ba87515fea
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>`.
The "module partitions" for the std module
The files in this directory contain the exported named declarations per header. These files are used for the following purposes:
- During testing exported named declarations are tested against the named declarations in the associated header. This excludes reserved names; they are not exported.
- Generate the module std.
These use cases require including the required headers for these "partitions" at different locations. This means the user of these "partitions" are responsible for including the proper header and validating whether the header can be loaded in the current libc++ configuration. For example "include " fails when locales are not available. The "partitions" use the libc++ feature macros to export the declarations available in the current configuration. This configuration is available if the user includes the `__config' header.
We use .inc files that we include from the top-level module instead of
using real C++ module partitions. This is a lot faster than module partitions,
see this for details.