@lorentey, are Iterators Sequences? Is there a special lazy flatMap?

@rudkx, do you know of anything that changed here?

In 4.1, barIterator is inferred to be a FlattenIterator<LazyMapIterator<IndexingIterator<Array<Foo>>, [Bar]>>, whereas in 4.2 it is inferred to be a FlattenSequence<LazyMapCollection<[Foo], [Bar]>>.Iterator.

Offhand I don't know if that's a result of a standard library change or a something that happens differently during expression type checking.

Isn't `FlattenIterator<LazyMapIterator<IndexingIterator<Array<Foo>>, [Bar]>>` the desugared form of `FlattenSequence<LazyMapCollection<[Foo], [Bar]>>.Iterator`? In 4.2 we made type alias types preserve sugar in more cases. However this should not have changed behavior.

Actually I'm wrong, `FlattenSequence.Iterator` is now a bona-fide struct.

Ok I see. In 4.1, FlattenIterator conformed to Sequence too. We have a default implementation of Sequence for Self : IteratorProtocol where makeIterator() just returns self.

Adding this conformance to the original code example makes it compile:

extension FlattenSequence.Iterator : Sequence {}

However I think it makes more sense to change the last two lines to:

var barSequence = foos.lazy.flatMap { $0.bars }
bars.append(contentsOf: barSequence)

That is, just dropping the makeIterator() call.

@airspeedswift do you want to add the "FlattenSequence.Iterator : Sequence" conformance, or just close this as working as expected?

@slavapestov Thanks for the 1st workaround. I don't use the 2nd because I iterate barIterator using next() and stops when some condition is met and append the rest to bars. I can, by manually keeping track of indices(I'm actually iterating 2 such iterators, I'm doing merging), but I think the iterator version is more elegant. @airspeedswift I think Sequence.Iterator should be a sequence.

@slavapestov I have issues with the 1st workaround: since #if swift(>=4.2) tests language version instead of compiler version, how can I make my project compilable in both Xcode 9.4 and Xcode 10?

@swift-ci create