[11630] Iterator#flatMap#hasNext calls outer#hasNext 1 time, not 2-3 times

[joshlemer]

Fixes scala/bug#11630

When doing two calls in succession to Iterator#flatMap:

it.hasNext()
it.next()

the status quo is to have the flatmapped iterator call the sub-iterator's hasNext 3 times:

• once during the "advance to next non-empty sub-iterator, if required, and return it" phase
• a second time, to return that now-current sub-iterator's hasNext to the outside caller
• a third time, when calling next()

This PR replaces that logic with a system where these 3 calls are replaced with a single call during the it.hasNext phase, which is achieved by caching the state of the iterator in a new private field _hasNext: Int which can either be:

• 0 = known to be false (hasNext is known to be false)
• 1 = known to be true (hasNext is known to be true true)
• -1 = unknown (not known if hasNext would be true or false)

The resulting state of the iterator after a hasNext call is either 0 or 1. After a call to next() we won't know if we have exhausted the entire iterator or not, since we are not forcing any more work to be done, so we set our state to -1.

This partially was a port of the old 2.12 implementation, but that implementation still required 2 calls to cur.hasNext, whereas this PR represents the additional improvement to only call cur.hasNext once during a hasNext(); next(); call sequence.

TODO

• Write some unit tests

[hrhino]

I would have expected the "regression" label to be a badge of shame attached to released PRs.

[lrytz]

Thanks, @joshlemer!