merge(3p/absl): subtree merge of Abseil up to e19260f r/1889

... notably, this includes Abseil's own StatusOr type, which
conflicted with our implementation (that was taken from TensorFlow).

Change-Id: Ie7d6764b64055caaeb8dc7b6b9d066291e6b538f

1 files changed, 7 insertions, 1 deletions

diff --git a/third_party/abseil_cpp/absl/container/flat_hash_map.h b/third_party/abseil_cpp/absl/container/flat_hash_map.h
index fcb70d861f39..74def0df0e33 100644
--- a/third_party/abseil_cpp/absl/container/flat_hash_map.h
+++ b/third_party/abseil_cpp/absl/container/flat_hash_map.h
@@ -234,7 +234,8 @@ class flat_hash_map : public absl::container_internal::raw_hash_map<
   //
   // size_type erase(const key_type& key):
   //
-  //   Erases the element with the matching key, if it exists.
+  //   Erases the element with the matching key, if it exists, returning the
+  //   number of elements erased (0 or 1).
   using Base::erase;
 
   // flat_hash_map::insert()
@@ -383,6 +384,11 @@ class flat_hash_map : public absl::container_internal::raw_hash_map<
   //   key value and returns a node handle owning that extracted data. If the
   //   `flat_hash_map` does not contain an element with a matching key, this
   //   function returns an empty node handle.
+  //
+  // NOTE: when compiled in an earlier version of C++ than C++17,
+  // `node_type::key()` returns a const reference to the key instead of a
+  // mutable reference. We cannot safely return a mutable reference without
+  // std::launder (which is not available before C++17).
   using Base::extract;
 
   // flat_hash_map::merge()