[Bug libstdc++/122224] std::prev when called with non-bidirectional iterators

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122224

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to M Welinder from comment #0)
> Created attachment 62525 [details]
> Preprocessed source
> 
> Calling std::prev on a iterator that isn't bidirectional is wrong.

That's unclear: https://cplusplus.github.io/LWG/issue3197

> Should this compile?  (See also bug 122194.)

I think so, yes. std::prev(it, n) is equivalent to std::advance(it, -n) and so
if you do std::prev(it, -1) then it's the same as ++it which doesn't need a
bidirectional iterator.

So we accept calling std::prev(it, n) on input iterators, but assert if n >= 0.

> For me, it compiles.  Without -O, it asserts.  With -O, I get an infinite
> loop.  The former is better than the latter but a compile error would be
> better.

Without -O you get -D_GLIBCXX_ASSERTIONS by default, and there's an assertion
that