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### 3405. `common_view`'s converting constructor is bad, too

Section: 26.7.19.2 [range.common.view] Status: WP Submitter: Casey Carter Opened: 2020-02-20 Last modified: 2020-11-09

Priority: 0

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Discussion:

LWG 3280 struck the problematic/extraneous converting constructor templates from the `meow_view` range adaptor types in the standard library with the exception of `common_view`. The omission of `common_view` seems to have been simply an oversight: its converting constructor template is no less problematic or extraneous. We should remove `common_view`'s converting constructor template as well to finish the task. Both `cmcstl2` and `range-v3` removed the converting constructor template from `common_view` when removing the other converting constructor templates, so we have implementation experience that this change is good as well as consistent with the general thrust of LWG 3280.

[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 26.7.19.2 [range.common.view], class template `common_view` synopsis, as indicated:

   >       […]
   >       constexpr explicit common_view(V r);

   >       template<viewable_range R>
   >         requires (!common_range<R> && constructible_from<V, all_view<R>>)
   >       constexpr explicit common_view(R&& r);

   >       constexpr V base() const& requires copy_constructible<V> { return base_; }
   >       […]

   > […]

   >     template<viewable_range R>
   >       requires (!common_range<R> && constructible_from<V, all_view<R>>)
   >     constexpr explicit common_view(R&& r);

   > > -2- Effects: Initializes `base_` with `views::all(std::forward<R>(r))`.