Proposed resolution:

This wording is relative to N4830.

1. Modify [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); };
   

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

Status to Ready

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

disable_sized_range ([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); };

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);