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2407. packaged_task(allocator_arg_t, const Allocator&, F&&) should neither be constrained nor explicit

Section: 32.10.10.2 [futures.task.members] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

LWG 2097⁽ⁱ⁾'s resolution was slightly too aggressive. It constrained packaged_task(allocator_arg_t, const Allocator&, F&&), but that's unnecessary because packaged_task doesn't have any other three-argument constructors. Additionally, it's marked as explicit (going back to WP N2798 when packaged_task first appeared) which is unnecessary.

[2015-02 Cologne]

Handed over to SG1.

[2015-05 Lenexa, SG1 response]

Back to LWG; not an SG1 issue.

[2015-05 Lenexa]

STL improves proposed wording by restoring the constraint again.

Proposed resolution:

This wording is relative to N3936.

1. Change 32.10.10 [futures.task] p2, class template packaged_task as indicated:

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

2. Change 32.10.10.2 [futures.task.members] as indicated:

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

   […]

   -3- Remarks: These constructors shall not participate in overload resolution if decay_t<F> is the same type as std::packaged_task<R(ArgTypes...)>.