_LIBCPP_BEGIN_NAMESPACE_STD
+#if defined(_LIBCPP_INCLUDED_C_LIBRARY_WCTYPE_H)
using ::wint_t _LIBCPP_USING_IF_EXISTS;
using ::wctrans_t _LIBCPP_USING_IF_EXISTS;
using ::wctype_t _LIBCPP_USING_IF_EXISTS;
using ::towupper _LIBCPP_USING_IF_EXISTS;
using ::towctrans _LIBCPP_USING_IF_EXISTS;
using ::wctrans _LIBCPP_USING_IF_EXISTS;
+#endif // _LIBCPP_INCLUDED_C_LIBRARY_WCTYPE_H
_LIBCPP_END_NAMESPACE_STD
#pragma GCC system_header
#endif
+// TODO:
+// In the future, we should unconditionally include_next <wctype.h> here and instead
+// have a mode under which the library does not need libc++'s <wctype.h> or <cwctype>
+// at all (i.e. a mode without wchar_t). As it stands, we need to do that to completely
+// bypass the using declarations in <cwctype> when we did not include <wctype.h>.
+// Otherwise, a using declaration like `using ::wint_t` in <cwctype> will refer to
+// nothing (with using_if_exists), and if we include another header that defines one
+// of these declarations (e.g. <wchar.h>), the second `using ::wint_t` with using_if_exists
+// will fail because it does not refer to the same declaration.
#if __has_include_next(<wctype.h>)
# include_next <wctype.h>
+# define _LIBCPP_INCLUDED_C_LIBRARY_WCTYPE_H
#endif
#ifdef __cplusplus