Proposed resolution:

This wording is relative to N4606.

1. Change [futures.async] p6 to:

   Throws: system_error if policy == launch::async and the implementation is unable to start a new thread, or std::bad_alloc if memory for the internal data structures could not be allocated.

2. Change [futures.task.members] p5 to:

   template <class F>
     packaged_task(F&& f);
   template <class F, class Allocator>
     packaged_task(allocator_arg_t, const Allocator& a, F&& f);
   

   Throws:

   1. (?) — Aany exceptions thrown by the copy or move constructor of f., or
   2. (?) — For the first version, std::bad_alloc if memory for the internal data structures could not be allocated.
   3. (?) — For the second version, any exceptions thrown by std::allocator_traits<Allocator>::template rebind_traits<unspecified>::allocate.

[ 2016-08 Chicago ]

Wed PM: Move to Tentatively Ready

[ 2016-07 Chicago ]

Alisdair thinks the third bullet is not quite right.

Previous resolution [SUPERSEDED]:

1. Change [futures.async] p6 to:

   Throws: system_error if policy == launch::async and the implementation is unable to start a new thread, or std::bad_alloc if memory for the internal data structures could not be allocated.

2. Change [futures.task.members] p5 to:

   template <class F>
     packaged_task(F&& f);
   template <class F, class Allocator>
     packaged_task(allocator_arg_t, const Allocator& a, F&& f);
   

   Throws:

   1. (?) — Aany exceptions thrown by the copy or move constructor of f., or
   2. (?) — For the first version, std::bad_alloc if memory for the internal data structures could not be allocated.
   3. (?) — For the second version, any exceptions thrown by std::allocator_traits<Allocator>::template rebind<unspecified>::allocate.

std::async is a request from the user for type erasure; as any given function gets passed to async which returns only future<ReturnType>. Therefore, it needs to be able to allocate memory, as other issues (e.g. LWG 2202) indicate. However, async's Throws clause doesn't allow an implementation to do this, as it permits only future_error.

std::packaged_task's constructor allocates memory using a user supplied allocator. An implementation needs to call the user's allocate to allocate such memory. The user's allocate function is not constrained to throwing only bad_alloc; it can raise whatever it wants, but packaged_task's constructor prohibits this.