3039. Unnecessary decay in thread and packaged_task

Section: 32.4.3.3 [thread.thread.constr], 32.10.10.2 [futures.task.members] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2017-11-17 Last modified: 2021-02-25

Priority: 0

View all other issues in [thread.thread.constr].

View all issues with C++20 status.

Discussion:

Following P0777R1 "Treating Unnecessary decay", more occurrences can be fixed. When constraints are checking for the same type as thread or a specialization of packaged_task, decaying functions to function pointers and arrays to object pointers can't affect the result.

[28-Nov-2017 Moved to Tentatively Ready after five positive votes on the ML.]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

Wording relative to N4700.

1. Edit 32.4.3.3 [thread.thread.constr] as indicated:

   template <class F, class... Args> explicit thread(F&& f, Args&&... args);
   

   -3- Requires: […]

   -4- Remarks: This constructor shall not participate in overload resolution if decay_tremove_cvref_t<F> is the same type as std::thread.

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

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

   -2- Requires: […]

   -3- Remarks: This constructor shall not participate in overload resolution if decay_tremove_cvref_t<F> is the same type as packaged_task<R(ArgTypes...)>.