Case simplify produces invalid BOM

[kavon]

Seems there's a bug in case simplify? I'm using d5dfda20e063ce4c49f82555121ff95fafae1c76

➤ ../trunk-pmlc/bin/pmlc -Cbom.check-all=true fib-datatype.pml 
***** Bogus BOM in Main after case-simplify *****
** invalid cast:(_t<D762>#2:int) = (int)left<F8FC>#5:any
** invalid cast:(_t<D773>#1:int) = (int)right<F8FD>#7:any
** type mismatch in return from fib<F8FA>#3.3
==   expected  any
==   but found int
** type mismatch in return from fib<F8FA>#3.3
==   expected  any
==   but found int
** invalid cast:(_t<D751>#1:int) = (int)right<F8FD>#7:any
** type mismatch in return from fib<F8FA>#3.3
==   expected  any
==   but found int
** type mismatch in return from fib<F8FA>#3.3
==   expected  any
==   but found int
** invalid cast:(_t<D78A>#1:int) = (int)ans<F90E>#3:any
broken BOM dumped to broken-BOM
bom/check-bom.sml:544.11-544.28: Fail: broken BOM

See fib-datatype.pml below:

datatype fib_result
  = Zero
  | One
  | Val of int

fun add a b = (case (a, b)
  of (Zero, b) => b
   | (a, Zero) => a
   | (One, One) => Val 2
   | (One, Val b) => Val (b+1)
   | (Val a, One) => Val (a+1)
   | (Val a, Val b) => Val (a+b)
  (* end case *))

fun show a = (case a
  of Zero => "0"
   | One => "1"
   | Val a => Int.toString a
  (* end case *))

fun fib n = (case n
  of 0 => Zero
   | 1 => One
   | _ => let
        val left = fib (n-1)
        val right = fib (n-2)
      in
        add left right
      end
  (* end case *))

val i2s = Int.toString

val n = 30  (* https://oeis.org/A000045/list *)
val ans = fib n

val _ = print ("fib(" ^ i2s n ^ ") = " ^ show ans ^ "\n")

[kavon]

Replacing the definition of add in the above with

fun asInt a = (case a
  of Zero => 0
   | One => 1
   | Val a => a
  (* end case *))

fun add a b = Val ((asInt a) + (asInt b))

changes the number of type mismatches, but does not solve the issue

➤ ../trunk-pmlc/bin/pmlc -Cbom.check-all=true fib-datatype.pml 
***** Bogus BOM in Main after case-simplify *****
** invalid cast:(_t<D85C>#1:int) = (int)left<F8C4>#3:any
** invalid cast:(_t<D862>#1:int) = (int)right<F8C5>#3:any
** type mismatch in return from fib<F8C2>#3.3
==   expected  any
==   but found int
** invalid cast:(_t<D73A>#1:int) = (int)ans<F8D1>#3:any
broken BOM dumped to broken-BOM
bom/check-bom.sml:544.11-544.28: Fail: broken BOM

-Cbom.enable-unbox=false doesn't change this message either.