[iterator.operations] clarify preconditions on std::distance

[RedBeard0531]

The prior wording implied that the two sides of the "or" were independent, so as long as last was reachable from first, first didn't need to be reachable from last even if passing a RandomAccessIterator. It seems clear that it is really trying to express two different possible preconditions depending on whether a RandomAccessIterator is passed or not. I used the same phrasing as in the "Effects:" clause to separate the cases.

Alternatively, this precondition seems redundant with http://eel.is/c++draft/iterators#iterator.requirements.general-12, so maybe it should just be removed? This seems to be the only function in [iterators], [ranges], and [algorithms] that has an explicit precondition that iterators passed to it are reachable.

[jwakely]

Reachable is defined in terms of ++, so it's wrong to require that first is reachable from last for all random access iterators.

[jwakely]

The prior wording implied that the two sides of the "or" were independent, so as long as last was reachable from first, first didn't need to be reachable from last even if passing a RandomAccessIterator.

Which is correct. Unless they are equal (in which case they are both reachable from the other by zero applications of ++) then it's impossible for them both to be reachable from the other (except in the special case of a circular buffer with no end iterator?)

[RedBeard0531]

Ahh, I think I see now. It is trying to make distance(vector.end(), vector.begin()) well defined, without making distance(list.end(), list.begin()) well defined. Maybe something like this would make that more clear?

Either [first, last) is a valid range, or [last, first) is a valid range and InputIterator meets the Cpp17RandomAccessIterator requirements

[CaseyCarter]

Maybe something like this would make that more clear?

Either [first, last) is a valid range, or [last, first) is a valid range and InputIterator meets the Cpp17RandomAccessIterator requirements

"Either" suggests these two cases are exclusive, which suggests to me that distance(x, x) might be invalid since both [first, last) and [last, first) are valid ranges. I do think that replacing "woof is reachable from meow" with "[meow, woof) is a valid range" is an improvement. Could you live with this sentence without the "Either" at the beginning?

[tkoeppe]

@RedBeard0531: could you make the changes as Jon and Casey suggest?

[tkoeppe]

@RedBeard0531 Ping :-)