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### 3358. §[span.cons] is mistaken that `to_address` can throw

Section: 24.7.3.2 [span.cons] Status: C++20 Submitter: Casey Carter Opened: 2019-12-10 Last modified: 2021-02-25

Priority: 0

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Discussion:

[span.cons] paragraphs 6 and 9:

> Throws: When and what `to_address(first)` throws.

could equivalently be "Throws: Nothing." since all overloads of `std::to_address` are `noexcept`. However, paragraph 9 fails to account for the fact that paragraph 8:

> Effects: Initializes `data_` with `to_address(first)` and `size_` with `last - first`.

must evaluate `last - first`.

[2020-01-14 Status set to Tentatively Ready after ten positive votes on the reflector.]

[2020-01-14; Daniel comments]

The fixed wording in 24.7.3.2 [span.cons] p9 depends on the no-throw-guarantee of integer-class conversions to integral types. This guarantee is specified by LWG 3367.

Proposed resolution:

This wording is relative to N4842.

1. Modify 24.7.3.2 [span.cons] paragraphs 6 and 9 as indicated:

   > [Drafting note:

   > 1. The missing paragraph number of the Preconditions element at p7/p8 has already been reported as editorial issue

   > 2. The effective change to "Throws: Nothing." in p6 has already been applied editorially.

   > ]

   >     template<class It>
   >       constexpr span(It first, size_type count);

   > > […]

   > > -4- Preconditions: […]

   > > -5- Effects: Initializes `data_` with `to_address(first)` and `size_` with `count`.

   > > -6- Throws: When and what `to_address(first)` throws Nothing.

   >     template<class It, class End>
   >       constexpr span(It first, End last);

   > > […]

   > > -?- Preconditions: […]

   > > -8- Effects: Initializes `data_` with `to_address(first)` and `size_` with `last - first`.

   > > -9- Throws: When and what `to_address(first)last - first` throws.