unexpected "var requires evaluation" error from topdown

[tsandall]

Given the following policy:

package x

p[a] {
    a := q
}

q[b] {
    b := 1
}

This query generates an error:

$ opa eval -b . -i input.json 'data.x.p[_]'
{
  "errors": [
    {
      "message": "var requires evaluation"
    }
  ]
}

[srenatus]

I'm looking into this. 🔍