[libc++][RFC] Always define internal feature test macros (#89178)
Currently, the library-internal feature test macros are only defined if
the feature is not available, and always have the prefix
`_LIBCPP_HAS_NO_`. This patch changes that, so that they are always
defined and have the prefix `_LIBCPP_HAS_` instead. This changes the
canonical use of these macros to `#if _LIBCPP_HAS_FEATURE`, which means
that using an undefined macro (e.g. due to a missing include) is
diagnosed now. While this is rather unlikely currently, a similar change
in `<__configuration/availability.h>` caught a few bugs. This also
improves readability, since it removes the double-negation of `#ifndef
_LIBCPP_HAS_NO_FEATURE`.
The current patch only touches the macros defined in `<__config>`. If
people are happy with this approach, I'll make a follow-up PR to also
change the macros defined in `<__config_site>`.
diff --git a/libcxx/include/__string/char_traits.h b/libcxx/include/__string/char_traits.h
index 9b7d567..107f0a9 100644
--- a/libcxx/include/__string/char_traits.h
+++ b/libcxx/include/__string/char_traits.h
@@ -257,7 +257,7 @@
};
#endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
-#ifndef _LIBCPP_HAS_NO_CHAR8_T
+#if _LIBCPP_HAS_CHAR8_T
template <>
struct _LIBCPP_TEMPLATE_VIS char_traits<char8_t>
@@ -277,7 +277,7 @@
}
};
-#endif // _LIBCPP_HAS_NO_CHAR8_T
+#endif // _LIBCPP_HAS_CHAR8_T
template <>
struct _LIBCPP_TEMPLATE_VIS char_traits<char16_t>
