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3264. sized_range and ranges::size redundantly use disable_sized_range

Section: 25.4.3 [range.sized] Status: C++20 Submitter: Casey Carter Opened: 2019-08-26 Last modified: 2021-02-25

Priority: 1

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Discussion:

disable_sized_range (25.4.3 [range.sized]) is an opt-out trait that users may specialize when their range type conforms to the syntax of sized_range but not its semantics, or when the type is so poorly suited to the Standard Library that even testing the validity of the expressions r.size() or size(r) for a range r is impossible. The library inspects disable_sized_range in two places. (1) In the definition of the sized_range concept: 

template<class T>
  concept sized_range =
    range<T> &&
    !disable_sized_range<remove_cvref_t<T>> &&
    requires(T& t) { ranges::size(t); };
If the pertinent specialization of disable_sized_range is true, we avoid checking the validity of the expression ranges::size(t) in the requires-expression. (2) In the definition of the ranges::size CPO itself (25.3.10 [range.prim.size]), the validity of the expressions decay-copy(E.size()) and decay-copy(size(E)) is not checked if the pertinent specialization of disable_sized_range is true.

disable_sized_range is effectively checked twice when evaluating sized_range. This redundancy could be forgiven, if it did not permit the existence of non-sized_ranges for which ranges::size returns a valid size: 

struct mytype {};
using A = mytype[42];

template <>
constexpr bool std::ranges::disable_sized_range<A> = true;

static_assert(std::ranges::range<A>);
static_assert(!std::ranges::sized_range<A>);
static_assert(std::ranges::size(A{}) == 42);

struct myrange {
    constexpr int* begin() const { return nullptr; }
    constexpr int* end() const { return nullptr; }
};

template <>
constexpr bool std::ranges::disable_sized_range<myrange> = true;

static_assert(std::ranges::range<myrange>);
static_assert(!std::ranges::sized_range<myrange>);
static_assert(std::ranges::size(myrange{}) == 0);
We should remove this gap between ranges::size and sized_range by checking disable_sized_range only in the definition of ranges::size, and continuing to rely on the validity of ranges::size in the sized_range concept.

[2019-09-14 Priority set to 1 based on reflector discussion]

[2019-11 Wednesday night issue processing in Belfast.]

Status to Ready

Proposed resolution:

This wording is relative to N4830.

  1. Modify 25.4.3 [range.sized] as follows:

    template<class T>
  concept sized_range =
    range<T> &&
    !disable_sized_range<remove_cvref_t<T>> &&
    requires(T& t) { ranges::size(t); };