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### 2298. [CD] `is_nothrow_constructible` is always false because of `create<>`

Section: 20.15.5.4 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2013-09-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US 18

The trait `is_constructible<T, Args...>` is defined in terms of a helper template, `create<>`, that is identical to `std::declval<>` except for the latter's `noexcept` clause.

If the absence of `noexcept` is critical to this definition, insert a Note of explanation; otherwise, excise `create<>` and reformulate in terms of `declval<>` the definition of `is_constructible`.

[2013-09-24 Daniel comments and provides resolution suggestion]

Replacing `create<>` by `std::declval<>` would make the situation worse, because the definition of `is_constructible` is based on a well-formed variable definition and there is no way to specify a variable definition without odr-using its initializer arguments. It should also be added, that there is another problem with the specification of all existing `is_trivially_*` traits, because neither `create<>` nor `std::declval<>` are considered as trivial functions, but this should be solved by a different issue.

[2013-09-26 Nico improves wording]

The additional change is just to keep both places were `create()` is defined consistent.

[2013-09 Chicago]

No objections, so moved to Immediate.

Accept for Working Paper

Proposed resolution:

This wording is relative to N3691.

1. Change 20.15.5.4 [meta.unary.prop] around p6 as indicated:

   > -6- Given the following function prototype:

   > >     template <class T>
   > >       typename add_rvalue_reference<T>::type create() noexcept;

   > the predicate condition for a template specialization `is_constructible<T, Args...>` shall be satisfied if and only if the following variable definition would be well-formed for some invented variable `t`:

   > >     T t(create<Args>()...);

   > […]

2. Change 20.15.5.4 [meta.unary.prop] around p4 as indicated:

   > -4- Given the following function prototype:

   > >     template <class T>
   > >       typename add_rvalue_reference<T>::type create() noexcept;

   > the predicate condition for a template specialization `is_convertible<From, To>` shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:

   > >     To test() {
   > >       return create<From>();
   > >     }

   > […]