[range.approximately.sized]

Given an lvalue t of type remove_reference_t<T>, T models approximately_sized_range only if

(2.1)
ranges::reserve_hint(t) is amortized $O (1), does not modify t, and has a value that is not negative and is representable in range_difference_t < T >, and$
(2.2)
if iterator_t<T> models forward_iterator, ranges::reserve_hint(t) is well-defined regardless of the evaluation of ranges::begin(t).
[Note 1:
ranges::reserve_hint(t) is otherwise not required to be well-defined after evaluating ranges::begin(t).

For example, it is possible for ranges::reserve_hint(t) to be well-defined for an approximately_sized_range whose iterator type does not model forward_iterator only if evaluated before the first call to ranges::begin(t).
— end note]