[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
NOKEYCHECK=True
GitOrigin-RevId: 8ff2d6af6906261567d8c10be62711ce899fb485
diff --git a/include/regex b/include/regex
index 8361b41..3899892 100644
--- a/include/regex
+++ b/include/regex
@@ -778,7 +778,7 @@
#include <vector>
#include <version>
-#ifndef _LIBCPP_REMOVE_TRANSITIVE_INCLUDES
+#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
# include <iterator>
# include <utility>
#endif