Defined in header
inline constexpr bool disable_sized_range = false;
1) The SizedRange concept specifies the requirements of a
Rangetype that knows its size in constant time with the size function.
- has non-amortized constant-time complexity,
- does not modify t, and
- is equal to ranges::distance(t), and
- if iterator_t<T> models
ForwardIterator, ranges::size(t) is well-defined regardless of the evaluation of ranges::begin(t) (in other words, a single-pass sized range may support a call to size only before the first call to begin, but a forward range must support size at all times)
2) disable_sized_range exists to allows use of range types that satisfy but do not in fact model SizedRange.