Issue #560

-- cue.mod/module.cue --
module: "example.com"

-- in.cue --
import "strings"

#User: {
    tags_str: string
    tags_map: {
         for k, v in strings.Split(tags_str, " ") {
             "\(v)": string
         }
         "{a}": string
    }
}

user: {
    #User
    tags_str: "b {c}"
}
-- out/eval --
(struct){
  #User: (#struct){
    tags_str: (string){ string }
    tags_map: (_|_){
      // [incomplete] error in call to strings.Split: non-concrete value string:
      //     ./in.cue:6:22
      "{a}": (string){ string }
    }
  }
  user: (#struct){
    tags_str: (string){ "b {c}" }
    tags_map: (#struct){
      "{a}": (string){ string }
      b: (string){ string }
      "{c}": (string){ string }
    }
  }
}
-- out/compile --
--- in.cue
{
  #User: {
    tags_str: string
    tags_map: {
      for k, v in 〈import;strings〉.Split(〈1;tags_str〉, " ") {
        "\(〈1;v〉)": string
      }
      "{a}": string
    }
  }
  user: {
    〈1;#User〉
    tags_str: "b {c}"
  }
}