Unexpected behavior of `NonEmptyList.reduce` for tuples of lists

[kiendang]

NonEmptyList.reduce used the wrong, or at least unexpected Semigroup instance for tuples of lists. I'm not sure if it's due to cats itself or the compiler though.

---

Let's say I want to semigroup combine a couple of tuples of lists

Tuple(List(1, 2)).combine(Tuple(List(5)))

I would get Tuple(List(1, 2, 5)), which is expected.

However if I do

NonEmptyList.of(Tuple(List(1, 2)), Tuple(List(5))).reduce

I would get Tuple(<function>). Looks like my tuples of lists not as tuples of List were inferred as tuples of (Int => Int) which is surprising. I would expect the same Semigroup instance would be used in both examples.

I even got

NonEmptyList.of((List(1, 2), List(1, 2), List(5, 6)), (List(4), List(5), List(7))).reduce

to produce Tuple(List(1, 2, 4),List(1, 2, 5),<function1>) on my local machine using scala-cli on Scala 3.7.4 but I couldn't reproduce elsewhere.

Weirder still, this workaround that looks like it would do nothing

def reduce[T: Semigroup](xs: NonEmptyList[T]) =
  xs.reduce

reduce(Tuple(List(1, 2)).combine(Tuple(List(5))))

produces Tuple(List(1, 2, 5)).

[TheBugYouCantFix]

Are you sure your last code sample works?

scala> def reduce[T: Semigroup](xs: NonEmptyList[T]) =
     |   xs.reduce
     | 
def reduce
  [T]
    (xs: cats.data.NonEmptyList[T])
      (using evidence$1: cats.kernel.Semigroup[T]): T
                                                               
scala> reduce(Tuple(List(1, 2)).combine(Tuple(List(5))))
-- [E007] Type Mismatch Error: -------------------------------------------------
1 |reduce(Tuple(List(1, 2)).combine(Tuple(List(5))))
  |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |       Found:    Tuple1[List[Int]]
  |       Required: cats.data.NonEmptyList[Any]