Crash due to "unknown constant"

[marcoeilers]

The following program crashes Silicon:

import <decreases/int.vpr>
import <decreases/set.vpr>
import <decreases/seq.vpr>

field left : Ref // true case
field right: Ref // false case
field id   : Int // var from code
field val  : Bool

function isLeaf(r:Ref) : Bool // type information

method allocLeaf() returns (r: Ref)
  ensures isLeaf(r) && acc(r.val) 
  decreases 0

method allocCompound() returns (r: Ref)
  ensures !isLeaf(r) && acc(r.left) && acc(r.right) && acc(r.id)
  decreases 0

define nodePerms(r) 
  (isLeaf(r) ? acc(r.val) : acc(r.left) && acc(r.right) && acc(r.id))

define mapInv(m) (forall k1: Ref, k2: Ref :: {k1 in m,k2 in m}
  (k1 != k2 && k1 in m && k2 in m ==> m[k1] != m[k2])) // no dups
  && forall k: Ref :: {k in m}{k in range(m)} k in m ==> nodePerms(m[k]) && m[k] == k && 
  (!isLeaf(k) ==> k.left in m && k.right in m)


field table : Map[Ref,Ref]

define BDD(this) acc(this.table) && mapInv(this.table)


// we use this to abstract over the hashMap get function itself
function tableGet(this:Ref, node:Ref) : Ref
  requires BDD(this) && (!(node in this.table) ==> nodePerms(node))
  ensures result != null ==> result in range(this.table) && toKNode(result) == toKNode(node)
  ensures result == null ==> !isLeaf(node) && forall k:Ref :: {k in this.table} k in this.table ==> toKNode(k) != toKNode(node)

method getNode(this:Ref, node:Ref) returns (res:Ref)
  requires BDD(this) && (nodePerms(node)) && (!isLeaf(node) ==> node.left in this.table && node.right in this.table) // TODO? Maybe (!(toKNode(node) in this.table) ==> nodePerms(node))
  ensures BDD(this) && res in this.table && forall k: Ref :: {k in this.table} old(k in this.table) ==> k in this.table && this.table[k] == old(this.table[k])
  ensures isLeaf(node) ==> isLeaf(res) && res.val == old(node.val)
  decreases 0 // termination is trivial
  {
    var returned : Bool := false// models whether we hit "return" already
    if(!isLeaf(node)) {         // if (node instanceof CompoundNode) {
      var that : Ref := node    // no need to "cast" - type info is in logic
      if (that.left == that.right) {// if (that.low == that.high) 
        res := that.left;        // return that.low;
        returned := true         // skip rest of code
      }
    } 
    if(!returned) {              // to model early return
      res := tableGet(this, node) // Node result = table.get(node);
      
        if (res == null) {
            node.id := |this.table|  // table.size();
            this.table := this.table[node := node] // table.put(node, node);
            res := node         // return node;
        }
      }
    }
  


///BOUNDARY///

//

function isTrue(n: Ref): Bool
  requires nodePerms(n)
{
  isLeaf(n) && n.val
}

function isFalse(n: Ref): Bool
  requires nodePerms(n)
{
  isLeaf(n) && !n.val
}

method node1(this: Ref, v: Bool) returns (res: Ref)
  requires BDD(this)
  ensures BDD(this) && res in range(this.table) && isLeaf(res) && res.val == v
  ensures forall k: Ref :: {k in this.table} old(k in this.table) ==> k in this.table && this.table[k] == old(this.table[k])
  decreases 1
{
  var l: Ref
  l := allocLeaf()
  l.val := v
  res := getNode(this, l)
}

method node2(this: Ref, v: Int, l: Ref, r: Ref) returns (res: Ref)
  requires BDD(this) && l in range(this.table) && r in range(this.table)
  ensures BDD(this) 
  //ensures res in range(this.table) && !isLeaf(res) // && TODO
  decreases 1
{
  var c: Ref
  c := allocCompound()
  c.left := l
  c.right := r
  c.id := v
  res := getNode(this, c)
}

method not(this: Ref, node: Ref) returns (res: Ref)
  requires BDD(this) && node in range(this.table)
  ensures BDD(this)
  decreases 2
  //ensures forall s: State :: eval(toPNode(this, res), s) == !(eval(toPNode(this, node), s))
{
  if (isTrue(node)) {
    assert forall s: State :: eval(toPNode(this, node), s)
    res := node1(this, false)
    //assert forall s: State :: eval(toPNode(this, node), s)
    //assert forall s: State :: !eval(toPNode(this, res), s) 

  } else {
    if (isFalse(node)) {
      res := node1(this, true)
      assert forall s: State :: eval(toPNode(this, res), s) 
    } else {
      res := node2(this, node.id, node.left, node.right)
    }
  }
}

function toKNode(r: Ref): KNode
  requires isLeaf(r) ==> acc(r.val)
  requires !isLeaf(r) ==> acc(r.left) && acc(r.right) && acc(r.id)
{
  isLeaf(r) ?
    (KLeaf(r.val)) :
    (KCompound(r.id, r.left, r.right))
}


adt KNode {
  KLeaf(v: Bool)
  KCompound(i: Int, kleft: Ref, kright: Ref)
}

adt PNode {
    PLeafTrue()
    PLeafFalse()
    PCompound(vl: Int, lft: PNode, rght: PNode)
}

function toPNode(bdd: Ref, n: Ref): PNode
  requires BDD(bdd) && n in range(bdd.table)
{
  isLeaf(n) ?
    (n.val ? PLeafTrue() : PLeafFalse()) :
    (PCompound(n.id, toPNode(bdd, n.left), toPNode(bdd, n.right)))
}

domain State {
  function getVal(s: State, v: Int): Bool
}

domain Evaluator {
  
  function eval(n: PNode, s: State): Bool

  axiom {
    forall s: State :: { eval(PLeafTrue(), s) } eval(PLeafTrue(), s)
  }

  axiom {
    forall s: State :: { eval(PLeafFalse(), s) } !eval(PLeafFalse(), s)
  }

  axiom {
    forall s: State, n1: PNode, n2: PNode, vall: Int :: { eval(PCompound(vall, n1, n2), s) } 
      eval(PCompound(vall, n1, n2), s) == (getVal(s, vall) ? eval(n1, s) : eval(n2, s) )
  }
}

If I remember correctly, the issue was that some heap-dependent expression (this.table?) occurs twice in the postcondition of tableGet, once under a quantifier. When translating the other occurrence, the guards refer to the quantified variable, which does not exist in this context.