This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of WP status.

### 3404. Finish removing `subrange`'s conversions from `pair-like`

Section: 24.5.4 [range.subrange] Status: WP Submitter: Casey Carter Opened: 2020-02-20 Last modified: 2020-11-09

Priority: 0

View all other issues in [range.subrange].

View all issues with WP status.

Discussion:

Both LWG 3281 "Conversion from `pair-like` types to `subrange` is a silent semantic promotion" and LWG 3282 "`subrange` converting constructor should disallow derived to base conversions" removed `subrange`'s hated implicit conversions from `pair-like` types. Notably, neither issue removed the two "`iterator-sentinel-pair`" deduction guides which target the removed constructors nor the exposition-only `iterator-sentinel-pair` concept itself, all of which are now useless.

[2020-03-11 Issue Prioritization]

Status set to Tentatively Ready after seven positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 24.5.4 [range.subrange] as indicated:

   >     […]
   >     template<class T, class U, class V>
   >       concept pair-like-convertible-from = // exposition only
   >         !range<T> && pair-like<T> && constructible_from<T, U, V>;

   >     template<class T>
   >       concept iterator-sentinel-pair = // exposition only
   >         !range<T> && pair-like<T> &&
   >         sentinel_for<tuple_element_t<1, T>, tuple_element_t<0, T>>;

   >     […]

   >     template<iterator-sentinel-pair P>
   >       subrange(P) -> subrange<tuple_element_t<0, P>, tuple_element_t<1, P>>;

   >     template<iterator-sentinel-pair P>
   >       subrange(P, make-unsigned-like-t(iter_difference_t<tuple_element_t<0, P>>)) ->
   >         subrange<uple_element_t<0, P>, tuple_element_t<1, P>, subrange_kind::sized>;
   >     […]