[coroutines] Add std::experimental::task<T> type

Summary:
Adds the coroutine `std::experimental::task<T>` type described in proposal P1056R0.
See https://wg21.link/P1056R0.

This implementation allows customization of the allocator used to allocate the
coroutine frame by passing std::allocator_arg as the first argument, followed by
the allocator to use.

This supports co_awaiting the same task multiple times. The second and
subsequent times it returns a reference to the already-computed value.

This diff also adds some implementations of other utilities that have potential for
standardization as helpers within the test/... area:
- `sync_wait(awaitable)` - See P1171R0
- `manual_reset_event`

Move the definition of the __aligned_allocation_size helper function
from <experimental/memory_resource> to <experimental/__memory>
so it can be more widely used without pulling in memory_resource.

Outstanding work:
- Use C++14 keywords directly rather than macro versions
  eg. use `noexcept` instead of `_NOEXCEPT`).
- Add support for overaligned coroutine frames.
  This may need wording in the Coroutines TS to support passing the extra `std::align_val_t`.
- Eliminate use of `if constexpr` if we want it to compile under C++14.

Patch by @lewissbaker (Lewis Baker).

llvm-svn: 357010

1 files changed, 70 insertions, 0 deletions

diff --git a/libcxx/test/std/experimental/task/task.basic/task_of_value.pass.cpp b/libcxx/test/std/experimental/task/task.basic/task_of_value.pass.cpp
new file mode 100644
index 00000000000..0d5a8c9f619
--- /dev/null
+++ b/libcxx/test/std/experimental/task/task.basic/task_of_value.pass.cpp
@@ -0,0 +1,70 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+#include <experimental/task>
+#include <string>
+#include <vector>
+#include <memory>
+#include "../sync_wait.hpp"
+
+void test_returning_move_only_type()
+{
+  auto move_only_async =
+   [](bool x) -> std::experimental::task<std::unique_ptr<int>> {
+     if (x) {
+       auto p = std::make_unique<int>(123);
+       co_return p; // Should be implicit std::move(p) here.
+     }
+
+     co_return std::make_unique<int>(456);
+   };
+
+  assert(*sync_wait(move_only_async(true)) == 123);
+  assert(*sync_wait(move_only_async(false)) == 456);
+}
+
+void test_co_return_with_curly_braces()
+{
+  auto t = []() -> std::experimental::task<std::tuple<int, std::string>>
+  {
+    co_return { 123, "test" };
+  }();
+
+  auto result = sync_wait(std::move(t));
+
+  assert(std::get<0>(result) == 123);
+  assert(std::get<1>(result) == "test");
+}
+
+void test_co_return_by_initialiser_list()
+{
+  auto t = []() -> std::experimental::task<std::vector<int>>
+  {
+    co_return { 2, 10, -1 };
+  }();
+
+  auto result = sync_wait(std::move(t));
+
+  assert(result.size() == 3);
+  assert(result[0] == 2);
+  assert(result[1] == 10);
+  assert(result[2] == -1);
+}
+
+int main()
+{
+  test_returning_move_only_type();
+  test_co_return_with_curly_braces();
+  test_co_return_by_initialiser_list();
+
+  return 0;
+}