Issue 2097: packaged_task constructors should be constrained

2097. `packaged_task` constructors should be constrained

Section: 32.10.10.2 [futures.task.members] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

With the proposed resolution of 2067, this no longer selects the copy constructor:

std::packaged_task<void()> p1;
std::packaged_task<void()> p2(p1);

Instead this constructor is a better match:

template <class F>
 explicit packaged_task(F&& f);

This attempts to package a packaged_task, which internally tries to copy p2, which fails because the copy constructor is deleted. For at least one implementation the resulting error message is much less helpful than the expected "cannot call deleted function" because it happens after instantiating several more templates rather than in the context where the constructor is called.

I believe the solution is to constrain to the template constructors so the template argument F cannot be deduced as (possibly cv) packaged_task& or packaged_task. It could be argued this constraint is already implied because packaged_task is not copyable and the template constructors require that "invoking a copy of f shall behave the same as invoking f".

Daniel points out that the variadic constructor of std::thread described in 32.4.3.3 [thread.thread.constr] has a similar problem and suggests a similar wording change, which has been integrated below.

An alternative is to declare thread(thread&) and packaged_task(packaged_task&) as deleted.

[2012, Portland]

This issue appears to be more about library specification than technical concurrency issues, so should be handled in LWG.

[2013, Chicago]

Move to Immediate resolution.

Howard volunteered existing implementation experience with the first change, and saw no issue that the second would introduce any new issue.

Proposed resolution:

This wording is relative to the FDIS.

Insert a new Remarks element to 32.4.3.3 [thread.thread.constr] around p3 as indicated:
```
template <class F, class ...Args> explicit thread(F&& f, Args&&... args);
```
-3- Requires: F and each Ti in Args shall satisfy the MoveConstructible requirements. INVOKE(DECAY_COPY ( std::forward<F>(f)), DECAY_COPY (std::forward<Args>(args))...) (20.8.2) shall be a valid expression.
-?- Remarks: This constructor shall not participate in overload resolution if decay<F>::type is the same type as std::thread.
Insert a new Remarks element to 32.10.10.2 [futures.task.members] around p2 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);
```
-2- Requires: INVOKE(f, t1, t2, ..., tN, R), where t1, t2, ..., tN are values of the corresponding types in ArgTypes..., shall be a valid expression. Invoking a copy of f shall behave the same as invoking f.
-?- Remarks: These constructors shall not participate in overload resolution if decay<F>::type is the same type as std::packaged_task<R(ArgTypes...)>.

2097. packaged_task constructors should be constrained

2097. `packaged_task` constructors should be constrained