[cpp-threads] Asynchronous Function Proposal

[Howard Hinnant]

I agree with Peter's comments regarding collapsing the number of  
signatures, though he did a better job of it than I would have.  At  
the very least we don't need:

async(fully_threaded policy, F f);
async(fully_threaded policy, F&& f, Args&&...);

Just one or the other.  The first if you want to go with Herb's  
suggestion, else just the second.

Either by value (F) or by rvalue ref (F&&) will work, but if the  
latter you probably want to work decay into the effects clause similar  
to that suggested by:

http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#929

or

http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#817

The use of the decay trait enables one to get the "decay behavior" for  
things like string literals.

> Requires: F and each type Ti in Args shall be CopyConstructible if  
> an lvalue and otherwise MoveConstructible.

I believe MoveConstructible is sufficient and should be specified with  
concepts.  The requirements you're trying to specify (how to bind an  
argument to the parameter) are the caller's concern and not the async  
function's concern.  But async is going to need to move construct from  
the host thread's parameter into a bit of memory that the child thread  
(and future) can access, so needs MoveConstructible there.

-Howard