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Section: 26.7.21 [range.reverse] Status: SG9 Submitter: Barry Revzin Opened: 2023-11-25 Last modified: 2024-06-28
Priority: 3
View all other issues in [range.reverse].
Discussion:
Consider the following:
auto a = views::iota(0) | views::reverse; auto b = views::repeat(42) | views::reverse;
Here, views::iota(0) and views::repeat(42) are both non-common bidirectional (even random-access) ranges.
They are also infinite ranges, even if the standard doesn't really recognize that.
views::reverse on a non-common range will actually compute the end iterator for you. So while both declarations
of a and b above compile, attempting to use either in any way will lead to an infinite loop when you
try a.begin() or b.begin().
A reddit post suggested disallowing reversing a non-common range but that likely breaks reasonable use-cases. We could at
the very least recognize ranges whose sentinel is unreachable_t and reject those from consideration. For instance,
we could change 25.4.4.4 [range.iter.op.next]/3 to Mandate that S is not unreachable_t.
[2024-03-11; Reflector poll]
Set priority to 3 after reflector poll. Ask SG9 to look. Probably needs a paper.
Infinite ranges are invalid and giving an invalid range to the library is undefined. But this is not a particularly satisfactory answer given that we provide such ranges ourselves...
[St. Louis 2024-06-28; LWG and SG9 joint session]
Poll: SG9 and LWG believe this is not a defect?
|SF| F| N| A|SA| | 3| 3| 0| 2| 0|Weak consensus => needs to go to LEWG
Poll: SG9 and LWG agree that standard could do something to create fewer sharp edges here, and we encourage a paper exploring options.
|SF| F| N| A|SA| | 4| 2| 2| 0| 0|
Proposed resolution: