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### 2488. Placeholders should be allowed and encouraged to be `constexpr`

Section: 20.14.15.5 [func.bind.place] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30

Priority: 2

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Discussion:

`piecewise_construct` (20.4.5 [pair.piecewise]), `allocator_arg` (20.10.6 [allocator.tag]), and `adopt_lock`/`defer_lock`/`try_to_lock` (32.5.5 [thread.lock]) are all required to be `constexpr` with internal linkage. `bind()`'s placeholders should be allowed to follow this modern practice, for increased consistency and reduced implementer headaches (header-only is easier than separately-compiled).

[2015-05-07 Lenexa: Move to Immediate]

STL: I'd like this one immediate.

Jonathan: I want to think about forcing constexpr, but the current issue, I have no objections. I'd say ready, but I won't object to immediate if STL wants it.

Marshall: You should report in Kona how it worked out.

STL: We went around a bit on the reflector about how to phrase the encouragement.

Jonathan: I think the shall may be not quite right.

Marshall: I see, you can change your implementation, but you don't.

Jonathan: There's precedent for the shall, but it's wrong, see editorial issue 493.

STL: I would prefer not to ask Daniel to reword, and 493 can fix it later.

Marshall: I remove my objection to immediate because it doesn't force implementations to change.

Marshall: Any other discussion?

Marshall: Immediate vote: 6. Opposed, 0. Abstain, 1.

Proposed resolution:

This wording is relative to N4296.

1. Change 20.14 [function.objects] p2 "Header `<functional>` synopsis" as depicted:

   >     namespace placeholders {
   >       // M is the implementation-defined number of placeholders
   >       see belowextern unspecified _1;
   >       see belowextern unspecified _2;
   >       ...
   >       see belowextern unspecified _M;
   >     }

2. Change 20.14.15.5 [func.bind.place] p2 as depicted:

   >     namespace std::placeholders {
   >       // M is the implementation-defined number of placeholders
   >       see belowextern unspecified _1;
   >       see belowextern unspecified _2;
   >                  .
   >                  .
   >                  .
   >       see belowextern unspecified _M;
   >     }

   > -1- All placeholder types shall be `DefaultConstructible` and `CopyConstructible`, and their default constructors and copy/move constructors shall not throw exceptions. It is implementation-defined whether placeholder types are `CopyAssignable`. `CopyAssignable` placeholders' copy assignment operators shall not throw exceptions.

   > -?- Placeholders should be defined as:

   > >     constexpr unspecified _1{};

   > If they are not, they shall be declared as:

   > >     extern unspecified _1;