Failure of postcondition shown as failure of the assertion in the branch

[vkuncak]

Trying to verify this:

import stainless.lang.* 
import stainless.collection.*
import stainless.annotation.*

def foldListConc[T,S](l1: List[T], l2: List[T], s0: S, seqop: (S,T) => S): Unit = {
  l1 match
    case Nil() => ()
    case Cons(t1,l1rest) => {
      assert(l1 ++ l2 == Cons(t1, l1rest ++ l2), "head of concat")
      assert((l1 ++ l2).foldLeft(s0)(seqop) == 
             (l1rest ++ l2).foldLeft(seqop(s0, t1))(seqop))
      foldListConc(l1rest, l2, s0, seqop)
    }
} ensuring(_ => (l1 ++ l2).foldLeft(s0)(seqop) == 
                l2.foldLeft(l1.foldLeft(s0)(seqop))(seqop))

gives the error that misleading suggests that an assert has failed, which is not the case, rather the postcondition fails:

warning:  - Result for 'postcondition' VC for foldListConc @11:7:
warning: l1.isInstanceOf[Nil] || ++[T](l1, l2) != Cons[T](l1.h, ++[T](l1.t, l2)) || foldLeft[T, S](++[T](l1, l2), s0, seqop) != foldLeft[T, S](++[T](l1.t, l2), seqop(s0, l1.h), seqop) || {
  val _$1: Unit = foldListConc[T, S](l1.t, l2, s0, seqop)
  foldLeft[T, S](++[T](l1, l2), s0, seqop) == foldLeft[T, S](l2, foldLeft[T, S](l1, s0, seqop), seqop)
}
PositionsError.scala:11:7: warning:  => TIMEOUT
                 assert(l1 ++ l2 == Cons(t1, l1rest ++ l2), "head of concat")
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^