[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/src/stdexcept.cpp b/libcxx/src/stdexcept.cpp
index 134d28e..0ee438b 100644
--- a/libcxx/src/stdexcept.cpp
+++ b/libcxx/src/stdexcept.cpp
@@ -20,7 +20,7 @@
_LIBCPP_BEGIN_NAMESPACE_STD
void __throw_runtime_error(const char* msg) {
-#ifndef _LIBCPP_HAS_NO_EXCEPTIONS
+#if _LIBCPP_HAS_EXCEPTIONS
throw runtime_error(msg);
#else
_LIBCPP_VERBOSE_ABORT("runtime_error was thrown in -fno-exceptions mode with message \"%s\"", msg);
