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### 1071. `is_bind_expression` should derive from `integral_constant<bool>`

Section: 20.14.15.2 [func.bind.isbind] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-19 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [func.bind.isbind].

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Discussion:

Class template is_bind_expression 20.14.15.2 [func.bind.isbind]:

>     namespace std {
>       template<class T> struct is_bind_expression {
>         static const bool value = see below;
>       };
>     }

`is_bind_expression` should derive from `std::integral_constant<bool>` like other similar trait types.

[ Daniel adds: ]

> We need the same thing for the trait `is_placeholder` as well.

[ 2009-03-22 Daniel provided wording. ]

[ Batavia (2009-05): ]

> We recommend this be deferred until after the next Committee Draft is issued.

> Move to Open.

[ 2009-05-31 Peter adds: ]

> I am opposed to the proposed resolution and to the premise of the issue in general. The traits's default definitions should NOT derive from `integral_constant`, because this is harmful, as it misleads people into thinking that `is_bind_expression<E>` always derives from `integral_constant`, whereas it may not.

> `is_bind_expression` and `is_placeholder` allow user specializations, and in fact, this is their primary purpose. Such user specializations may not derive from `integral_constant`, and the places where `is_bind_expression` and `is_placeholder` are used intentionally do not require such derivation.

> The long-term approach here is to switch to `BindExpression<E>` and `Placeholder<P>` explicit concepts, of course, but until that happens, I say leave them alone.

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready. We are comfortable with requiring user specializations to derive from `integral_constant`.

Proposed resolution:

1. In 20.14.15.2 [func.bind.isbind] change as indicated:

   >     namespace std {
   >      template<class T> struct is_bind_expression : integral_constant<bool, see below> { };{
   >        static const bool value = see below;
   >      };
   >     }

2. In 20.14.15.2 [func.bind.isbind]/2 change as indicated:

   >     static const bool value;

   > > -2- `true` if `T` is a type returned from `bind`, `false` otherwise. If `T` is a type returned from `bind`, `is_bind_expression<T>` shall be publicly derived from `integral_constant<bool, true>`, otherwise it shall be publicly derived from `integral_constant<bool, false>`.

3. In 20.14.15.3 [func.bind.isplace] change as indicated:

   >     namespace std {
   >      template<class T> struct is_placeholder : integral_constant<int, see below> { };{
   >        static const int value = see below;
   >      };
   >     }

4. In 20.14.15.3 [func.bind.isplace]/2 change as indicated:

   >     static const int value;

   > > -2- value is `J` if `T` is the type of `std::placeholders::_J`, 0 otherwise. If `T` is the type of `std::placeholders::_J`, `is_placeholder<T>` shall be publicly derived from `integral_constant<int, J>` otherwise it shall be publicly derived from `integral_constant<int, 0>`.