PE with shallow inlining producing incorrect result for some function calls

[tsandall]

Given the following policy:

package x

p {
  y = f(1)
  count(y)
}

f(x) = [] {
    _ = input  # anything dependent on an unknown will do
}

PE w/ shallow inlining produces the following:

$ docker run -it --rm -v  $PWD:/src -w /src openpolicyagent/opa:0.30.2 eval -f pretty -d ./x.rego 'data.x.p' -p  -u input  --explain=full --shallow-inlining
query:1        Enter data.x.p
query:1        | Eval data.x.p
query:1        | Index data.x.p (matched 1 rule)
./x.rego:3     | Enter data.x.p
./x.rego:4     | | Eval data.x.f(1, __local1__)
./x.rego:4     | | Index data.x.f (matched 1 rule)
./x.rego:8     | | Enter data.x.f
./x.rego:9     | | | Eval _ = input
./x.rego:9     | | | Save _ = input
./x.rego:8     | | | Exit data.x.f
./x.rego:8     | | Redo data.x.f
./x.rego:9     | | | Redo _ = input
./x.rego:4     | | Save data.partial.x.f(1, __local1__)
./x.rego:4     | | Eval y = __local1__
./x.rego:5     | | Eval count(y)
./x.rego:5     | | Fail count(y)
./x.rego:4     | | Redo y = __local1__
./x.rego:4     | | Redo data.x.f(1, __local1__)
query:1        | Save data.partial.x.p = _
query:1        | Save _
query:1        | Exit data.x.p
query:1        Redo data.x.p
query:1        | Fail data.x.p
+-----------+------------------------------+
| Query 1   | data.partial.x.p = _term_0_0 |
|           | _term_0_0                    |
+-----------+------------------------------+
| Support 1 | package partial.x            |
|           |                              |
|           | f(__local0__2) = [] {        |
|           |   _ = input                  |
|           | }                            |
+-----------+------------------------------+

Running PE with --strict-builtin-errors reveals the following:

1 error occurred: ./x.rego:5: eval_type_error: count: operand 1 must be one of {array, object, set} but got var

The problem with the trace above is here:

./x.rego:4     | | Eval y = __local1__
./x.rego:5     | | Eval count(y)
./x.rego:5     | | Fail count(y)

In this case, the Fail on L5 is actually due to the type error above. However, count() should either be invoked with [] or it should not be invoked at all and it should be saved.

Note, evaluating without shallow inlining produces the expected answer (_ = input); PE also returns the correct answer when the output contains unknowns if shallow inlining is off (count([input])).