[libcxx] Slightly improved policy for handling experimental features

[ldionne]

Summary

Following the discussion on the libcxx-dev mailing list
(http://lists.llvm.org/pipermail/libcxx-dev/2019-May/000358.html),
this implements the new policy for handling experimental features and
their deprecation. We basically add a deprecation warning for
std::experimental::filesystem, and we remove a bunch of <experimental/*>
headers that were now empty.

[ldionne]

Basically, we're comfortable with giving this a try and seeing whether we still get into trouble. If so, we might push again for a -fexperimental flag, however for now the simple approach taken here might be sufficient.

Also note that we might want a -fexperimental flag just to control whether -lc++experimental is being linked, however this is left as a separate change if we decide we want to do it.

[daphnediane]

Not all package systems are good about cleaning up removed headers so the approach given by this patch could result in deprecated headers being left behind after an update instead of removed.

I wonder if something should be done with __has_include also as it seems like this could be a more generic problem for deprecated headers for more than just the TS experimental headers. Maybe have __has_include immediately check after finding a matching file if a non-empty file with the suffix replaced with .rmh exists in the same path evaluate to zero. Likewise if #include ( but not #include_next ) falls to find a file it could search for a the file with the suffix replaced with .rmh and if found print an error message about a deprecated header possibly including some content from the .rmh header with details. This would allow a header to be deprecated but still give useful messages if used while not breaking __has_include. It would also help with cases where an updated library was installed where headers were removed and the old headers did not get rewritten with an installed package.

Edit: Tweaked wording, avoiding __ in __has_include acting as markup.

[ldionne]

In D62428#1518405, @daphnediane wrote:

Not all package systems are good about cleaning up removed headers so the approach given by this patch could result in deprecated headers being left behind after an update instead of removed.

Thanks a lot for raising this point.

However, I'll admit that I'm personally uncomfortable with working around broken package managers with that sort of hack, unless we have an extremely compelling reason to do it (e.g. an important package manager doesn't do it, and can't be fixed because of <insert reasons>).

[mclow.lists]

The deletions are fine, the doc updates are good, but I think deprecation warnings are not.
Things that are going to happen in the future (a year from now, say) or never (if people don't update their tools) don't belong in the warning spew for every single build

[jfb]

In D62428#1518488, @mclow.lists wrote:

The deletions are fine, the doc updates are good, but I think deprecation warnings are not.
Things that are going to happen in the future (a year from now, say) or never (if people don't update their tools) don't belong in the warning spew for every single build

Similar deprecation warnings are pretty common coming from clang (both for standard things as well as extensions / builtins). What Louis proposes seems consistent with what the compiler does. Do you think there's a better approach that libc++ could take?

[jfb]

libcxx/docs/DesignDocs/ExperimentalFeatures.rst

26

I generally capitalize "Standard".

40

I think you want to specify: when implemented fully. Sometimes we can ship something that's not complete yet, and we can't start the deprecation clock until the implementation is complete.

47

2 releases of LLVM? What do we call "open source public Official LLVM"?

140

toto africa?

libcxx/docs/UsingLibcxx.rst

89

Link to the policy from here?

[ldionne]

In D62428#1518488, @mclow.lists wrote:

The deletions are fine, the doc updates are good, but I think deprecation warnings are not.
Things that are going to happen in the future (a year from now, say) or never (if people don't update their tools) don't belong in the warning spew for every single build

Okay, so we've had offline discussions about this, and the general idea I got is that you would prefer that these warnings be in a tool like clang-tidy, that users can run only when they so desire. Please correct me if that's incorrect.

However, the key thing here is that I would like for these warnings to be opt-OUT, not opt-IN. This is really important for me, as otherwise most users are not even aware of the situation and they don't turn the warnings on (aka run the tool). Hence, using clang-tidy or any similar solution that is NOT in the mandatory compilation workflow is not sufficient.

Furthermore, I think this is, at the end of the day, a vendor decision. I don't think having these warnings (or not having them) makes much of a difference for libc++ as an open source project, however it makes a big difference for vendors. I feel like this issue is important enough that we would do this downstream if we were only given that choice. However, one thing I would suggest in order to resolve the stalemate is to guard these deprecation warnings behind a CMake configuration option (for example). That way, vendors can decide whether they want their libc++ to contain the warnings or not, and upstream libc++ can decide to do differently. The downside with this solution is that we add yet another configuration that must be tested, but it's better than nothing.

@mclow.lists How do you feel about that compromise?

[mclow.lists]

In D62428#1519664, @jfb wrote:

In D62428#1518488, @mclow.lists wrote:

The deletions are fine, the doc updates are good, but I think deprecation warnings are not.
Things that are going to happen in the future (a year from now, say) or never (if people don't update their tools) don't belong in the warning spew for every single build

Similar deprecation warnings are pretty common coming from clang (both for standard things as well as extensions / builtins). What Louis proposes seems consistent with what the compiler does. Do you think there's a better approach that libc++ could take?

I think they're wrong there, too.
I've enumerated why I believe so on the ML, and in Slack (several times), so I don't see the point of doing it again here.
[ Though I will note that no one has argued that my reasons are incorrect or invalid. ]

[dexonsmith]

In D62428#1525189, @mclow.lists wrote:

In D62428#1519664, @jfb wrote:

In D62428#1518488, @mclow.lists wrote:

The deletions are fine, the doc updates are good, but I think deprecation warnings are not.
Things that are going to happen in the future (a year from now, say) or never (if people don't update their tools) don't belong in the warning spew for every single build

I don't understand your point here Marshall. Why would this cause spew for every build? Users can choose to change their code, or to opt-out of the diagnostic.

If this opt-out mechanism isn't quite right, we can invent new ones. For example, we could use __attribute__((deprecated)) or extend __attribute__((availability)) somehow so that the diagnostics are bucketed under -Wdeprecated. We could create a special -Wdeprecated-experimental diagnostic so users can opt-out of (or -in to, via -Wno-deprecated -Wdeprecated-experimental) just those.

Similar deprecation warnings are pretty common coming from clang (both for standard things as well as extensions / builtins). What Louis proposes seems consistent with what the compiler does. Do you think there's a better approach that libc++ could take?

I think they're wrong there, too.
I've enumerated why I believe so on the ML, and in Slack (several times), so I don't see the point of doing it again here.
[ Though I will note that no one has argued that my reasons are incorrect or invalid. ]

I think everyone respects your opinion, and there isn't only one right answer here. I can understand why some vendors would prefer to relegate deprecation notices entirely to release notes, or to make them opt-in. I can also understand why some users would choose to opt-out of all deprecation notices (e.g., via -Wno-deprecated).

As a vendor, we want our users to know immediately about deprecations so that they have maximum time to plan how to update their code. We want users to be able to suppress deprecation warnings (a) outright, in case they don't want this kind of notice, or (b) temporarily, to avoid noise their builds while they make plans for how to update them.

It's clear that you personally disagree with this policy, and I want to acknowledge that that's a valid opinion. Nevertheless, given that this policy is fully supported by Clang and we've followed it in Libc++ in the past (e.g., when we added diagnostics to the libstdc++ compatibility headers), it's surprising that you feel so strongly that you might block this entirely, even as a vendor configuration option.

I'm curious, as a thought experiment, let's say you agreed to support vendors that wanted to continue with this policy. Do you have thoughts on this way of configuring whether vendors adopt it? Do have you have ideas for different or better opt-out mechanisms (through some combination of -W or -D flags, or otherwise)?

libcxx/include/experimental/filesystem

241 ↗

(On Diff #201336)

IMO, we should find a way to allow users to opt-out via -Wno-deprecated, in addition to (or instead of) macros.

[ldionne]

Folks, I just spoke to Marshall over the phone and I believe one of the reasons for the opposition is that I didn't make it clear we were not warning on ALL uses of experimental features. The new proposal only adds deprecation warnings when we standardize a feature and we know FOR SURE we're going to remove the experimental version in the near future. I didn't make that change from the initial proposal clear, and I apologize for that. With that thing being clear, the proposal seemed to be more reasonable.

Now, it's not to say that this clears out ALL opposition. We agreed to hold a wider-audience discussion about the place of warnings (and deprecation warnings in particular) inside the compiler versus a separate tool (e.g. clang-tidy). I'm very much looking forward to that discussion because right now we only have a hammer (the compiler), and as a result everything looks like a nail. However, we decided not to make that discussion a prerequisite for the policy that is the object of this patch.

Marshall, I'm looking forward to your comments on the proposal, and to the discussion on cfe-dev about deprecation warnings in the compiler in general!

[mclow.lists]

In D62428#1525385, @ldionne wrote:

Folks, I just spoke to Marshall over the phone and I believe one of the reasons for the opposition is that I didn't make it clear we were not warning on ALL uses of experimental features. The new proposal only adds deprecation warnings when we standardize a feature and we know FOR SURE we're going to remove the experimental version in the near future. I didn't make that change from the initial proposal clear, and I apologize for that. With that thing being clear, the proposal seemed to be more reasonable.

I still think that this is the wrong place for this functionality, but this solves a problem today, and we can fix it "better" later.
P.S. I like @dexonsmith 's suggestion to hook into -Wno-deprecated somehow.

[ldionne]

Address reviewer's comments

[ldionne]

libcxx/docs/DesignDocs/ExperimentalFeatures.rst

47

I think saying "2 releases of LLVM" is unambiguous, as vendors are downstream (and hence in some sense invisible to the eyes of this document).

libcxx/include/experimental/filesystem

241 ↗

(On Diff #201336)

We definitely want to allow users to disable warnings for each feature, because otherwise when they disable the warnings for one feature, they stop getting warnings for all other experimental features, which defeats the purpose of this proposal.

However, using the [[deprecated]] attribute does make sense since it _allows_ users to turn the warnings off globally if they want to do so, but they can also turn off just the warnings they want.

libcxx/docs/DesignDocs/ExperimentalFeatures.rst

+=====================
+Experimental Features
+=====================
+
+.. contents::
+   :local:
+
+.. _experimental features:
+
+Overview
+========
+
+Libc++ implements technical specifications (TSes) and ships them as experimental
+features that users are free to try out. The goal is to allow getting feedback
+on those experimental features.
+
+However, libc++ does not provide the same guarantees about those features as
+it does for the rest of the library. In particular, no ABI or API stability
+is guaranteed, and experimental features are deprecated once the non-experimental
+equivalent has shipped in the library. This document outlines the details of
+that process.
+
+Background
+==========
+
+The "end game" of a Technical Specification (TS) is to have the features in
+there added to a future version of the C++ Standard. When this happens, the TS
+can be retired. Sometimes, only part of at TS is added to the standard, and
+the rest of the features may be incorporated into the next version of the TS.
+
+Adoption leaves library implementors with two implementations of a feature,
+one in namespace ``std``, and the other in namespace ``std::experimental``.
+The first one will continue to evolve (via issues and papers), while the other
+will not. Gradually they will diverge. It's not good for users to have two
+(subtly) different implementations of the same functionality in the same library.
+
+Design
+======
+
+When a feature is adopted into the main standard, we implement it in namespace
+``std``. Once that implementation is complete, we then create a deprecation
+warning for the corresponding experimental feature warning users to move off
+of it and to the now-standardized feature.
+
+These deprecation warnings are guarded by a macro of the form
+``_LIBCPP_NO_EXPERIMENTAL_DEPRECATION_WARNING_<FEATURE>``, which
+can be defined by users to disable the deprecation warning. Whenever
+possible, deprecation warnings are put on a per-declaration basis
+using the ``[[deprecated]]`` attribute, which also allows disabling
+the warnings using ``-Wno-deprecated-declarations``.
+
+After **2 releases** of LLVM, the experimental feature is removed completely
+(and the deprecation notice too). Using the experimental feature simply becomes
+an error. Furthermore, when an experimental header becomes empty due to the
+removal of the corresponding experimental feature, the header is removed.
+Keeping the header around creates incorrect assumptions from users and breaks
+``__has_include``.
+
+
+Status of TSes
+==============
+
+Library Fundamentals TS `V1 <https://wg21.link/N4480>`__ and `V2 <https://wg21.link/N4617>`__
+---------------------------------------------------------------------------------------------
+
+Most (but not all) of the features of the LFTS were accepted into C++17.
+
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| Section | Feature                                               | Shipped in ``std`` | To be removed from ``std::experimental`` | Notes                   |
++=========+=======================================================+====================+==========================================+=========================+
+| 2.1     | ``uses_allocator construction``                       | 5.0                | 7.0                                      |                         |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 3.1.2   | ``erased_type``                                       |                    | n/a                                      | Not part of C++17       |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 3.2.1   | ``tuple_size_v``                                      | 5.0                | 7.0                                      | Removed                 |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 3.2.2   | ``apply``                                             | 5.0                | 7.0                                      | Removed                 |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 3.3.1   | All of the ``_v`` traits in ``<type_traits>``         | 5.0                | 7.0                                      | Removed                 |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 3.3.2   | ``invocation_type`` and ``raw_invocation_type``       |                    | n/a                                      | Not part of C++17       |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 3.3.3   | Logical operator traits                               | 5.0                | 7.0                                      | Removed                 |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 3.3.3   | Detection Idiom                                       | 5.0                |                                          | Only partially in C++17 |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 3.4.1   | All of the ``_v`` traits in ``<ratio>``               | 5.0                | 7.0                                      | Removed                 |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 3.5.1   | All of the ``_v`` traits in ``<chrono>``              | 5.0                | 7.0                                      | Removed                 |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 3.6.1   | All of the ``_v`` traits in ``<system_error>``        | 5.0                | 7.0                                      | Removed                 |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 3.7     | ``propagate_const``                                   |                    | n/a                                      | Not part of C++17       |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 4.2     | Enhancements to ``function``                          | Not yet            |                                          |                         |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 4.3     | searchers                                             | 7.0                | 9.0                                      |                         |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 5       | optional                                              | 5.0                | 7.0                                      | Removed                 |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 6       | ``any``                                               | 5.0                | 7.0                                      | Removed                 |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 7       | ``string_view``                                       | 5.0                | 7.0                                      | Removed                 |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 8.2.1   | ``shared_ptr`` enhancements                           | Not yet            | Never added                              |                         |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 8.2.2   | ``weak_ptr`` enhancements                             | Not yet            | Never added                              |                         |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 8.5     | ``memory_resource``                                   | Not yet            |                                          |                         |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 8.6     | ``polymorphic_allocator``                             | Not yet            |                                          |                         |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 8.7     | ``resource_adaptor``                                  |                    | n/a                                      | Not part of C++17       |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 8.8     | Access to program-wide ``memory_resource`` objects    | Not yet            |                                          |                         |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 8.9     | Pool resource classes                                 | Not yet            |                                          |                         |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 8.10    | ``monotonic_buffer_resource``                         | Not yet            |                                          |                         |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 8.11    | Alias templates using polymorphic memory resources    | Not yet            |                                          |                         |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 8.12    | Non-owning pointers                                   |                    | n/a                                      | Not part of C++17       |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 11.2    | ``promise``                                           |                    | n/a                                      | Not part of C++17       |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 11.3    | ``packaged_task``                                     |                    | n/a                                      | Not part of C++17       |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 12.2    | ``search``                                            | 7.0                | 9.0                                      |                         |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 12.3    | ``sample``                                            | 5.0                | 7.0                                      | Removed                 |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 12.4    | ``shuffle``                                           |                    |                                          | Not part of C++17       |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 13.1    | ``gcd`` and ``lcm``                                   | 5.0                | 7.0                                      | Removed                 |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 13.2    | Random number generation                              |                    |                                          | Not part of C++17       |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+| 14      | Reflection Library                                    |                    |                                          | Not part of C++17       |
++---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+
+
+`FileSystem TS <https://wg21.link/N4100>`__
+-------------------------------------------
+The FileSystem TS was accepted (in totality) for C++17.
+The FileSystem TS implementation was shipped in namespace ``std`` in LLVM 7.0, and will be removed in LLVM 11.0 (due to the lack of deprecation warnings before LLVM 9.0).
+
+Parallelism TS `V1 <https://wg21.link/N4507>`__ and `V2 <https://wg21.link/N4706>`__
+------------------------------------------------------------------------------------
+Some (most) of the Parallelism TS was accepted for C++17.
+We have not yet shipped an implementation of the Parallelism TS.
+
+`Coroutines TS <https://wg21.link/N4680>`__
+-------------------------------------------
+The Coroutines TS is not yet part of a shipping standard.
+We are shipping (as of v5.0) an implementation of the Coroutines TS in namespace ``std::experimental``.
+
+`Networking TS <https://wg21.link/N4656>`__
+-------------------------------------------
+The Networking TS is not yet part of a shipping standard.
+We have not yet shipped an implementation of the Networking TS.
+
+`Ranges TS <https://wg21.link/N4685>`__
+---------------------------------------
+The Ranges TS is not yet part of a shipping standard.
+We have not yet shipped an implementation of the Ranges TS.
+
+`Concepts TS <https://wg21.link/N4641>`__
+-----------------------------------------
+The Concepts TS is not yet part of a shipping standard, but it has been adopted into the C++20 working draft.
+We have not yet shipped an implementation of the Concepts TS.
+
+`Concurrency TS <https://wg21.link/P0159>`__
+--------------------------------------------
+The Concurrency TS was adopted in Kona (2015).
+None of the Concurrency TS was accepted for C++17.
+We have not yet shipped an implementation of the Concurrency TS.
+
+.. +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+.. | Section | Feature                                               | Shipped in ``std`` | To be removed from ``std::experimental`` | Notes                   |
+.. +=========+=======================================================+====================+==========================================+=========================+
+.. | 2.3     | class template ``future``                             |                    |                                          |                         |
+.. +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+.. | 2.4     | class template ``shared_future``                      |                    |                                          |                         |
+.. +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+.. | 2.5     | class template ``promise``                            |                    |                                          | Only using ``future``   |
+.. +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+.. | 2.6     | class template ``packaged_task``                      |                    |                                          | Only using ``future``   |
+.. +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+.. | 2.7     | function template ``when_all``                        |                    |                                          | Not part of C++17       |
+.. +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+.. | 2.8     | class template ``when_any_result``                    |                    |                                          | Not part of C++17       |
+.. +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+.. | 2.9     | function template ``when_any``                        |                    |                                          | Not part of C++17       |
+.. +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+.. | 2.10    | function template ``make_ready_future``               |                    |                                          | Not part of C++17       |
+.. +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+.. | 2.11    | function template ``make_exeptional_future``          |                    |                                          | Not part of C++17       |
+.. +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+.. | 3       | ``latches`` and ``barriers``                          |                    |                                          | Not part of C++17       |
+.. +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
+.. | 4       | Atomic Smart Pointers                                 |                    |                                          | Adopted for C++20       |
+.. +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+

libcxx/docs/UsingLibcxx.rst

 Also see the `Experimental Library Implementation Status <http://libcxx.llvm.org/ts1z_status.html>`__
 page.
 
 .. warning::
   Experimental libraries are Experimental.
     * The contents of the ``<experimental/...>`` headers and ``libc++experimental.a``
       library will not remain compatible between versions.
     * No guarantees of API or ABI stability are provided.
+    * When we implement the standardized version of an experimental feature,
+      the experimental feature is removed two releases after the non-experimental
+      version has shipped. The full policy is explained :ref:`here <experimental features>`.
 
 Using libc++ on Linux
 =====================
 
 On Linux libc++ can typically be used with only '-stdlib=libc++'. However
 some libc++ installations require the user manually link libc++abi themselves.
 If you are running into linker errors when using libc++ try adding '-lc++abi'
 to the link line.  For example:

libcxx/docs/index.rst

 
 .. toctree::
    :maxdepth: 1
 
    DesignDocs/AvailabilityMarkup
    DesignDocs/DebugMode
    DesignDocs/CapturingConfigInfo
    DesignDocs/ABIVersioning
+   DesignDocs/ExperimentalFeatures
    DesignDocs/VisibilityMacros
    DesignDocs/ThreadingSupportAPI
    DesignDocs/FileTimeType
    DesignDocs/FeatureTestMacros
 
 * `<atomic> design <http://libcxx.llvm.org/atomic_design.html>`_
 * `<type_traits> design <http://libcxx.llvm.org/type_traits_design.html>`_
 * `Notes by Marshall Clow`__

libcxx/include/CMakeLists.txt

set(files
   cwchar
   cwctype
   deque
   errno.h
   exception
   experimental/__config
   experimental/__memory
   experimental/algorithm
-  experimental/any
-  experimental/chrono
   experimental/coroutine
   experimental/deque
   experimental/filesystem
   experimental/forward_list
   experimental/functional
   experimental/iterator
   experimental/list
   experimental/map
   experimental/memory_resource
-  experimental/numeric
-  experimental/optional
   experimental/propagate_const
-  experimental/ratio
   experimental/regex
   experimental/set
   experimental/simd
   experimental/string
-  experimental/string_view
-  experimental/system_error
-  experimental/tuple
   experimental/type_traits
   experimental/unordered_map
   experimental/unordered_set
   experimental/utility
   experimental/vector
   ext/__hash
   ext/hash_map
   ext/hash_set

libcxx/include/experimental/__config

 #define _LIBCPP_BEGIN_NAMESPACE_LFTS_PMR _LIBCPP_BEGIN_NAMESPACE_LFTS namespace pmr {
 #define _LIBCPP_END_NAMESPACE_LFTS_PMR _LIBCPP_END_NAMESPACE_LFTS }
 #define _VSTD_LFTS_PMR _VSTD_LFTS::pmr
 
 #define _LIBCPP_BEGIN_NAMESPACE_CHRONO_LFTS _LIBCPP_BEGIN_NAMESPACE_STD        \
   namespace chrono { namespace experimental { inline namespace fundamentals_v1 {
 #define _LIBCPP_END_NAMESPACE_CHRONO_LFTS _LIBCPP_END_NAMESPACE_STD } } }
 
+#if defined(_LIBCPP_NO_EXPERIMENTAL_DEPRECATION_WARNING_FILESYSTEM)
+#   define _LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM /* nothing */
+#else
+#   define _LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM __attribute__((deprecated("std::experimental::filesystem has now been deprecated in favor of C++17's std::filesystem. Please stop using it and start using std::filesystem. This experimental version will be removed in LLVM 11. You can remove this warning by defining the _LIBCPP_NO_EXPERIMENTAL_DEPRECATION_WARNING_FILESYSTEM macro.")))
+#endif
+
 #define _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL_FILESYSTEM \
-    _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL namespace filesystem { \
+    _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL namespace filesystem _LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM { \
     inline namespace v1 {
 
 #define _LIBCPP_END_NAMESPACE_EXPERIMENTAL_FILESYSTEM \
     } } _LIBCPP_END_NAMESPACE_EXPERIMENTAL
 
 #define _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL_COROUTINES \
   _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL inline namespace coroutines_v1 {
 

libcxx/include/experimental/any

-// -*- C++ -*-
-//===------------------------------- any ----------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-#ifndef _LIBCPP_EXPERIMENTAL_ANY
-#define _LIBCPP_EXPERIMENTAL_ANY
-
-#include <__config>
-
-#ifdef _LIBCPP_WARNING
-_LIBCPP_WARNING("<experimental/any> has been removed. Use <any> instead.")
-#else
-# warning "<experimental/any> has been removed. Use <any> instead."
-#endif
-
-#endif // _LIBCPP_EXPERIMENTAL_ANY

libcxx/include/experimental/chrono

-// -*- C++ -*-
-//===---------------------------- chrono ----------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-#ifndef _LIBCPP_EXPERIMENTAL_CHRONO
-#define _LIBCPP_EXPERIMENTAL_CHRONO
-
-#include <__config>
-
-#ifdef _LIBCPP_WARNING
-_LIBCPP_WARNING("<experimental/chrono> has been removed. Use <chrono> instead.")
-#else
-# warning "<experimental/chrono> has been removed. Use <chrono> instead."
-#endif
-
-#endif // _LIBCPP_EXPERIMENTAL_CHRONO

libcxx/include/experimental/numeric

-// -*- C++ -*-
-//===--------------------------- numeric ----------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-#ifndef _LIBCPP_EXPERIMENTAL_NUMERIC
-#define _LIBCPP_EXPERIMENTAL_NUMERIC
-
-#include <__config>
-
-#ifdef _LIBCPP_WARNING
-_LIBCPP_WARNING("<experimental/numeric> has been removed. Use <numeric> instead.")
-#else
-# warning "<experimental/numeric> has been removed. Use <numeric> instead."
-#endif
-
-#endif // _LIBCPP_EXPERIMENTAL_NUMERIC

libcxx/include/experimental/optional

-// -*- C++ -*-
-//===-------------------------- optional ----------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-#ifndef _LIBCPP_EXPERIMENTAL_OPTIONAL
-#define _LIBCPP_EXPERIMENTAL_OPTIONAL
-
-#include <__config>
-
-#ifdef _LIBCPP_WARNING
-_LIBCPP_WARNING("<experimental/optional> has been removed. Use <optional> instead.")
-#else
-# warning "<experimental/optional> has been removed. Use <optional> instead."
-#endif
-
-#endif // _LIBCPP_EXPERIMENTAL_OPTIONAL

libcxx/include/experimental/ratio

-// -*- C++ -*-
-//===----------------------------- ratio ----------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-#ifndef _LIBCPP_EXPERIMENTAL_RATIO
-#define _LIBCPP_EXPERIMENTAL_RATIO
-
-#include <__config>
-
-#ifdef _LIBCPP_WARNING
-_LIBCPP_WARNING("<experimental/ratio> has been removed. Use <ratio> instead.")
-#else
-# warning "<experimental/ratio> has been removed. Use <ratio> instead."
-#endif
-
-#endif // _LIBCPP_EXPERIMENTAL_RATIO

libcxx/include/experimental/string_view

-// -*- C++ -*-
-//===------------------------ string_view ---------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-#ifndef _LIBCPP_EXPERIMENTAL_STRING_VIEW
-#define _LIBCPP_EXPERIMENTAL_STRING_VIEW
-
-#include <__config>
-
-#ifdef _LIBCPP_WARNING
-_LIBCPP_WARNING("<experimental/string_view> has been removed. Use <string_view> instead.")
-#else
-# warning "<experimental/string_view> has been removed. Use <string_view> instead."
-#endif
-
-#endif // _LIBCPP_EXPERIMENTAL_STRING_VIEW

libcxx/include/experimental/system_error

-// -*- C++ -*-
-//===-------------------------- system_error ------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-#ifndef _LIBCPP_EXPERIMENTAL_SYSTEM_ERROR
-#define _LIBCPP_EXPERIMENTAL_SYSTEM_ERROR
-
-#include <__config>
-
-#ifdef _LIBCPP_WARNING
-_LIBCPP_WARNING("<experimental/system_error> has been removed. Use <system_error> instead.")
-#else
-# warning "<experimental/system_error> has been removed. Use <system_error> instead."
-#endif
-
-#endif // _LIBCPP_EXPERIMENTAL_SYSTEM_ERROR

libcxx/include/experimental/tuple

-// -*- C++ -*-
-//===----------------------------- tuple ----------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-#ifndef _LIBCPP_EXPERIMENTAL_TUPLE
-#define _LIBCPP_EXPERIMENTAL_TUPLE
-
-#include <__config>
-
-#ifdef _LIBCPP_WARNING
-_LIBCPP_WARNING("<experimental/tuple> has been removed. Use <tuple> instead.")
-#else
-# warning "<experimental/tuple> has been removed. Use <tuple> instead."
-#endif
-
-#endif // _LIBCPP_EXPERIMENTAL_TUPLE

libcxx/test/libcxx/experimental/diagnostics/syserr/use_header_warning.fail.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// REQUIRES: verify-support
-
-// <experimental/system_error>
-
-#include <experimental/system_error>
-
-// expected-error@experimental/system_error:* {{"<experimental/system_error> has been removed. Use <system_error> instead."}}
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/diagnostics/syserr/version.pass.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// <experimental/system_error>
-
-#ifdef __GNUC__
-#pragma GCC diagnostic ignored "-W#warnings"
-#endif
-#include <experimental/system_error>
-
-#ifndef _LIBCPP_VERSION
-#error _LIBCPP_VERSION not defined
-#endif
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/filesystem/deprecated.fail.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: verify-support
+// UNSUPPORTED: c++98, c++03
+
+// <experimental/filesystem>
+
+#include <experimental/filesystem>
+
+using namespace std::experimental::filesystem; // expected-error {{'filesystem' is deprecated: std::experimental::filesystem has now been deprecated in favor of C++17's std::filesystem. Please stop using it and start using std::filesystem. This experimental version will be removed in LLVM 11. You can remove this warning by defining the _LIBCPP_NO_EXPERIMENTAL_DEPRECATION_WARNING_FILESYSTEM macro.}}
+
+int main(int, char**) {
+  return 0;
+}

libcxx/test/libcxx/experimental/filesystem/version.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // <experimental/filesystem>
 
+#define _LIBCPP_NO_EXPERIMENTAL_DEPRECATION_WARNING_FILESYSTEM
+
 #include <experimental/filesystem>
 
 #ifndef _LIBCPP_VERSION
 #error _LIBCPP_VERSION not defined
 #endif
 
 int main(int, char**)
 {
 
   return 0;
 }

libcxx/test/libcxx/experimental/numerics/numeric.ops/use_header_warning.fail.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// REQUIRES: verify-support
-
-// <experimental/numeric>
-
-#include <experimental/numeric>
-
-// expected-error@experimental/numeric:* {{"<experimental/numeric> has been removed. Use <numeric> instead."}}
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/numerics/numeric.ops/version.pass.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// <experimental/numeric>
-
-#ifdef __GNUC__
-#pragma GCC diagnostic ignored "-W#warnings"
-#endif
-#include <experimental/numeric>
-
-#ifndef _LIBCPP_VERSION
-#error _LIBCPP_VERSION not defined
-#endif
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/strings/string.view/use_header_warning.fail.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// REQUIRES: verify-support
-
-// <experimental/string_view>
-
-#include <experimental/string_view>
-
-// expected-error@experimental/string_view:* {{"<experimental/string_view> has been removed. Use <string_view> instead."}}
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/strings/string.view/version.pass.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// <experimental/string_view>
-
-#ifdef __GNUC__
-#pragma GCC diagnostic ignored "-W#warnings"
-#endif
-#include <experimental/string_view>
-
-#ifndef _LIBCPP_VERSION
-#error _LIBCPP_VERSION not defined
-#endif
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/utilities/any/use_header_warning.fail.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// REQUIRES: verify-support
-
-// <experimental/any>
-
-#include <experimental/any>
-
-// expected-error@experimental/any:* {{"<experimental/any> has been removed. Use <any> instead."}}
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/utilities/any/version.pass.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// <experimental/any>
-
-#ifdef __GNUC__
-#pragma GCC diagnostic ignored "-W#warnings"
-#endif
-#include <experimental/any>
-
-#ifndef _LIBCPP_VERSION
-#error _LIBCPP_VERSION not defined
-#endif
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/utilities/optional/use_header_warning.fail.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// REQUIRES: verify-support
-
-// <experimental/optional>
-
-#include <experimental/optional>
-
-// expected-error@experimental/optional:* {{"<experimental/optional> has been removed. Use <optional> instead."}}
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/utilities/optional/version.pass.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// <experimental/optional>
-
-#ifdef __GNUC__
-#pragma GCC diagnostic ignored "-W#warnings"
-#endif
-#include <experimental/optional>
-
-#ifndef _LIBCPP_VERSION
-#error _LIBCPP_VERSION not defined
-#endif
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/utilities/ratio/use_header_warning.fail.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// REQUIRES: verify-support
-
-// <experimental/ratio>
-
-#include <experimental/ratio>
-
-// expected-error@experimental/ratio:* {{"<experimental/ratio> has been removed. Use <ratio> instead."}}
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/utilities/ratio/version.pass.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// <experimental/ratio>
-
-#ifdef __GNUC__
-#pragma GCC diagnostic ignored "-W#warnings"
-#endif
-#include <experimental/ratio>
-
-#ifndef _LIBCPP_VERSION
-#error _LIBCPP_VERSION not defined
-#endif
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/utilities/time/use_header_warning.fail.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// REQUIRES: verify-support
-
-// <experimental/chrono>
-
-#include <experimental/chrono>
-
-// expected-error@experimental/chrono:* {{"<experimental/chrono> has been removed. Use <chrono> instead."}}
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/utilities/time/version.pass.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// <experimental/chrono>
-
-#ifdef __GNUC__
-#pragma GCC diagnostic ignored "-W#warnings"
-#endif
-#include <experimental/chrono>
-
-#ifndef _LIBCPP_VERSION
-#error _LIBCPP_VERSION not defined
-#endif
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/utilities/tuple/use_header_warning.fail.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// REQUIRES: verify-support
-
-// <experimental/tuple>
-
-#include <experimental/tuple>
-
-// expected-error@experimental/tuple:* {{"<experimental/tuple> has been removed. Use <tuple> instead."}}
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/libcxx/experimental/utilities/tuple/version.pass.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// <experimental/tuple>
-
-#ifdef __GNUC__
-#pragma GCC diagnostic ignored "-W#warnings"
-#endif
-#include <experimental/tuple>
-
-#ifndef _LIBCPP_VERSION
-#error _LIBCPP_VERSION not defined
-#endif
-
-int main(int, char**) {
-  return 0;
-}

libcxx/test/std/experimental/filesystem/lit.local.cfg

+config.test_format.cxx.compile_flags += ['-D_LIBCPP_NO_EXPERIMENTAL_DEPRECATION_WARNING_FILESYSTEM']

libcxx/www/TS_deprecation.html

-<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
-          "http://www.w3.org/TR/html4/strict.dtd">
-<!-- Material used from: HTML 4.01 specs: http://www.w3.org/TR/html401/ -->
-<html>
-<head>
-  <meta http-equiv="content-type" content="text/html; charset=utf-8">
-  <title>libc++ Technical Specification Status</title>
-  <link type="text/css" rel="stylesheet" href="menu.css">
-  <link type="text/css" rel="stylesheet" href="content.css">
-</head>
-
-<body>
-<div id="menu">
-  <div>
-    <a href="https://llvm.org/">LLVM Home</a>
-  </div>
-
-  <div class="submenu">
-    <label>libc++ Info</label>
-    <a href="/index.html">About</a>
-  </div>
-
-  <div class="submenu">
-    <label>Quick Links</label>
-    <a href="https://lists.llvm.org/mailman/listinfo/cfe-dev">cfe-dev</a>
-    <a href="https://lists.llvm.org/mailman/listinfo/cfe-commits">cfe-commits</a>
-    <a href="https://bugs.llvm.org/">Bug Reports</a>
-    <a href="https://github.com/llvm/llvm-project/tree/master/libcxx/">Browse Sources</a>
-  </div>
-</div>
-
-<div id="content">
-<p>The "end game" of a Technical Specification (TS) is to have the features in there added to a future version of the C++ standard. When this happens, the TS can be retired. Sometimes, only part of at TS is added to the standard, and the rest of the features may be incorporated into the next version of the TS.</p>
-
-<p>Adoption leaves library implementors with two implementations of a feature, one in namespace <tt>std</tt>, and the other in namespace <tt>std::experimental</tt>. The first one will continue to evolve (via issues and papers), while the other will not. Gradually they will diverge. It's not good for users to have two (subtly) different implementations of the same functionality in the same library.</p>
-
-<p>As features are adopted into the main standard, we will implement them in namespace <tt>std</tt>, and then remove the versions in <tt>std::experimental</tt>. The removal will not happen immediately, because that would be unhelpful for users - giving them no chance to update their code.</p>
-
-<p>The rule of thumb that libc++ will follow is: <b>one year</b>. <br/>One year after we ship an implementation of a feature in <tt>std</tt>, we will remove it from <tt>std::experimental</tt>.</p>
-
-<p>A specific example: The first release of clang/libc++ that officially supported C++17 was 5.0. For the 7.0 release (one year after 5.0), we will remove the features that were adopted into C++17 from the TSes, <i>and</i> that were present in namespace <tt>std</tt> in the 5.0 release.</p>
-
-<h2>Library Fundamentals TS <a href="https://wg21.link/N4480">V1</a> and <a href="https://wg21.link/N4617">V2</a></h2>
-
-<p>Most (but not all) of the features of the LFTS were accepted into C++17.</p>
-
-<table id="LFTS" border="1">
-<tr><th>Section</th><th>Feature</th><th>shipped in<br/><tt>std</tt></th><th>To be removed from<br/><tt>std::experimental</tt></th><th>Notes</th></tr>
-<tr><td>2.1</td><td>uses_allocator construction</td><td><center>5.0</center></td><td><center>7.0</center></td><td></td></tr>
-<tr><td>3.1.2</td><td><tt>erased_type</tt></td><td></td><td><center>n/a</center></td><td><i>Not part of C++17</i></td></tr>
-<tr><td>3.2.1</td><td><tt>tuple_size_v</tt></td><td><center>5.0</center></td><td><center>7.0</center></td><td><i>Removed</i></td></tr>
-<tr><td>3.2.2</td><td><tt>apply</tt></td><td><center>5.0</center></td><td><center>7.0</center></td><td><i>Removed</i></td></tr>
-<tr><td>3.3.1</td><td>All of the '_v' traits in &lt;type_traits&gt;</td><td><center>5.0</center></td><td><center>7.0</center></td><td><i>Removed</i></td></tr>
-<tr><td>3.3.2</td><td><tt>invocation_type</tt> and <tt>raw_invocation_type</tt></td><td></td><td><center>n/a</center></td><td><I>Not part of C++17</I></td></tr>
-<tr><td>3.3.3</td><td>Logical operator traits</td><td><center>5.0</center></td><td><center>7.0</center></td><td><i>Removed</i></td></tr>
-<tr><td>3.3.3</td><td>Detection Idiom</td><td><center>5.0</center></td><td></td><td><I>Only partially in C++17</I></td></tr>
-<tr><td>3.4.1</td><td>All of the '_v' traits in &lt;ratio&gt;</td><td><center>5.0</center></td><td><center>7.0</center></td><td><i>Removed</i></td></tr>
-<tr><td>3.5.1</td><td>All of the '_v' traits in &lt;chrono&gt;</td><td><center>5.0</center></td><td><center>7.0</center></td><td><i>Removed</i></td></tr>
-<tr><td>3.6.1</td><td>All of the '_v' traits in &lt;system_error&gt;</td><td><center>5.0</center></td><td><center>7.0</center></td><td><i>Removed</i></td></tr>
-<tr><td>3.7</td><td><tt>propagate_const</tt></td><td></td><td><center>n/a</center></td><td><I>Not part of C++17</I></td></tr>
-<tr><td>4.2</td><td>Enhancements to <tt>function</tt></td><td><center>Not yet</center></td><td></td><td></td></tr>
-<tr><td>4.3</td><td>searchers</td><td><center>7.0</center></td><td><center>9.0</center></td><td></td></tr>
-<tr><td>5</td><td><tt>optional</tt></td><td><center>5.0</center></td><td><center>7.0</center></td><td><i>Removed</i></td></tr>
-<tr><td>6</td><td><tt>any</tt></td><td><center>5.0</center></td><td><center>7.0</center></td><td><i>Removed</i></td></tr>
-<tr><td>7</td><td><tt>string_view</tt></td><td><center>5.0</center></td><td><center>7.0</center></td><td><i>Removed</i></td></tr>
-<tr><td>8.2.1</td><td><tt>shared_ptr</tt> enhancements</td><td><center>Not yet</center></td><td><center>Never added</center></td><td></td></tr>
-<tr><td>8.2.2</td><td><tt>weak_ptr</tt> enhancements</td><td><center>Not yet</center></td><td><center>Never added</center></td><td></td></tr>
-<tr><td>8.5</td><td><tt>memory_resource</tt></td><td><center>Not yet</center></td><td></td><td></td></tr>
-<tr><td>8.6</td><td><tt>polymorphic_allocator</tt></td><td><center>Not yet</center></td><td></td><td></td></tr>
-<tr><td>8.7</td><td><tt>resource_adaptor</tt></tt></td><td><center></center></td><td><center>n/a</center></td><td><I>Not part of C++17</I></td></tr>
-<tr><td>8.8</td><td>Access to program-wide <tt>memory_resource</tt> objects</td><td><center>Not yet</center></td><td></td><td></td></tr>
-<tr><td>8.9</td><td>Pool resource classes</td><td><center>Not yet</center></td><td></td><td></td></tr>
-<tr><td>8.10</td><td><tt>monotonic_buffer_resource</tt></td><td><center>Not yet</center></td><td></td><td></td></tr>
-<tr><td>8.11</td><td>Alias templates using polymorphic memory resources</td><td><center>Not yet</center></td><td></td><td></td></tr>
-<tr><td>8.12</td><td>Non-owning pointers</td><td></td><td><center>n/a</center></td><td><I>Not part of C++17</I></td></tr>
-<tr><td>11.2</td><td><tt>promise</tt></td><td></td><td><center>n/a</center></td><td><I>Not part of C++17</I></td></tr>
-<tr><td>11.3</td><td><tt>packaged_task</tt></td><td></td><td><center>n/a</center></td><td><I>Not part of C++17</I></td></tr>
-<tr><td>12.2</td><td><tt>search</tt></td><td><center>7.0</center></td><td><center>9.0</center></td><td><I></I></td></tr>
-<tr><td>12.3</td><td><tt>sample</tt></td><td><center>5.0</center></td><td><center>7.0</center></td><td><i>Removed</i></td></tr>
-<tr><td>12.4</td><td><tt>shuffle</tt></td><td></td><td></td><td><I>Not part of C++17</I></td></tr>
-<tr><td>13.1</td><td><tt>gcd</tt> and <tt>lcm</tt></td><td><center>5.0</center></td><td><center>7.0</center></td><td><i>Removed</i></td></tr>
-<tr><td>13.2</td><td>Random number generation</td><td></td><td><td><I>Not part of C++17</I></td></tr>
-<tr><td>14</td><td>Reflection Library</td><td></td><td><td><I>Not part of C++17</I></td></tr>
-</table>
-
-<h2><a href="https://wg21.link/N4100">FileSystem TS</a></h2>
-<p>The FileSystem TS was accepted (in toto) for C++17.</p>
-<p>The FileSystem TS implementation is still (as of v6.0) in namespace <tt>std::experimental</tt>.</p>
-
-<h2>Parallelism TS <a href="https://wg21.link/N4507">V1</a> and <a href="https://wg21.link/N4706">V2</a></h2>
-<p>Some (most) of the Parallelism TS was accepted for C++17.</p>
-<p>We have not yet shipped an implementation of the Parallelism TS.</p>
-
-<h2><a href="https://wg21.link/N4680">Coroutines TS</a></h2>
-<p>The Coroutines TS is not yet part of a shipping standard.</p>
-<p>We are shipping (as of v5.0) an implementation of the Coroutines TS in namespace <tt>std::experimental</tt>.</p>
-
-<h2><a href="https://wg21.link/N4656">Networking TS</a></h2>
-<p>The Networking TS is not yet part of a shipping standard.</p>
-<p>We have not yet shipped an implementation of the Networking TS.</p>
-
-<h2><a href="https://wg21.link/N4685">Ranges TS</a></h2>
-<p>The Ranges TS is not yet part of a shipping standard.</p>
-<p>We have not yet shipped an implementation of the Ranges TS.</p>
-
-<h2><a href="https://wg21.link/N4641">Concepts TS</a></h2>
-<p>The Concepts TS is not yet part of a shipping standard, but it has been adopted into the C++20 working draft.</p>
-<p>We have not yet shipped an implementation of the Concepts TS.</p>
-
-<h2><a href="https://wg21.link/P0159">Concurrency TS</a></h2>
-<!-- The Concurrency TS was adopted in Kona (2015). -->
-<p>None of the Concurrency TS was accepted for C++17.</p>
-<p>We have not yet shipped an implementation of the Concurrency TS.</p>
-
-<!--
-><table id="Concurrency" border="1">
-<tr><th>Section</th><th>Feature</th><th>shipped in <tt>std</tt></th><th>To be removed from <tt>std::experimental</tt></th><th>Notes</th></tr>
-<tr><td>2.3</td><td>class template <tt>future</tt></td><td></td><td><td></td></tr>
-<tr><td>2.4</td><td>class template <tt>shared_future</tt></td><td></td><td><td></td></tr>
-<tr><td>2.5</td><td>class template <tt>promise</tt></td><td></td><td><td><center><I>Only using <tt>future</tt></I></center></td></tr>
-<tr><td>2.6</td><td>class template <tt>packaged_task</tt></td><td></td><td><td><center><I>Only using <tt>future</tt></I></center></td></tr>
-<tr><td>2.7</td><td>function template <tt>when_all</tt></td><td></td><td><td><center><I>Not part of C++17</I></center></td></tr>
-<tr><td>2.8</td><td>class template <tt>when_any_result</tt></td><td></td><td><td><center><I>Not part of C++17</I></center></td></tr>
-<tr><td>2.9</td><td>function template <tt>when_any</tt></td><td></td><td><td><center><I>Not part of C++17</I></center></td></tr>
-<tr><td>2.10</td><td>function template <tt>make_ready_future</tt></td><td></td><td><td><center><I>Not part of C++17</I></center></td></tr>
-<tr><td>2.11</td><td>function template <tt>make_exeptional_future</tt></td><td></td><td><td><center><I>Not part of C++17</I></center></td></tr>
-<tr><td>3</td><td><tt>latches</tt> and <tt>barriers</tt></td><td></td><td><td><center><I>Not part of C++17</I></center></td></tr>
-<tr><td>4</td><td>Atomic Smart Pointers</td><td></td><td><td><center><I>Adopted for C++20</I></center></td></tr>
--->
-</table>
-
-
-  <hr/>
-  <p>Last Updated: 8-Feb-2018</p>
-</div>
-</body>
-</html>

libcxx/www/index.html

<div id="content">
    <p>libc++'s C++17 implementation is not yet complete. A list of features and changes
       for C++17 can be found <a href="cxx1z_status.html">here</a>.</p>
    <p>A list of features and changes for the next C++ standard, known here as
       "C++2a" (probably to be C++20) can be found <a href="cxx2a_status.html">here</a>.</p>
    <p>Implementation of the post-C++14 Technical Specifications is in progress. A list of features
       and the current status of these features can be found <a href="ts1z_status.html">here</a>.</p>
    <p>As features get moved from the Technical Specifications into the main standard, we
       will (after a period for migration) remove them from the TS implementation. This
-      process is detailed <a href="TS_deprecation.html">here</a>.</p>
+      process is detailed <a href="DesignDocs/ExperimentalFeatures.html">here</a>.</p>
 
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