Fatal error: exception (Failure "Type mismatch (simple checker broken B?)")

[aigarashi]

input:

create_nested_object() {
    let random_v = _ in
    if random_v then
        null
    else {
        let tail = create_nested_object() in
        mkref mkref tail
    }
}

{
    assert(1 = 1)
}

output:

Fatal error: exception (Failure "Type mismatch (simple checker broken B?)")

typechecker output:

/* () -> (μ '0.'0 ref ref) */
create_nested_object() {
  /* random_v: int */
  let:5 random_v = _ in 
  if:4 random_v then {
    /* __t0: (μ '0.'0 ref ref) */
    let:9 __t0 = null in 
    return:0 __t0
  } else {
    /* tail: (μ '0.'0 ref ref) */
    let:3 tail = create_nested_object:1() in 
    /* __t0: (μ '1.'1 ref ref) */
    let:8 __t0 = mkref tail in 
    /* __t1: (μ '0.'0 ref ref) */
    let:7 __t1 = mkref __t0 in 
    return:2 __t1
  }
  
}
{
 assert:6(1 = 1);
 /* __t0: int */
 let:11 __t0 = 0 in 
 ()
}