diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -880,6 +880,12 @@
 #  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) \
diff --git a/libcxx/include/cctype b/libcxx/include/cctype
--- a/libcxx/include/cctype
+++ b/libcxx/include/cctype
@@ -100,20 +100,20 @@
 #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
 
diff --git a/libcxx/include/cfenv b/libcxx/include/cfenv
--- a/libcxx/include/cfenv
+++ b/libcxx/include/cfenv
@@ -61,20 +61,20 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::fenv_t;
-using ::fexcept_t;
-
-using ::feclearexcept;
-using ::fegetexceptflag;
-using ::feraiseexcept;
-using ::fesetexceptflag;
-using ::fetestexcept;
-using ::fegetround;
-using ::fesetround;
-using ::fegetenv;
-using ::feholdexcept;
-using ::fesetenv;
-using ::feupdateenv;
+using ::fenv_t _LIBCPP_USING_IF_EXISTS;
+using ::fexcept_t _LIBCPP_USING_IF_EXISTS;
+
+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
 
diff --git a/libcxx/include/cinttypes b/libcxx/include/cinttypes
--- a/libcxx/include/cinttypes
+++ b/libcxx/include/cinttypes
@@ -244,13 +244,13 @@
 
 _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
 
diff --git a/libcxx/include/clocale b/libcxx/include/clocale
--- a/libcxx/include/clocale
+++ b/libcxx/include/clocale
@@ -43,11 +43,11 @@
 
 _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
 
diff --git a/libcxx/include/cmath b/libcxx/include/cmath
--- a/libcxx/include/cmath
+++ b/libcxx/include/cmath
@@ -318,215 +318,215 @@
 
 _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); }
diff --git a/libcxx/include/csetjmp b/libcxx/include/csetjmp
--- a/libcxx/include/csetjmp
+++ b/libcxx/include/csetjmp
@@ -39,8 +39,8 @@
 
 _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
 
diff --git a/libcxx/include/csignal b/libcxx/include/csignal
--- a/libcxx/include/csignal
+++ b/libcxx/include/csignal
@@ -48,9 +48,9 @@
 
 _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
 
diff --git a/libcxx/include/cstdarg b/libcxx/include/cstdarg
--- a/libcxx/include/cstdarg
+++ b/libcxx/include/cstdarg
@@ -40,7 +40,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::va_list;
+using ::va_list _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/cstddef b/libcxx/include/cstddef
--- a/libcxx/include/cstddef
+++ b/libcxx/include/cstddef
@@ -46,11 +46,11 @@
 
 _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 }; };
diff --git a/libcxx/include/cstdint b/libcxx/include/cstdint
--- a/libcxx/include/cstdint
+++ b/libcxx/include/cstdint
@@ -149,41 +149,41 @@
 
 _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::intptr_t;
-using::uintptr_t;
-
-using::intmax_t;
-using::uintmax_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 _LIBCPP_USING_IF_EXISTS;
+using ::uintptr_t _LIBCPP_USING_IF_EXISTS;
+
+using ::intmax_t _LIBCPP_USING_IF_EXISTS;
+using ::uintmax_t _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/cstdio b/libcxx/include/cstdio
--- a/libcxx/include/cstdio
+++ b/libcxx/include/cstdio
@@ -104,70 +104,70 @@
 
 _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
diff --git a/libcxx/include/cstdlib b/libcxx/include/cstdlib
--- a/libcxx/include/cstdlib
+++ b/libcxx/include/cstdlib
@@ -96,66 +96,66 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::size_t;
-using ::div_t;
-using ::ldiv_t;
+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;
+using ::lldiv_t _LIBCPP_USING_IF_EXISTS;
 #endif // _LIBCPP_HAS_NO_LONG_LONG
-using ::atof;
-using ::atoi;
-using ::atol;
+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;
+using ::atoll _LIBCPP_USING_IF_EXISTS;
 #endif // _LIBCPP_HAS_NO_LONG_LONG
-using ::strtod;
-using ::strtof;
-using ::strtold;
-using ::strtol;
+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;
+using ::strtoll _LIBCPP_USING_IF_EXISTS;
 #endif // _LIBCPP_HAS_NO_LONG_LONG
-using ::strtoul;
+using ::strtoul _LIBCPP_USING_IF_EXISTS;
 #ifndef _LIBCPP_HAS_NO_LONG_LONG
-using ::strtoull;
+using ::strtoull _LIBCPP_USING_IF_EXISTS;
 #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 ::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;
+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;
+using ::llabs _LIBCPP_USING_IF_EXISTS;
 #endif // _LIBCPP_HAS_NO_LONG_LONG
-using ::div;
-using ::ldiv;
+using ::div _LIBCPP_USING_IF_EXISTS;
+using ::ldiv _LIBCPP_USING_IF_EXISTS;
 #ifndef _LIBCPP_HAS_NO_LONG_LONG
-using ::lldiv;
+using ::lldiv _LIBCPP_USING_IF_EXISTS;
 #endif // _LIBCPP_HAS_NO_LONG_LONG
-using ::mblen;
-using ::mbtowc;
-using ::wctomb;
-using ::mbstowcs;
-using ::wcstombs;
+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
diff --git a/libcxx/include/cstring b/libcxx/include/cstring
--- a/libcxx/include/cstring
+++ b/libcxx/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
 
diff --git a/libcxx/include/ctime b/libcxx/include/ctime
--- a/libcxx/include/ctime
+++ b/libcxx/include/ctime
@@ -68,26 +68,26 @@
 
 _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
diff --git a/libcxx/include/cwchar b/libcxx/include/cwchar
--- a/libcxx/include/cwchar
+++ b/libcxx/include/cwchar
@@ -112,79 +112,79 @@
 
 _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
diff --git a/libcxx/include/cwctype b/libcxx/include/cwctype
--- a/libcxx/include/cwctype
+++ b/libcxx/include/cwctype
@@ -59,27 +59,27 @@
 
 _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