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Section: 26.5.4.2 [range.subrange.ctor] Status: C++20 Submitter: Casey Carter Opened: 2018-12-21 Last modified: 2021-02-25
Priority: 0
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Discussion:
The constructors of subrange place no requirements on the iterator and sentinel values from which a subrange is constructed. They allow constructions like subrange{myvec.end(), myvec.begin()} in which the resulting subrange isn't in the domain of the Range concept which requires that "[ranges::begin(r), ranges::end(r)) denotes a range" ([range.range]/3.1). We should forbid the construction of abominable values like this to enforce the useful semantic that values of subrange are always in the domain of Range.
(From ericniebler/stl2#597.)
[2019-01-20 Reflector prioritization]
Set Priority to 0 and status to Tentatively Ready
Proposed resolution:
This wording is relative to N4791.
Change 26.5.4.2 [range.subrange.ctor] as indicated:
constexpr subrange(I i, S s) requires (!StoreSize);-?- Expects: [i, s) is a valid range.
-1- Effects: Initializes begin_ with i and end_ with s.
constexpr subrange(I i, S s, iter_difference_t<I> n) requires (K == subrange_kind::sized);-2- Expects: [i, s) is a valid range, and n == ranges::distance(i, s).