[ 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...

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 [range.iter.op.next]/3 to Mandate that S is not unreachable_t.