subrange: must end be reachable from begin?

Is it kosher to construct a subrange for which end is not reachable from begin? Say, if begin is reachable from end instead? And size() returns a negative number?

Probably not. If that is the case, we should require that [begin,end) denotes a valid range.

Proposed Resolution

Change [range.subrange.ctor] as follows:

 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).

It it kosher to construct a subrange for which end is not reachable from begin?

Technically, yes it is. The resulting subrange wouldn't be in the domain of Range, however, since [range.range]/3.1 requires that "[ranges::begin(E), ranges::end(E)) denotes a range ([iterator.requirements.general])".

This seems like the sort of weirdness we should forbid.

PR for your perusal.

This issue is now LWG 3179.

Does this mean subrange(iota_view::iterator(0u), unreachable_sentinel) (or subrange(iota(0u))) is disallowed?

No, those are fine.

"Being a valid range" imposes the reachability condition, but [iota_view::iterator(0u), unreachable_sentinel) does not satisfy the reachability condition. Thus, after this PR is applied, subrange(iota_view::iterator(0u), unreachable_sentinel) becomes UB.

Or, I may be misunderstanding the validity of ranges / unreachable_sentinel.

We have a known issue regarding the definition of valid range in terms of reachability.

Ah, ok, understood. Thanks for the reply!

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subrange: must end be reachable from begin? #597

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subrange: must end be reachable from begin? #597

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CaseyCarter commented on Dec 19, 2018

CaseyCarter commented on Dec 21, 2018

morinmorin commented on Dec 30, 2018

ericniebler commented on Dec 30, 2018

morinmorin commented on Dec 30, 2018

ericniebler commented on Dec 30, 2018

morinmorin commented on Dec 30, 2018

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