[libc++] Reduces the number of transitive includes.

This defines a new policy for removal of transitive includes.
The goal of the policy it to make it relatively easy to remove
headers when needed, but avoid breaking developers using and
vendors shipping libc++.

The method used is to guard transitive includes based on the
C++ language version. For the upcoming C++23 we can remove
headers when we want, but for other language versions we try
to keep it to a minimum.

In this code the transitive include of `<chrono>` is removed
since D128577 introduces a header cycle between `<format>`
and `<chrono>`. This cycle is indirectly required by the
Standard. Our cycle dependency tool basically is a grep based
tool, so it needs some hints to ignore cycles. With the input
of our transitive include tests we can create a better tool.
However that's out of the scope of this patch.

Note the flag `_LIBCPP_REMOVE_TRANSITIVE_INCLUDES` remains
unchanged. So users can still opt-out of transitives includes
entirely.

Reviewed By: #libc, ldionne, philnik

Differential Revision: https://reviews.llvm.org/D132284

1 files changed, 2 insertions, 2 deletions

diff --git a/libcxx/include/any b/libcxx/include/any
index b256630..cd704a9 100644
--- a/libcxx/include/any
+++ b/libcxx/include/any
@@ -94,8 +94,8 @@ namespace std {
 #include <typeinfo>
 #include <version>
 
-#ifndef _LIBCPP_REMOVE_TRANSITIVE_INCLUDES
-#  include <chrono>
+#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 17
+#  include <chrono> // IGNORE-CYCLE due to <format>
 #endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)