commit 833f924d62810fb72e463f65538774e7f7deeae3
author Mark de Wever <koraq@xs4all.nl> Fri Jul 30 14:35:37 2021 -0400
committer Copybara-Service <copybara-worker@google.com> Fri Jul 30 18:28:12 2021 -0700
tree 867ecc7e48805bc3ab018dd4518a8b95443e1e19
parent 8a03e0573f6dea1fe4ca9f1de05897bc3007f587

[libc++] Improve LIBCXX_ENABLE_INCOMPLETE_FEATURES.

@tcanens pointed out the current behavior of the macro breaks the usage
pattern described in http://wg21.link/SD6
```
#  if __has_include(<optional>)
#    include <optional>
#    if __cpp_lib_optional >= 201606
#      define have_optional 1
#    endif
```

To support this usage pattern the hard errror is removed. Instead the
header includes nothing but the `<version>` header.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D107134

GitOrigin-RevId: d618a1cc5c3919d29ee27fa48de80bed4dcb3566