[libc++] Deprecate extension packaged_task::result_type

This extension is questionable and non-conforming. Perhaps we should
deprecate and then remove it.

libcxx/docs/ReleaseNotes/20.rst

@@ -146,6 +146,8 @@ Deprecations and Removals
   ``__undeclare_reachable`` have been removed from the library. These functions were never implemented in a non-trivial
   way, making it very unlikely that any binary depends on them.
 
+- Non-conforming extension ``packaged_task::result_type`` is deprecated. It will be removed in LLVM 21.
+
 Upcoming Deprecations and Removals
 ----------------------------------
 
@@ -164,6 +166,8 @@ LLVM 21
 - The ``_LIBCPP_VERBOSE_ABORT_NOT_NOEXCEPT`` macro will be removed in LLVM 21, making ``std::__libcpp_verbose_abort``
   unconditionally ``noexcept``.
 
+- Non-conforming extension ``packaged_task::result_type`` will be removed in LLVM 21.
+
 
 ABI Affecting Changes
 ---------------------

libcxx/include/future

@@ -1612,11 +1612,11 @@ inline _Rp __packaged_task_function<_Rp(_ArgTypes...)>::operator()(_ArgTypes...
 template <class _Rp, class... _ArgTypes>
 class _LIBCPP_TEMPLATE_VIS packaged_task<_Rp(_ArgTypes...)> {
 public:
-  typedef _Rp result_type; // extension
+  using result_type _LIBCPP_DEPRECATED = _Rp; // extension
 
 private:
-  __packaged_task_function<result_type(_ArgTypes...)> __f_;
-  promise<result_type> __p_;
+  __packaged_task_function<_Rp(_ArgTypes...)> __f_;
+  promise<_Rp> __p_;
 
 public:
   // construction and destruction
@@ -1653,7 +1653,7 @@ public:
   _LIBCPP_HIDE_FROM_ABI bool valid() const _NOEXCEPT { return __p_.__state_ != nullptr; }
 
   // result retrieval
-  _LIBCPP_HIDE_FROM_ABI future<result_type> get_future() { return __p_.get_future(); }
+  _LIBCPP_HIDE_FROM_ABI future<_Rp> get_future() { return __p_.get_future(); }
 
   // execution
   _LIBCPP_HIDE_FROM_ABI void operator()(_ArgTypes... __args);
@@ -1700,17 +1700,17 @@ template <class _Rp, class... _ArgTypes>
 void packaged_task<_Rp(_ArgTypes...)>::reset() {
   if (!valid())
     __throw_future_error(future_errc::no_state);
-  __p_ = promise<result_type>();
+  __p_ = promise<_Rp>();
 }
 
 template <class... _ArgTypes>
 class _LIBCPP_TEMPLATE_VIS packaged_task<void(_ArgTypes...)> {
 public:
-  typedef void result_type; // extension
+  using result_type _LIBCPP_DEPRECATED = void; // extension
 
 private:
-  __packaged_task_function<result_type(_ArgTypes...)> __f_;
-  promise<result_type> __p_;
+  __packaged_task_function<void(_ArgTypes...)> __f_;
+  promise<void> __p_;
 
 public:
   // construction and destruction
@@ -1745,7 +1745,7 @@ public:
   _LIBCPP_HIDE_FROM_ABI bool valid() const _NOEXCEPT { return __p_.__state_ != nullptr; }
 
   // result retrieval
-  _LIBCPP_HIDE_FROM_ABI future<result_type> get_future() { return __p_.get_future(); }
+  _LIBCPP_HIDE_FROM_ABI future<void> get_future() { return __p_.get_future(); }
 
   // execution
   _LIBCPP_HIDE_FROM_ABI void operator()(_ArgTypes... __args);
@@ -1804,7 +1804,7 @@ template <class... _ArgTypes>
 void packaged_task<void(_ArgTypes...)>::reset() {
   if (!valid())
     __throw_future_error(future_errc::no_state);
-  __p_ = promise<result_type>();
+  __p_ = promise<void>();
 }
 
 template <class _Rp, class... _ArgTypes>

libcxx/test/libcxx/thread/futures/futures.task/type.depr.verify.cpp

@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: no-threads
+// UNSUPPORTED: c++03
+
+// <future>
+
+// template<class R, class... ArgTypes>
+//     class packaged_task<R(ArgTypes...)>
+// {
+// public:
+//     typedef R result_type; // extension
+
+// This libc++ extension is deprecated. See https://github.com/llvm/llvm-project/issues/112856.
+
+#include <future>
+#include <type_traits>
+
+struct A {};
+
+using RA = std::packaged_task<A(int, char)>::result_type;    // expected-warning {{'result_type' is deprecated}}
+using RV = std::packaged_task<void(int, char)>::result_type; // expected-warning {{'result_type' is deprecated}}

libcxx/test/libcxx/thread/futures/futures.task/types.pass.cpp

@@ -19,16 +19,16 @@
 
 // This is a libc++ extension.
 
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 #include <future>
 #include <type_traits>
 
-#include "test_macros.h"
-
 struct A {};
 
-int main(int, char**)
-{
-    static_assert((std::is_same<std::packaged_task<A(int, char)>::result_type, A>::value), "");
+int main(int, char**) {
+  static_assert((std::is_same<std::packaged_task<A(int, char)>::result_type, A>::value), "");
+  static_assert((std::is_same<std::packaged_task<void(int, char)>::result_type, void>::value), "");
 
   return 0;
 }