Foldable.dropWhile_() is not working as advertised

[weihsiu]

this is what i get when using Foldable.dropWhile_() in the repl. the results do not conform with the spec.

scala> Foldable[List].dropWhile_(List(2, 4, 5))(_ % 2 == 0)
res46: List[Int] = List(2, 4, 5)

scala> // should return List(5)

scala> Foldable[List].dropWhile_(List(1, 2, 4, 5))(_ % 2 == 0)
res47: List[Int] = List(2, 4, 5)

scala> // should return List(1, 2, 4, 5)

[ceedubs]

Thanks for spotting this and creating an issue @weihsiu. The fix itself is pretty easy (adding a ! in the implementation). However when I went to make this fix, I noticed that this probably should have been caught by https://github.com/non/cats/blob/master/tests/src/test/scala/cats/tests/FoldableTests.scala#L32 but this FoldableCheck class isn't actually used anywhere. @non do you think those tests should be moved into the FoldableTests in the laws module? The Iterator requirement is a bit outside of the norm for law tests, but we should certainly add these sort of checks in one way or another. Another option is just using toList or toStreaming on the F[A] instance and checking for consistency instead of requiring an Iterator.