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3361. safe_range<SomeRange&> case

Section: 26.4.2 [range.range] Status: WP Submitter: Johel Ernesto Guerrero Peña Opened: 2019-12-19 Last modified: 2021-10-14

Priority: 3

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Discussion:

26.5.5 [range.dangling] p2 hints at how safe_range should allow lvalue ranges to model it. However, its wording doesn't take into account that case.

[2020-01 Priority set to 3 after review on the reflector.]

Previous resolution [SUPERSEDED]:

This wording is relative to N4842.

1. Modify 26.4.2 [range.range] as indicated:

   template<class T>
     concept safe_range =
       range<T> &&
         (is_lvalue_reference_v<T> || enable_safe_range<remove_cvref_t<T>>);
   

   -5- Given an expression E such that decltype((E)) is T,A type T models safe_range only if:

   1. (5.1) — is_lvalue_reference_v<T> is true, or

   2. (5.2) — given an expression E such that decltype((E)) is T, if the validity of iterators obtained from the object denoted by E is not tied to the lifetime of that object.

   -6- [Note: Since the validity of iterators is not tied to the lifetime of an object whose type models safe_range, aA function can accept arguments of such a type that models safe_range by value and return iterators obtained from it without danger of dangling. — end note]

[2021-05-19 Tim updates wording]

The new wording below attempts to keep the "borrowed" property generally applicable to all models of borrowed_range, instead of bluntly carving out lvalue reference types.

[2021-09-20; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll in June.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 26.4.2 [range.range] as indicated:

   template<class T>
     concept borrowed_range =
       range<T> &&
         (is_lvalue_reference_v<T> || enable_borrowed_range<remove_cvref_t<T>>);
   

   -5- Let U be remove_reference_t<T> if T is an rvalue reference type, and T otherwise. Given an expression E such that decltype((E)) is T a variable u of type U, T models borrowed_range only if the validity of iterators obtained from the object denoted by E u is not tied to the lifetime of that object variable.

   -6- [Note: Since the validity of iterators is not tied to the lifetime of an objecta variable whose type models borrowed_range, a function can accept arguments of with a parameter of such a type by value and can return iterators obtained from it without danger of dangling. — end note]