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Added sanity check to lub calculation to prevent invalid lubs from
emerging. The underlying cause of said lubs is that higher-order
type parameters are not handled correctly: this is why the issue
is seen so frequently in the collections. See pending test
pending/pos/those-kinds-are-high.scala for a demonstration. Until that's
fixed, we can at least raise the bar a bit.
Closes #2094, #2322, #4501. Also, some test cases in neg have been
promoted into working programs: #2179, #3774. (They're not in neg for
the "shouldn't work" reason, but out of despair.)
In some cases, such as the original reported ticket in #3528, this
only pushes the problem downfield: it still fails due to inferred type
parameters not conforming to bounds. I believe a similar issue with
higher-order type parameters underlies that.
Look at how far this takes us though. All kinds of stuff which did not
work, now works. None of these even compiled until now:
scala> :type List(mutable.Map(1 -> 1), immutable.Map(1 -> 1))
List[scala.collection.Map[Int,Int]]
scala> :type Set(List(1), mutable.Map(1 -> 1))
scala.collection.Set[Iterable[Any] with PartialFunction[Int,Int]]
scala> :type Stream(List(1), Set(1), 1 to 5)
Stream[Iterable[Int] with Int => AnyVal{def getClass(): Class[_ >: Int with Boolean <: AnyVal]}]
scala> :type Map(1 -> (1 to 10), 2 -> (1 to 10).toList)
scala.collection.immutable.Map[Int,scala.collection.immutable.Seq[Int]
]
PERFORMANCE: compiling quick.lib and quick.comp, this patch results in
an extra 27 subtype tests. Total. Time difference too small to measure.
However to be on the safe side I made it really easy to disable.
private final val verifyLubs = true // set to false
Review by moors, odersky.
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