[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, 57 insertions, 0 deletions

diff --git a/libcxx/test/std/experimental/task/task.lifetime/task_parameter_lifetime.pass.cpp b/libcxx/test/std/experimental/task/task.lifetime/task_parameter_lifetime.pass.cpp
new file mode 100644
index 00000000000..bc1bd432244
--- /dev/null
+++ b/libcxx/test/std/experimental/task/task.lifetime/task_parameter_lifetime.pass.cpp
@@ -0,0 +1,57 @@
+// -*- 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 <cassert>
+
+#include "../counted.hpp"
+#include "../sync_wait.hpp"
+
+DEFINE_COUNTED_VARIABLES();
+
+void test_parameter_lifetime()
+{
+  counted::reset();
+
+  auto f = [](counted c) -> std::experimental::task<std::size_t>
+  {
+    co_return c.id();
+  };
+
+  {
+    auto t = f({});
+
+    assert(counted::active_instance_count() == 1);
+    assert(counted::copy_constructor_count() == 0);
+    assert(counted::move_constructor_count() <= 2); // Ideally <= 1
+
+    auto id = sync_wait(t);
+    assert(id == 1);
+
+    assert(counted::active_instance_count() == 1);
+    assert(counted::copy_constructor_count() == 0);
+
+    // We are relying on C++17 copy-elision when passing the temporary counter
+    // into f(). Then f() must move the parameter into the coroutine frame by
+    // calling the move-constructor. This move could also potentially be
+    // elided by the
+    assert(counted::move_constructor_count() <= 1);
+  }
+
+  assert(counted::active_instance_count() == 0);
+}
+
+int main()
+{
+  test_parameter_lifetime();
+  return 0;
+}