When iota_view receives different argument types, .end() returns a sentinel instead of an iterator.
A sentinel is a type that's ==-comparable with an iterator, and optionally subtractible from it (the latter makes it a sized sentinel).
If it happens to not be subtractible, ranges::partition_point can't know the range size in advance, and has to increment the iterator step by step (almost as if it wasn't random-access).
For some reason, for iota_view<A,B> this sentinel is specified to overload - only if std::sized_sentinel_for<B, A> == true, which is false, because neither is an iterator in this case (int and long long).