Export of internal Abseil changes

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44b312da54263fc7491b5f1e115e49e0c1e2dc10 by Greg Falcon <gfalcon@google.com>:

Import of CCTZ from GitHub.

PiperOrigin-RevId: 315782632

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27618a3b195f75384ba44e9712ae0b0b7d85937e by Greg Falcon <gfalcon@google.com>:

Update Abseil's internal Invoke() implementation to follow C++17 semantics.

Starting in C++17, when invoke'ing a pointer-to-member, if the object representing the class is a reference_wrapper, that wrapper is unpacked.  Because we implement a number of functional APIs that closely match C++ standard proposals, it is better if we follow the standard's notion of what "invoking" means.

This also makes `absl::base_internal::Invoke()` match `std::invoke()` in C++17.  I intend to make this an alias in a follow-up CL.

PiperOrigin-RevId: 315750659

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059519ea402cd55b1b716403bb680504c6ff5808 by Xiaoyi Zhang <zhangxy@google.com>:

Internal change

PiperOrigin-RevId: 315597064

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5e2042c8520576b2508e2bfb1020a97c7db591da by Titus Winters <titus@google.com>:

Update notes on the delta between absl::Span vs. std::span.

PiperOrigin-RevId: 315518942

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9d3875527b93124f5de5d6a1d575c42199fbf323 by Abseil Team <absl-team@google.com>:

Internal cleanup

PiperOrigin-RevId: 315497633
GitOrigin-RevId: 44b312da54263fc7491b5f1e115e49e0c1e2dc10
Change-Id: I24573f317c8388bd693c0fdab395a7dd419b33b0

1 files changed, 5 insertions, 10 deletions

diff --git a/absl/types/span.h b/absl/types/span.h
index 734db695..4e450fc9 100644
--- a/absl/types/span.h
+++ b/absl/types/span.h
@@ -27,22 +27,17 @@
 // `Span<const T>` when such types may be difficult to identify due to issues
 // with implicit conversion.
 //
-// The C++ standards committee currently has a proposal for a `std::span` type,
-// (http://wg21.link/p0122), which is not yet part of the standard (though may
-// become part of C++20). As of August 2017, the differences between
-// `absl::Span` and this proposal are:
-//    * `absl::Span` uses `size_t` for `size_type`
-//    * `absl::Span` has no `operator()`
-//    * `absl::Span` has no constructors for `std::unique_ptr` or
-//      `std::shared_ptr`
+// The C++20 draft standard includes a `std::span` type. As of June 2020, the
+// differences between `absl::Span` and `std::span` are:
+//    * `absl::Span` has `operator==` (which is likely a design bug,
+//       per https://abseil.io/blog/20180531-regular-types)
 //    * `absl::Span` has the factory functions `MakeSpan()` and
 //      `MakeConstSpan()`
-//    * `absl::Span` has `front()` and `back()` methods
 //    * bounds-checked access to `absl::Span` is accomplished with `at()`
 //    * `absl::Span` has compiler-provided move and copy constructors and
 //      assignment. This is due to them being specified as `constexpr`, but that
 //      implies const in C++11.
-//    * `absl::Span` has no `element_type` or `index_type` typedefs
+//    * `absl::Span` has no `element_type` typedef
 //    * A read-only `absl::Span<const T>` can be implicitly constructed from an
 //      initializer list.
 //    * `absl::Span` has no `bytes()`, `size_bytes()`, `as_bytes()`, or