3948. possibly-const-range and as-const-pointer should be noexcept

Section: 25.2 [ranges.syn], 25.3.14 [range.prim.cdata] Status: WP Submitter: Jiang An Opened: 2023-06-20 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

As of P2278R4, several range access CPOs are specified with possibly-const-range and as-const-pointer. These helper functions never throw exceptions, but are not marked with noexcept. As a result, implementations are currently allowed to make a call to ranges::ccpo potentially throwing while the underlying ranges::cpo call is non-throwing, which doesn't seem to be intended.

[2023-10-27; Reflector poll]

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

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 25.2 [ranges.syn], header <ranges> synopsis, as indicated:

   […]
   // 25.7.22 [range.as.const], as const view
   template<input_range R>
     constexpr auto& possibly-const-range(R& r) noexcept {      // exposition only
       if constexpr (constant_range<const R> && !constant_range<R>) {
         return const_cast<const R&>(r);
       } else {
         return r;
       }
     }
   […]
   

2. Modify 25.3.14 [range.prim.cdata] before p1 as indicated:

   template<class T>
   constexpr auto as-const-pointer(const T* p) noexcept { return p; }    // exposition only