-
-using ::sin;
-using ::sinf;
-using ::sinh;
-using ::sinhf;
-
-using ::sqrt;
-using ::sqrtf;
-using ::tan;
-using ::tanf;
-
-using ::tanh;
-using ::tanhf;
-
-using ::acosh;
-using ::acoshf;
-using ::asinh;
-using ::asinhf;
-using ::atanh;
-using ::atanhf;
-using ::cbrt;
-using ::cbrtf;
-
-using ::copysign;
-using ::copysignf;
-
-using ::erf;
-using ::erff;
-using ::erfc;
-using ::erfcf;
-using ::exp2;
-using ::exp2f;
-using ::expm1;
-using ::expm1f;
-using ::fdim;
-using ::fdimf;
-using ::fmaf;
-using ::fma;
-using ::fmax;
-using ::fmaxf;
-using ::fmin;
-using ::fminf;
-using ::hypot;
-using ::hypotf;
-using ::ilogb;
-using ::ilogbf;
-using ::lgamma;
-using ::lgammaf;
-using ::llrint;
-using ::llrintf;
-using ::llround;
-using ::llroundf;
-using ::log1p;
-using ::log1pf;
-using ::log2;
-using ::log2f;
-using ::logb;
-using ::logbf;
-using ::lrint;
-using ::lrintf;
-using ::lround;
-using ::lroundf;
-
-using ::nan;
-using ::nanf;
-
-using ::nearbyint;
-using ::nearbyintf;
-using ::nextafter;
-using ::nextafterf;
-using ::nexttoward;
-using ::nexttowardf;
-using ::remainder;
-using ::remainderf;
-using ::remquo;
-using ::remquof;
-using ::rint;
-using ::rintf;
-using ::round;
-using ::roundf;
-using ::scalbln;
-using ::scalblnf;
-using ::scalbn;
-using ::scalbnf;
-using ::tgamma;
-using ::tgammaf;
-using ::trunc;
-using ::truncf;
-
-using ::acosl;
-using ::asinl;
-using ::atanl;
-using ::atan2l;
-using ::ceill;
-using ::cosl;
-using ::coshl;
-using ::expl;
-using ::fabsl;
-using ::floorl;
-using ::fmodl;
-using ::frexpl;
-using ::ldexpl;
-using ::logl;
-using ::log10l;
-using ::modfl;
-using ::powl;
-using ::sinl;
-using ::sinhl;
-using ::sqrtl;
-using ::tanl;
-
-using ::tanhl;
-using ::acoshl;
-using ::asinhl;
-using ::atanhl;
-using ::cbrtl;
-
-using ::copysignl;
-
-using ::erfl;
-using ::erfcl;
-using ::exp2l;
-using ::expm1l;
-using ::fdiml;
-using ::fmal;
-using ::fmaxl;
-using ::fminl;
-using ::hypotl;
-using ::ilogbl;
-using ::lgammal;
-using ::llrintl;
-using ::llroundl;
-using ::log1pl;
-using ::log2l;
-using ::logbl;
-using ::lrintl;
-using ::lroundl;
-using ::nanl;
-using ::nearbyintl;
-using ::nextafterl;
-using ::nexttowardl;
-using ::remainderl;
-using ::remquol;
-using ::rintl;
-using ::roundl;
-using ::scalblnl;
-using ::scalbnl;
-using ::tgammal;
-using ::truncl;
+using ::signbit _LIBCPP_USING_IF_EXISTS;
+using ::fpclassify _LIBCPP_USING_IF_EXISTS;
+using ::isfinite _LIBCPP_USING_IF_EXISTS;
+using ::isinf _LIBCPP_USING_IF_EXISTS;
+using ::isnan _LIBCPP_USING_IF_EXISTS;
+using ::isnormal _LIBCPP_USING_IF_EXISTS;
+using ::isgreater _LIBCPP_USING_IF_EXISTS;
+using ::isgreaterequal _LIBCPP_USING_IF_EXISTS;
+using ::isless _LIBCPP_USING_IF_EXISTS;
+using ::islessequal _LIBCPP_USING_IF_EXISTS;
+using ::islessgreater _LIBCPP_USING_IF_EXISTS;
+using ::isunordered _LIBCPP_USING_IF_EXISTS;
+using ::isunordered _LIBCPP_USING_IF_EXISTS;
+
+using ::float_t _LIBCPP_USING_IF_EXISTS;
+using ::double_t _LIBCPP_USING_IF_EXISTS;
+
+using ::abs _LIBCPP_USING_IF_EXISTS;
+
+using ::acos _LIBCPP_USING_IF_EXISTS;
+using ::acosf _LIBCPP_USING_IF_EXISTS;
+using ::asin _LIBCPP_USING_IF_EXISTS;
+using ::asinf _LIBCPP_USING_IF_EXISTS;
+using ::atan _LIBCPP_USING_IF_EXISTS;
+using ::atanf _LIBCPP_USING_IF_EXISTS;
+using ::atan2 _LIBCPP_USING_IF_EXISTS;
+using ::atan2f _LIBCPP_USING_IF_EXISTS;
+using ::ceil _LIBCPP_USING_IF_EXISTS;
+using ::ceilf _LIBCPP_USING_IF_EXISTS;
+using ::cos _LIBCPP_USING_IF_EXISTS;
+using ::cosf _LIBCPP_USING_IF_EXISTS;
+using ::cosh _LIBCPP_USING_IF_EXISTS;
+using ::coshf _LIBCPP_USING_IF_EXISTS;
+
+using ::exp _LIBCPP_USING_IF_EXISTS;
+using ::expf _LIBCPP_USING_IF_EXISTS;
+
+using ::fabs _LIBCPP_USING_IF_EXISTS;
+using ::fabsf _LIBCPP_USING_IF_EXISTS;
+using ::floor _LIBCPP_USING_IF_EXISTS;
+using ::floorf _LIBCPP_USING_IF_EXISTS;
+
+using ::fmod _LIBCPP_USING_IF_EXISTS;
+using ::fmodf _LIBCPP_USING_IF_EXISTS;
+
+using ::frexp _LIBCPP_USING_IF_EXISTS;
+using ::frexpf _LIBCPP_USING_IF_EXISTS;
+using ::ldexp _LIBCPP_USING_IF_EXISTS;
+using ::ldexpf _LIBCPP_USING_IF_EXISTS;
+
+using ::log _LIBCPP_USING_IF_EXISTS;
+using ::logf _LIBCPP_USING_IF_EXISTS;
+
+using ::log10 _LIBCPP_USING_IF_EXISTS;
+using ::log10f _LIBCPP_USING_IF_EXISTS;
+using ::modf _LIBCPP_USING_IF_EXISTS;
+using ::modff _LIBCPP_USING_IF_EXISTS;
+
+using ::pow _LIBCPP_USING_IF_EXISTS;
+using ::powf _LIBCPP_USING_IF_EXISTS;
+
+using ::sin _LIBCPP_USING_IF_EXISTS;
+using ::sinf _LIBCPP_USING_IF_EXISTS;
+using ::sinh _LIBCPP_USING_IF_EXISTS;
+using ::sinhf _LIBCPP_USING_IF_EXISTS;
+
+using ::sqrt _LIBCPP_USING_IF_EXISTS;
+using ::sqrtf _LIBCPP_USING_IF_EXISTS;
+using ::tan _LIBCPP_USING_IF_EXISTS;
+using ::tanf _LIBCPP_USING_IF_EXISTS;
+
+using ::tanh _LIBCPP_USING_IF_EXISTS;
+using ::tanhf _LIBCPP_USING_IF_EXISTS;
+
+using ::acosh _LIBCPP_USING_IF_EXISTS;
+using ::acoshf _LIBCPP_USING_IF_EXISTS;
+using ::asinh _LIBCPP_USING_IF_EXISTS;
+using ::asinhf _LIBCPP_USING_IF_EXISTS;
+using ::atanh _LIBCPP_USING_IF_EXISTS;
+using ::atanhf _LIBCPP_USING_IF_EXISTS;
+using ::cbrt _LIBCPP_USING_IF_EXISTS;
+using ::cbrtf _LIBCPP_USING_IF_EXISTS;
+
+using ::copysign _LIBCPP_USING_IF_EXISTS;
+using ::copysignf _LIBCPP_USING_IF_EXISTS;
+
+using ::erf _LIBCPP_USING_IF_EXISTS;
+using ::erff _LIBCPP_USING_IF_EXISTS;
+using ::erfc _LIBCPP_USING_IF_EXISTS;
+using ::erfcf _LIBCPP_USING_IF_EXISTS;
+using ::exp2 _LIBCPP_USING_IF_EXISTS;
+using ::exp2f _LIBCPP_USING_IF_EXISTS;
+using ::expm1 _LIBCPP_USING_IF_EXISTS;
+using ::expm1f _LIBCPP_USING_IF_EXISTS;
+using ::fdim _LIBCPP_USING_IF_EXISTS;
+using ::fdimf _LIBCPP_USING_IF_EXISTS;
+using ::fmaf _LIBCPP_USING_IF_EXISTS;
+using ::fma _LIBCPP_USING_IF_EXISTS;
+using ::fmax _LIBCPP_USING_IF_EXISTS;
+using ::fmaxf _LIBCPP_USING_IF_EXISTS;
+using ::fmin _LIBCPP_USING_IF_EXISTS;
+using ::fminf _LIBCPP_USING_IF_EXISTS;
+using ::hypot _LIBCPP_USING_IF_EXISTS;
+using ::hypotf _LIBCPP_USING_IF_EXISTS;
+using ::ilogb _LIBCPP_USING_IF_EXISTS;
+using ::ilogbf _LIBCPP_USING_IF_EXISTS;
+using ::lgamma _LIBCPP_USING_IF_EXISTS;
+using ::lgammaf _LIBCPP_USING_IF_EXISTS;
+using ::llrint _LIBCPP_USING_IF_EXISTS;
+using ::llrintf _LIBCPP_USING_IF_EXISTS;
+using ::llround _LIBCPP_USING_IF_EXISTS;
+using ::llroundf _LIBCPP_USING_IF_EXISTS;
+using ::log1p _LIBCPP_USING_IF_EXISTS;
+using ::log1pf _LIBCPP_USING_IF_EXISTS;
+using ::log2 _LIBCPP_USING_IF_EXISTS;
+using ::log2f _LIBCPP_USING_IF_EXISTS;
+using ::logb _LIBCPP_USING_IF_EXISTS;
+using ::logbf _LIBCPP_USING_IF_EXISTS;
+using ::lrint _LIBCPP_USING_IF_EXISTS;
+using ::lrintf _LIBCPP_USING_IF_EXISTS;
+using ::lround _LIBCPP_USING_IF_EXISTS;
+using ::lroundf _LIBCPP_USING_IF_EXISTS;
+
+using ::nan _LIBCPP_USING_IF_EXISTS;
+using ::nanf _LIBCPP_USING_IF_EXISTS;
+
+using ::nearbyint _LIBCPP_USING_IF_EXISTS;
+using ::nearbyintf _LIBCPP_USING_IF_EXISTS;
+using ::nextafter _LIBCPP_USING_IF_EXISTS;
+using ::nextafterf _LIBCPP_USING_IF_EXISTS;
+using ::nexttoward _LIBCPP_USING_IF_EXISTS;
+using ::nexttowardf _LIBCPP_USING_IF_EXISTS;
+using ::remainder _LIBCPP_USING_IF_EXISTS;
+using ::remainderf _LIBCPP_USING_IF_EXISTS;
+using ::remquo _LIBCPP_USING_IF_EXISTS;
+using ::remquof _LIBCPP_USING_IF_EXISTS;
+using ::rint _LIBCPP_USING_IF_EXISTS;
+using ::rintf _LIBCPP_USING_IF_EXISTS;
+using ::round _LIBCPP_USING_IF_EXISTS;
+using ::roundf _LIBCPP_USING_IF_EXISTS;
+using ::scalbln _LIBCPP_USING_IF_EXISTS;
+using ::scalblnf _LIBCPP_USING_IF_EXISTS;
+using ::scalbn _LIBCPP_USING_IF_EXISTS;
+using ::scalbnf _LIBCPP_USING_IF_EXISTS;
+using ::tgamma _LIBCPP_USING_IF_EXISTS;
+using ::tgammaf _LIBCPP_USING_IF_EXISTS;
+using ::trunc _LIBCPP_USING_IF_EXISTS;
+using ::truncf _LIBCPP_USING_IF_EXISTS;
+
+using ::acosl _LIBCPP_USING_IF_EXISTS;
+using ::asinl _LIBCPP_USING_IF_EXISTS;
+using ::atanl _LIBCPP_USING_IF_EXISTS;
+using ::atan2l _LIBCPP_USING_IF_EXISTS;
+using ::ceill _LIBCPP_USING_IF_EXISTS;
+using ::cosl _LIBCPP_USING_IF_EXISTS;
+using ::coshl _LIBCPP_USING_IF_EXISTS;
+using ::expl _LIBCPP_USING_IF_EXISTS;
+using ::fabsl _LIBCPP_USING_IF_EXISTS;
+using ::floorl _LIBCPP_USING_IF_EXISTS;
+using ::fmodl _LIBCPP_USING_IF_EXISTS;
+using ::frexpl _LIBCPP_USING_IF_EXISTS;
+using ::ldexpl _LIBCPP_USING_IF_EXISTS;
+using ::logl _LIBCPP_USING_IF_EXISTS;
+using ::log10l _LIBCPP_USING_IF_EXISTS;
+using ::modfl _LIBCPP_USING_IF_EXISTS;
+using ::powl _LIBCPP_USING_IF_EXISTS;
+using ::sinl _LIBCPP_USING_IF_EXISTS;
+using ::sinhl _LIBCPP_USING_IF_EXISTS;
+using ::sqrtl _LIBCPP_USING_IF_EXISTS;
+using ::tanl _LIBCPP_USING_IF_EXISTS;
+
+using ::tanhl _LIBCPP_USING_IF_EXISTS;
+using ::acoshl _LIBCPP_USING_IF_EXISTS;
+using ::asinhl _LIBCPP_USING_IF_EXISTS;
+using ::atanhl _LIBCPP_USING_IF_EXISTS;
+using ::cbrtl _LIBCPP_USING_IF_EXISTS;
+
+using ::copysignl _LIBCPP_USING_IF_EXISTS;
+
+using ::erfl _LIBCPP_USING_IF_EXISTS;
+using ::erfcl _LIBCPP_USING_IF_EXISTS;
+using ::exp2l _LIBCPP_USING_IF_EXISTS;
+using ::expm1l _LIBCPP_USING_IF_EXISTS;
+using ::fdiml _LIBCPP_USING_IF_EXISTS;
+using ::fmal _LIBCPP_USING_IF_EXISTS;
+using ::fmaxl _LIBCPP_USING_IF_EXISTS;
+using ::fminl _LIBCPP_USING_IF_EXISTS;
+using ::hypotl _LIBCPP_USING_IF_EXISTS;
+using ::ilogbl _LIBCPP_USING_IF_EXISTS;
+using ::lgammal _LIBCPP_USING_IF_EXISTS;
+using ::llrintl _LIBCPP_USING_IF_EXISTS;
+using ::llroundl _LIBCPP_USING_IF_EXISTS;
+using ::log1pl _LIBCPP_USING_IF_EXISTS;
+using ::log2l _LIBCPP_USING_IF_EXISTS;
+using ::logbl _LIBCPP_USING_IF_EXISTS;
+using ::lrintl _LIBCPP_USING_IF_EXISTS;
+using ::lroundl _LIBCPP_USING_IF_EXISTS;
+using ::nanl _LIBCPP_USING_IF_EXISTS;
+using ::nearbyintl _LIBCPP_USING_IF_EXISTS;
+using ::nextafterl _LIBCPP_USING_IF_EXISTS;
+using ::nexttowardl _LIBCPP_USING_IF_EXISTS;
+using ::remainderl _LIBCPP_USING_IF_EXISTS;
+using ::remquol _LIBCPP_USING_IF_EXISTS;
+using ::rintl _LIBCPP_USING_IF_EXISTS;
+using ::roundl _LIBCPP_USING_IF_EXISTS;
+using ::scalblnl _LIBCPP_USING_IF_EXISTS;
+using ::scalbnl _LIBCPP_USING_IF_EXISTS;
+using ::tgammal _LIBCPP_USING_IF_EXISTS;
+using ::truncl _LIBCPP_USING_IF_EXISTS;
 
 #if _LIBCPP_STD_VER > 14
 inline _LIBCPP_INLINE_VISIBILITY float       hypot(       float x,       float y,       float z ) { return sqrt(x*x + y*y + z*z); }
 inline _LIBCPP_INLINE_VISIBILITY double      hypot(      double x,      double y,      double z ) { return sqrt(x*x + y*y + z*z); }
 inline _LIBCPP_INLINE_VISIBILITY long double hypot( long double x, long double y, long double z ) { return sqrt(x*x + y*y + z*z); }
 
 template <class _A1, class _A2, class _A3>
 inline _LIBCPP_INLINE_VISIBILITY

libcxx/include/csetjmp

 #include <setjmp.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::jmp_buf;
-using ::longjmp;
+using ::jmp_buf _LIBCPP_USING_IF_EXISTS;
+using ::longjmp _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CSETJMP

libcxx/include/csignal

 #include <signal.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::sig_atomic_t;
-using ::signal;
-using ::raise;
+using ::sig_atomic_t _LIBCPP_USING_IF_EXISTS;
+using ::signal _LIBCPP_USING_IF_EXISTS;
+using ::raise _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CSIGNAL

libcxx/include/cstdarg

 #include <stdarg.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::va_list;
+using ::va_list _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CSTDARG

libcxx/include/cstddef

 #endif
 
 // Don't include our own <stddef.h>; we don't want to declare ::nullptr_t.
 #include_next <stddef.h>
 #include <__nullptr>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::ptrdiff_t;
-using ::size_t;
+using ::ptrdiff_t _LIBCPP_USING_IF_EXISTS;
+using ::size_t _LIBCPP_USING_IF_EXISTS;
 
 #if !defined(_LIBCPP_CXX03_LANG)
-using ::max_align_t;
+using ::max_align_t _LIBCPP_USING_IF_EXISTS;
 #endif
 
 template <class _Tp> struct __libcpp_is_integral                     { enum { value = 0 }; };
 template <>          struct __libcpp_is_integral<bool>               { enum { value = 1 }; };
 template <>          struct __libcpp_is_integral<char>               { enum { value = 1 }; };
 template <>          struct __libcpp_is_integral<signed char>        { enum { value = 1 }; };
 template <>          struct __libcpp_is_integral<unsigned char>      { enum { value = 1 }; };
 template <>          struct __libcpp_is_integral<wchar_t>            { enum { value = 1 }; };

libcxx/include/cstdint

 #include <stdint.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using::int8_t;
-using::int16_t;
-using::int32_t;
-using::int64_t;
-
-using::uint8_t;
-using::uint16_t;
-using::uint32_t;
-using::uint64_t;
-
-using::int_least8_t;
-using::int_least16_t;
-using::int_least32_t;
-using::int_least64_t;
-
-using::uint_least8_t;
-using::uint_least16_t;
-using::uint_least32_t;
-using::uint_least64_t;
-
-using::int_fast8_t;
-using::int_fast16_t;
-using::int_fast32_t;
-using::int_fast64_t;
-
-using::uint_fast8_t;
-using::uint_fast16_t;
-using::uint_fast32_t;
-using::uint_fast64_t;
+using ::int8_t _LIBCPP_USING_IF_EXISTS;
+using ::int16_t _LIBCPP_USING_IF_EXISTS;
+using ::int32_t _LIBCPP_USING_IF_EXISTS;
+using ::int64_t _LIBCPP_USING_IF_EXISTS;
+
+using ::uint8_t _LIBCPP_USING_IF_EXISTS;
+using ::uint16_t _LIBCPP_USING_IF_EXISTS;
+using ::uint32_t _LIBCPP_USING_IF_EXISTS;
+using ::uint64_t _LIBCPP_USING_IF_EXISTS;
+
+using ::int_least8_t _LIBCPP_USING_IF_EXISTS;
+using ::int_least16_t _LIBCPP_USING_IF_EXISTS;
+using ::int_least32_t _LIBCPP_USING_IF_EXISTS;
+using ::int_least64_t _LIBCPP_USING_IF_EXISTS;
+
+using ::uint_least8_t _LIBCPP_USING_IF_EXISTS;
+using ::uint_least16_t _LIBCPP_USING_IF_EXISTS;
+using ::uint_least32_t _LIBCPP_USING_IF_EXISTS;
+using ::uint_least64_t _LIBCPP_USING_IF_EXISTS;
+
+using ::int_fast8_t _LIBCPP_USING_IF_EXISTS;
+using ::int_fast16_t _LIBCPP_USING_IF_EXISTS;
+using ::int_fast32_t _LIBCPP_USING_IF_EXISTS;
+using ::int_fast64_t _LIBCPP_USING_IF_EXISTS;
+
+using ::uint_fast8_t _LIBCPP_USING_IF_EXISTS;
+using ::uint_fast16_t _LIBCPP_USING_IF_EXISTS;
+using ::uint_fast32_t _LIBCPP_USING_IF_EXISTS;
+using ::uint_fast64_t _LIBCPP_USING_IF_EXISTS;
 
-using::intptr_t;
-using::uintptr_t;
+using ::intptr_t _LIBCPP_USING_IF_EXISTS;
+using ::uintptr_t _LIBCPP_USING_IF_EXISTS;
 
-using::intmax_t;
-using::uintmax_t;
+using ::intmax_t _LIBCPP_USING_IF_EXISTS;
+using ::uintmax_t _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CSTDINT

libcxx/include/cstdio

 #include <stdio.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::FILE;
-using ::fpos_t;
-using ::size_t;
-
-using ::fclose;
-using ::fflush;
-using ::setbuf;
-using ::setvbuf;
-using ::fprintf;
-using ::fscanf;
-using ::snprintf;
-using ::sprintf;
-using ::sscanf;
-using ::vfprintf;
-using ::vfscanf;
-using ::vsscanf;
-using ::vsnprintf;
-using ::vsprintf;
-using ::fgetc;
-using ::fgets;
-using ::fputc;
-using ::fputs;
-using ::getc;
-using ::putc;
-using ::ungetc;
-using ::fread;
-using ::fwrite;
+using ::FILE _LIBCPP_USING_IF_EXISTS;
+using ::fpos_t _LIBCPP_USING_IF_EXISTS;
+using ::size_t _LIBCPP_USING_IF_EXISTS;
+
+using ::fclose _LIBCPP_USING_IF_EXISTS;
+using ::fflush _LIBCPP_USING_IF_EXISTS;
+using ::setbuf _LIBCPP_USING_IF_EXISTS;
+using ::setvbuf _LIBCPP_USING_IF_EXISTS;
+using ::fprintf _LIBCPP_USING_IF_EXISTS;
+using ::fscanf _LIBCPP_USING_IF_EXISTS;
+using ::snprintf _LIBCPP_USING_IF_EXISTS;
+using ::sprintf _LIBCPP_USING_IF_EXISTS;
+using ::sscanf _LIBCPP_USING_IF_EXISTS;
+using ::vfprintf _LIBCPP_USING_IF_EXISTS;
+using ::vfscanf _LIBCPP_USING_IF_EXISTS;
+using ::vsscanf _LIBCPP_USING_IF_EXISTS;
+using ::vsnprintf _LIBCPP_USING_IF_EXISTS;
+using ::vsprintf _LIBCPP_USING_IF_EXISTS;
+using ::fgetc _LIBCPP_USING_IF_EXISTS;
+using ::fgets _LIBCPP_USING_IF_EXISTS;
+using ::fputc _LIBCPP_USING_IF_EXISTS;
+using ::fputs _LIBCPP_USING_IF_EXISTS;
+using ::getc _LIBCPP_USING_IF_EXISTS;
+using ::putc _LIBCPP_USING_IF_EXISTS;
+using ::ungetc _LIBCPP_USING_IF_EXISTS;
+using ::fread _LIBCPP_USING_IF_EXISTS;
+using ::fwrite _LIBCPP_USING_IF_EXISTS;
 #ifndef _LIBCPP_HAS_NO_FGETPOS_FSETPOS
-using ::fgetpos;
+using ::fgetpos _LIBCPP_USING_IF_EXISTS;
 #endif
-using ::fseek;
+using ::fseek _LIBCPP_USING_IF_EXISTS;
 #ifndef _LIBCPP_HAS_NO_FGETPOS_FSETPOS
-using ::fsetpos;
+using ::fsetpos _LIBCPP_USING_IF_EXISTS;
 #endif
-using ::ftell;
-using ::rewind;
-using ::clearerr;
-using ::feof;
-using ::ferror;
-using ::perror;
+using ::ftell _LIBCPP_USING_IF_EXISTS;
+using ::rewind _LIBCPP_USING_IF_EXISTS;
+using ::clearerr _LIBCPP_USING_IF_EXISTS;
+using ::feof _LIBCPP_USING_IF_EXISTS;
+using ::ferror _LIBCPP_USING_IF_EXISTS;
+using ::perror _LIBCPP_USING_IF_EXISTS;
 
 #ifndef _LIBCPP_HAS_NO_GLOBAL_FILESYSTEM_NAMESPACE
-using ::fopen;
-using ::freopen;
-using ::remove;
-using ::rename;
-using ::tmpfile;
-using ::tmpnam;
+using ::fopen _LIBCPP_USING_IF_EXISTS;
+using ::freopen _LIBCPP_USING_IF_EXISTS;
+using ::remove _LIBCPP_USING_IF_EXISTS;
+using ::rename _LIBCPP_USING_IF_EXISTS;
+using ::tmpfile _LIBCPP_USING_IF_EXISTS;
+using ::tmpnam _LIBCPP_USING_IF_EXISTS;
 #endif
 
 #ifndef _LIBCPP_HAS_NO_STDIN
-using ::getchar;
+using ::getchar _LIBCPP_USING_IF_EXISTS;
 #if _LIBCPP_STD_VER <= 11 && !defined(_LIBCPP_C_HAS_NO_GETS)
-using ::gets;
+using ::gets _LIBCPP_USING_IF_EXISTS;
 #endif
-using ::scanf;
-using ::vscanf;
+using ::scanf _LIBCPP_USING_IF_EXISTS;
+using ::vscanf _LIBCPP_USING_IF_EXISTS;
 #endif
 
 #ifndef _LIBCPP_HAS_NO_STDOUT
-using ::printf;
-using ::putchar;
-using ::puts;
-using ::vprintf;
+using ::printf _LIBCPP_USING_IF_EXISTS;
+using ::putchar _LIBCPP_USING_IF_EXISTS;
+using ::puts _LIBCPP_USING_IF_EXISTS;
+using ::vprintf _LIBCPP_USING_IF_EXISTS;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CSTDIO

libcxx/include/cstdlib

 #ifdef __GNUC__
 #define _LIBCPP_UNREACHABLE() __builtin_unreachable()
 #else
 #define _LIBCPP_UNREACHABLE() _VSTD::abort()
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::size_t;
-using ::div_t;
-using ::ldiv_t;
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
-using ::lldiv_t;
-#endif // _LIBCPP_HAS_NO_LONG_LONG
-using ::atof;
-using ::atoi;
-using ::atol;
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
-using ::atoll;
-#endif // _LIBCPP_HAS_NO_LONG_LONG
-using ::strtod;
-using ::strtof;
-using ::strtold;
-using ::strtol;
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
-using ::strtoll;
-#endif // _LIBCPP_HAS_NO_LONG_LONG
-using ::strtoul;
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
-using ::strtoull;
-#endif // _LIBCPP_HAS_NO_LONG_LONG
-using ::rand;
-using ::srand;
-using ::calloc;
-using ::free;
-using ::malloc;
-using ::realloc;
-using ::abort;
-using ::atexit;
-using ::exit;
-using ::_Exit;
+using ::size_t _LIBCPP_USING_IF_EXISTS;
+using ::div_t _LIBCPP_USING_IF_EXISTS;
+using ::ldiv_t _LIBCPP_USING_IF_EXISTS;
+#ifndef _LIBCPP_HAS_NO_LONG_LONG
+using ::lldiv_t _LIBCPP_USING_IF_EXISTS;
+#endif // _LIBCPP_HAS_NO_LONG_LONG
+using ::atof _LIBCPP_USING_IF_EXISTS;
+using ::atoi _LIBCPP_USING_IF_EXISTS;
+using ::atol _LIBCPP_USING_IF_EXISTS;
+#ifndef _LIBCPP_HAS_NO_LONG_LONG
+using ::atoll _LIBCPP_USING_IF_EXISTS;
+#endif // _LIBCPP_HAS_NO_LONG_LONG
+using ::strtod _LIBCPP_USING_IF_EXISTS;
+using ::strtof _LIBCPP_USING_IF_EXISTS;
+using ::strtold _LIBCPP_USING_IF_EXISTS;
+using ::strtol _LIBCPP_USING_IF_EXISTS;
+#ifndef _LIBCPP_HAS_NO_LONG_LONG
+using ::strtoll _LIBCPP_USING_IF_EXISTS;
+#endif // _LIBCPP_HAS_NO_LONG_LONG
+using ::strtoul _LIBCPP_USING_IF_EXISTS;
+#ifndef _LIBCPP_HAS_NO_LONG_LONG
+using ::strtoull _LIBCPP_USING_IF_EXISTS;
+#endif // _LIBCPP_HAS_NO_LONG_LONG
+using ::rand _LIBCPP_USING_IF_EXISTS;
+using ::srand _LIBCPP_USING_IF_EXISTS;
+using ::calloc _LIBCPP_USING_IF_EXISTS;
+using ::free _LIBCPP_USING_IF_EXISTS;
+using ::malloc _LIBCPP_USING_IF_EXISTS;
+using ::realloc _LIBCPP_USING_IF_EXISTS;
+using ::abort _LIBCPP_USING_IF_EXISTS;
+using ::atexit _LIBCPP_USING_IF_EXISTS;
+using ::exit _LIBCPP_USING_IF_EXISTS;
+using ::_Exit _LIBCPP_USING_IF_EXISTS;
 #ifndef _LIBCPP_WINDOWS_STORE_APP
-using ::getenv;
-using ::system;
+using ::getenv _LIBCPP_USING_IF_EXISTS;
+using ::system _LIBCPP_USING_IF_EXISTS;
 #endif
-using ::bsearch;
-using ::qsort;
-using ::abs;
-using ::labs;
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
-using ::llabs;
-#endif // _LIBCPP_HAS_NO_LONG_LONG
-using ::div;
-using ::ldiv;
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
-using ::lldiv;
-#endif // _LIBCPP_HAS_NO_LONG_LONG
-using ::mblen;
-using ::mbtowc;
-using ::wctomb;
-using ::mbstowcs;
-using ::wcstombs;
+using ::bsearch _LIBCPP_USING_IF_EXISTS;
+using ::qsort _LIBCPP_USING_IF_EXISTS;
+using ::abs _LIBCPP_USING_IF_EXISTS;
+using ::labs _LIBCPP_USING_IF_EXISTS;
+#ifndef _LIBCPP_HAS_NO_LONG_LONG
+using ::llabs _LIBCPP_USING_IF_EXISTS;
+#endif // _LIBCPP_HAS_NO_LONG_LONG
+using ::div _LIBCPP_USING_IF_EXISTS;
+using ::ldiv _LIBCPP_USING_IF_EXISTS;
+#ifndef _LIBCPP_HAS_NO_LONG_LONG
+using ::lldiv _LIBCPP_USING_IF_EXISTS;
+#endif // _LIBCPP_HAS_NO_LONG_LONG
+using ::mblen _LIBCPP_USING_IF_EXISTS;
+using ::mbtowc _LIBCPP_USING_IF_EXISTS;
+using ::wctomb _LIBCPP_USING_IF_EXISTS;
+using ::mbstowcs _LIBCPP_USING_IF_EXISTS;
+using ::wcstombs _LIBCPP_USING_IF_EXISTS;
 #if !defined(_LIBCPP_CXX03_LANG) && defined(_LIBCPP_HAS_QUICK_EXIT)
-using ::at_quick_exit;
-using ::quick_exit;
+using ::at_quick_exit _LIBCPP_USING_IF_EXISTS;
+using ::quick_exit _LIBCPP_USING_IF_EXISTS;
 #endif
 #if _LIBCPP_STD_VER > 14 && defined(_LIBCPP_HAS_ALIGNED_ALLOC)
-using ::aligned_alloc;
+using ::aligned_alloc _LIBCPP_USING_IF_EXISTS;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CSTDLIB

libcxx/include/cstring

 #include <string.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _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
 
 #endif // _LIBCPP_CSTRING

libcxx/include/ctime

 #   include <sys/cdefs.h>
 #   if defined(_LIBCPP_HAS_TIMESPEC_GET) && (__DARWIN_C_LEVEL < __DARWIN_C_FULL)
 #       define _LIBCPP_HAS_TIMESPEC_GET_NOT_ACTUALLY_PROVIDED
 #   endif
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::clock_t;
-using ::size_t;
-using ::time_t;
-using ::tm;
+using ::clock_t _LIBCPP_USING_IF_EXISTS;
+using ::size_t _LIBCPP_USING_IF_EXISTS;
+using ::time_t _LIBCPP_USING_IF_EXISTS;
+using ::tm _LIBCPP_USING_IF_EXISTS;
 #if _LIBCPP_STD_VER > 14 && defined(_LIBCPP_HAS_TIMESPEC_GET)
-using ::timespec;
+using ::timespec _LIBCPP_USING_IF_EXISTS;
 #endif
-using ::clock;
-using ::difftime;
-using ::mktime;
-using ::time;
+using ::clock _LIBCPP_USING_IF_EXISTS;
+using ::difftime _LIBCPP_USING_IF_EXISTS;
+using ::mktime _LIBCPP_USING_IF_EXISTS;
+using ::time _LIBCPP_USING_IF_EXISTS;
 #ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
-using ::asctime;
-using ::ctime;
-using ::gmtime;
-using ::localtime;
+using ::asctime _LIBCPP_USING_IF_EXISTS;
+using ::ctime _LIBCPP_USING_IF_EXISTS;
+using ::gmtime _LIBCPP_USING_IF_EXISTS;
+using ::localtime _LIBCPP_USING_IF_EXISTS;
 #endif
-using ::strftime;
+using ::strftime _LIBCPP_USING_IF_EXISTS;
 #if _LIBCPP_STD_VER > 14 && defined(_LIBCPP_HAS_TIMESPEC_GET) && !defined(_LIBCPP_HAS_TIMESPEC_GET_NOT_ACTUALLY_PROVIDED)
-using ::timespec_get;
+using ::timespec_get _LIBCPP_USING_IF_EXISTS;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CTIME

libcxx/include/cwchar

 #include <wchar.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::mbstate_t;
-using ::size_t;
-using ::tm;
-using ::wint_t;
-using ::FILE;
-using ::fwprintf;
-using ::fwscanf;
-using ::swprintf;
-using ::vfwprintf;
-using ::vswprintf;
-using ::swscanf;
-using ::vfwscanf;
-using ::vswscanf;
-using ::fgetwc;
-using ::fgetws;
-using ::fputwc;
-using ::fputws;
-using ::fwide;
-using ::getwc;
-using ::putwc;
-using ::ungetwc;
-using ::wcstod;
-using ::wcstof;
-using ::wcstold;
-using ::wcstol;
+using ::mbstate_t _LIBCPP_USING_IF_EXISTS;
+using ::size_t _LIBCPP_USING_IF_EXISTS;
+using ::tm _LIBCPP_USING_IF_EXISTS;
+using ::wint_t _LIBCPP_USING_IF_EXISTS;
+using ::FILE _LIBCPP_USING_IF_EXISTS;
+using ::fwprintf _LIBCPP_USING_IF_EXISTS;
+using ::fwscanf _LIBCPP_USING_IF_EXISTS;
+using ::swprintf _LIBCPP_USING_IF_EXISTS;
+using ::vfwprintf _LIBCPP_USING_IF_EXISTS;
+using ::vswprintf _LIBCPP_USING_IF_EXISTS;
+using ::swscanf _LIBCPP_USING_IF_EXISTS;
+using ::vfwscanf _LIBCPP_USING_IF_EXISTS;
+using ::vswscanf _LIBCPP_USING_IF_EXISTS;
+using ::fgetwc _LIBCPP_USING_IF_EXISTS;
+using ::fgetws _LIBCPP_USING_IF_EXISTS;
+using ::fputwc _LIBCPP_USING_IF_EXISTS;
+using ::fputws _LIBCPP_USING_IF_EXISTS;
+using ::fwide _LIBCPP_USING_IF_EXISTS;
+using ::getwc _LIBCPP_USING_IF_EXISTS;
+using ::putwc _LIBCPP_USING_IF_EXISTS;
+using ::ungetwc _LIBCPP_USING_IF_EXISTS;
+using ::wcstod _LIBCPP_USING_IF_EXISTS;
+using ::wcstof _LIBCPP_USING_IF_EXISTS;
+using ::wcstold _LIBCPP_USING_IF_EXISTS;
+using ::wcstol _LIBCPP_USING_IF_EXISTS;
 #ifndef _LIBCPP_HAS_NO_LONG_LONG
-using ::wcstoll;
+using ::wcstoll _LIBCPP_USING_IF_EXISTS;
 #endif // _LIBCPP_HAS_NO_LONG_LONG
-using ::wcstoul;
+using ::wcstoul _LIBCPP_USING_IF_EXISTS;
 #ifndef _LIBCPP_HAS_NO_LONG_LONG
-using ::wcstoull;
+using ::wcstoull _LIBCPP_USING_IF_EXISTS;
 #endif // _LIBCPP_HAS_NO_LONG_LONG
-using ::wcscpy;
-using ::wcsncpy;
-using ::wcscat;
-using ::wcsncat;
-using ::wcscmp;
-using ::wcscoll;
-using ::wcsncmp;
-using ::wcsxfrm;
-using ::wcschr;
-using ::wcspbrk;
-using ::wcsrchr;
-using ::wcsstr;
-using ::wmemchr;
-using ::wcscspn;
-using ::wcslen;
-using ::wcsspn;
-using ::wcstok;
-using ::wmemcmp;
-using ::wmemcpy;
-using ::wmemmove;
-using ::wmemset;
-using ::wcsftime;
-using ::btowc;
-using ::wctob;
-using ::mbsinit;
-using ::mbrlen;
-using ::mbrtowc;
-using ::wcrtomb;
-using ::mbsrtowcs;
-using ::wcsrtombs;
+using ::wcscpy _LIBCPP_USING_IF_EXISTS;
+using ::wcsncpy _LIBCPP_USING_IF_EXISTS;
+using ::wcscat _LIBCPP_USING_IF_EXISTS;
+using ::wcsncat _LIBCPP_USING_IF_EXISTS;
+using ::wcscmp _LIBCPP_USING_IF_EXISTS;
+using ::wcscoll _LIBCPP_USING_IF_EXISTS;
+using ::wcsncmp _LIBCPP_USING_IF_EXISTS;
+using ::wcsxfrm _LIBCPP_USING_IF_EXISTS;
+using ::wcschr _LIBCPP_USING_IF_EXISTS;
+using ::wcspbrk _LIBCPP_USING_IF_EXISTS;
+using ::wcsrchr _LIBCPP_USING_IF_EXISTS;
+using ::wcsstr _LIBCPP_USING_IF_EXISTS;
+using ::wmemchr _LIBCPP_USING_IF_EXISTS;
+using ::wcscspn _LIBCPP_USING_IF_EXISTS;
+using ::wcslen _LIBCPP_USING_IF_EXISTS;
+using ::wcsspn _LIBCPP_USING_IF_EXISTS;
+using ::wcstok _LIBCPP_USING_IF_EXISTS;
+using ::wmemcmp _LIBCPP_USING_IF_EXISTS;
+using ::wmemcpy _LIBCPP_USING_IF_EXISTS;
+using ::wmemmove _LIBCPP_USING_IF_EXISTS;
+using ::wmemset _LIBCPP_USING_IF_EXISTS;
+using ::wcsftime _LIBCPP_USING_IF_EXISTS;
+using ::btowc _LIBCPP_USING_IF_EXISTS;
+using ::wctob _LIBCPP_USING_IF_EXISTS;
+using ::mbsinit _LIBCPP_USING_IF_EXISTS;
+using ::mbrlen _LIBCPP_USING_IF_EXISTS;
+using ::mbrtowc _LIBCPP_USING_IF_EXISTS;
+using ::wcrtomb _LIBCPP_USING_IF_EXISTS;
+using ::mbsrtowcs _LIBCPP_USING_IF_EXISTS;
+using ::wcsrtombs _LIBCPP_USING_IF_EXISTS;
 
 #ifndef _LIBCPP_HAS_NO_STDIN
-using ::getwchar;
-using ::vwscanf;
-using ::wscanf;
+using ::getwchar _LIBCPP_USING_IF_EXISTS;
+using ::vwscanf _LIBCPP_USING_IF_EXISTS;
+using ::wscanf _LIBCPP_USING_IF_EXISTS;
 #endif
 
 #ifndef _LIBCPP_HAS_NO_STDOUT
-using ::putwchar;
-using ::vwprintf;
-using ::wprintf;
+using ::putwchar _LIBCPP_USING_IF_EXISTS;
+using ::vwprintf _LIBCPP_USING_IF_EXISTS;
+using ::wprintf _LIBCPP_USING_IF_EXISTS;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CWCHAR

libcxx/include/cwctype

 #include <wctype.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::wint_t;
-using ::wctrans_t;
-using ::wctype_t;
-using ::iswalnum;
-using ::iswalpha;
-using ::iswblank;
-using ::iswcntrl;
-using ::iswdigit;
-using ::iswgraph;
-using ::iswlower;
-using ::iswprint;
-using ::iswpunct;
-using ::iswspace;
-using ::iswupper;
-using ::iswxdigit;
-using ::iswctype;
-using ::wctype;
-using ::towlower;
-using ::towupper;
-using ::towctrans;
-using ::wctrans;
+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