Broken forEach on CovariantIdentityBoth

[felher]

Hey all,

using forEach on a bunch of data types seems to give the wrong result:

println(List(List(1, 2), List(3)).forEach(identity))

gives List(List(1, 2, 3, 3), List(1, 2, 3, 3)), while it should give List(List(1, 3), List(2, 3)) or something like that.

The problem seems to be in trait CovariantIdentityBoth in "coherent.scala":

def forEach[A, B, Collection[+Element] <: Iterable[Element]](in: Collection[A])(f: A => F[B])(implicit
    bf: BuildFrom[Collection[A], B, Collection[B]]
  ): F[Collection[B]] =
    in.foldLeft(bf.newBuilder(in).succeed)((bs, a) => bs.zipWith(f(a))(_ += _)).map(_.result())

Here, a mutable builder is used to collect the results. _ += _ is passed to zipWith which then passes it to map. But functions given to map on Covariant can be called multiple times, as is the case for List. Also, both resulting sublists are built from the same shared builder, since += on a builder returns the same mutable builder, as opposed to say :+ on Vector, which returns a new one.