Allow temporary string_view((const char*)nullptr) while cleanup occurs.

[EricWF]

Summary

Google is working to clean up a local patch to libc++ that allows
string_view to be constructed from null pointers. In order to do this we
need to intercept the call to char_traits::length. This temporary change
allows google to live at head with libc++ while it cleans up
non-conforming code.

[EricWF]

• Make code compile (Spell the parameter name correctly)

[ldionne]

I'll go ahead and land D121231, and we can land this patch on top if it makes your life easier in terms of upstream/downstream maintenance.

libcxx/include/string_view

320

With this patch, we are not asserting that __s != nullptr. I'd be OK with the patch if that were addressed. TBH I'm mostly neutral on how we solve this problem, as long as we get rid of the naked call to __libcpp_debug_function and we retain the _LIBCPP_ASSERT(__s != nullptr) upstream.

[ldionne]

I landed my patch last week -- I assume you resolved the merge conflict internally already. If we want to move forward with something like this, let's do it, otherwise, we can abandon the patch to clear up the review queue!

libcxx/include/__config

 #  define _LIBCPP_PACKED_BYTE_FOR_AIX_END  /* empty */
 #endif
 
+// Used to allow Google to transition to std::string_view while cleaning up
+// code that constructs it from `nullptr` -- which is undefined.
+// TODO(EricWF) Remove me!
+#ifndef _LIBCPP_CHAR_TRAITS_LENGTH_HOOK
+#  define _LIBCPP_CHAR_TRAITS_LENGTH_HOOK(_TraitsType, __str) _TraitsType::length(__str)
+#endif
+
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG

libcxx/include/__string

 }
 #endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
 
-template <class _Traits>
-_LIBCPP_INLINE_VISIBILITY
-_LIBCPP_CONSTEXPR
-inline size_t __char_traits_length_checked(const typename _Traits::char_type* __s) _NOEXCEPT {
-#if _LIBCPP_DEBUG_LEVEL >= 1
-  return __s ? _Traits::length(__s) : (_VSTD::__libcpp_debug_function(_VSTD::__libcpp_debug_info(__FILE__, __LINE__, "p == nullptr", "null pointer pass to non-null argument of char_traits<...>::length")), 0);
-#else
-  return _Traits::length(__s);
-#endif
-}
-
 #ifndef _LIBCPP_HAS_NO_CHAR8_T
 
 template <>

libcxx/include/string_view

 
     _LIBCPP_CONSTEXPR _LIBCPP_INLINE_VISIBILITY
     basic_string_view(const _CharT* __s)
-        : __data(__s), __size(_VSTD::__char_traits_length_checked<_Traits>(__s)) {}
+        : __data(__s), __size(_LIBCPP_CHAR_TRAITS_LENGTH_HOOK(_Traits, __s)) {}
 
 #if _LIBCPP_STD_VER > 20
     basic_string_view(nullptr_t) = delete;