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### 3596. `ranges::starts_with` and `ranges::ends_with` are underspecified

Section: 25.6.14 [alg.starts.with], 25.6.15 [alg.ends.with] Status: NAD Submitter: Michael Schellenberger Costa Opened: 2021-09-17 Last modified: 2021-09-24

Priority: Not Prioritized

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Discussion:

When implementing C++23 new `ranges::starts_with` and `ranges::ends_with` for MSVC we came around some issues regarding infinite ranges.

In short, there is no possible answers to `ends_with`, if the second range is infinite. Similarly, there is no meaningful answer to `starts_with` if both ranges are infinite.

We should constraint both algorithms to forbid those corner cases (preferred resolution), or explicitly specify a given return value for those (The reasonable choice would be "`return false;`")

[2021-09-24 infinite ranges are invalid by [iterator.requirements.general] p10, so undefined by p12. Might be nice to relax this in future, but needs a paper. Status changed: New → NAD.]

Proposed resolution: