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3351. ranges::enable_safe_range should not be constrained

Section: 25.2 [ranges.syn] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-12-05 Last modified: 2021-02-25

Priority: 0

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Discussion:

Currently ranges::enable_safe_range is constrained with ranges::range, which not only forces the compiler to do unnecessary satisfaction checking when it's used, but also creates a tricky dependency cycle (ranges::range depends on ranges::begin which depends on ranges::enable_safe_range which depends on ranges::range).

The only place the variable template is expected to be used is in the ranges::safe_range concept, which already checks range<T> before using enable_safe_range<T> anyway.

The constraint serves no purpose and should be removed.

[2019-12-12 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.

Proposed resolution:

This wording is relative to N4842.

1. Modify 25.2 [ranges.syn] as indicated:

   [Drafting note: The definition in 25.4.2 [range.range] p7 is already unconstrained, which contradicts the synopsis.]

   […]
   // 25.4.2 [range.range], ranges
   template<class T>
   concept range = see below;
   
   template<rangeclass T>
   inline constexpr bool enable_safe_range = false;
   
   template<class T>
   concept safe_range = see below;
   […]