! cue cmd ref
! stdout .
cmp stderr cmd_badfields.out

-- cmd_badfields.out --
command.ref.task.display.contents: invalid bytes argument: non-concrete value (string|bytes):
    ./task_tool.cue:6:8
    tool/file:17:3
command.ref.task.display.filename: invalid string argument: non-concrete value string:
    ./task_tool.cue:6:8
    ./task_tool.cue:7:9
    tool/file:15:3
-- task_tool.cue --
package home

import "tool/file"

command: ref: {
	task: display: file.Create & {
        filename: filename
	}
}