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### 3451. Inconsistently explicit deduction guides

Section: 23.4.3 [basic.string], 26.7.14.2 [range.join.view] Status: New Submitter: Johel Ernesto Guerrero Peña Opened: 2020-06-11 Last modified: 2020-07-17

Priority: 3

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Discussion:

The library inconsistently marks deduction guides as `explicit`. `join_view` and `basic_string` account for the only two occurrences of unconditionally explicit deduction guides. All other deduction guides have no `explicit`-specifier. Following is a list of unconditionally explicit constructors with their deduction guides.

>     template<class Y>
>       explicit shared_ptr(const weak_ptr<Y>& r);

>     template<class T>
>       shared_ptr(weak_ptr<T>) -> shared_ptr<T>;

>     template<class T>
>       constexpr explicit basic_string(const T& t, const Allocator& a = Allocator());

>     template<class charT,
>              class traits,
>              class Allocator = allocator<charT>>
>       explicit basic_string(basic_string_view<charT, traits>, const Allocator& = Allocator())
>         -> basic_string<charT, traits, Allocator>;

>     explicit queue(const Container&);
>     explicit queue(Container&&);

>     template<class Container>
>       queue(Container) -> queue<typename Container::value_type, Container>;

>     explicit stack(const Container&);
>     explicit stack(Container&&);

>     template<class Container>
>       stack(Container) -> stack<typename Container::value_type, Container>;

>     constexpr explicit join_view(V base);

>     template<class R>
>       explicit join_view(R&&) -> join_view<views::all_t<R>>;

>     constexpr explicit common_view(V r);

>     template<class R>
>       common_view(R&&) -> common_view<views::all_t<R>>;

>     constexpr explicit reverse_view(V r);

>     template<class R>
>       reverse_view(R&&) -> reverse_view<views::all_t<R>>;

>     explicit zoned_time(TimeZonePtr z);
>     explicit zoned_time(string_view name);

>     template<class TimeZonePtrOrName>
>       zoned_time(TimeZonePtrOrName&&)
>         -> zoned_time<seconds, time-zone-representation<TimeZonePtrOrName>>;

>     template<class C>
>     explicit stop_callback(const stop_token& st, C&& cb)
>         noexcept(is_nothrow_constructible_v<Callback, C>);
>     template<class C>
>     explicit stop_callback(stop_token&& st, C&& cb)
>         noexcept(is_nothrow_constructible_v<Callback, C>);

>     template<class Callback>
>       stop_callback(stop_token, Callback) -> stop_callback<Callback>;

[2020-07-17; Priority set to 3 in telecon]

Proposed resolution:

This wording is relative to N4861.

1. Modify 23.4.3 [basic.string], class template `basic_string` synopsis, as indicated:

   >     […]
   >     template<class charT,
   >              class traits,
   >              class Allocator = allocator<charT>>
   >       explicit basic_string(basic_string_view<charT, traits>, const Allocator& = Allocator())
   >         -> basic_string<charT, traits, Allocator>;
   >     […]

2. Modify 23.4.3.3 [string.cons] as indicated:

   >     […]
   >     template<class charT,
   >              class traits,
   >              class Allocator = allocator<charT>>
   >       explicit basic_string(basic_string_view<charT, traits>, const Allocator& = Allocator())
   >         -> basic_string<charT, traits, Allocator>;
   >     […]

   > > -22- Constraints: `Allocator` is a type that qualifies as an allocator (24.2.2.1 [container.requirements.general]).

3. Modify 26.7.14.2 [range.join.view], class template `join_view` synopsis, as indicated:

   >     […]
   >     template<class R>
   >       explicit join_view(R&&) -> join_view<views::all_t<R>>;
   >     […]