cmd/cue: regression: comprehension value must be struct, found *ast.StructLit

[cueckoo]

Originally opened by @myitcv in cuelang/cue#473

What version of CUE are you using (cue version)?

$ cue version
cue version +304b02e linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

-- cue.mod/module.cue --
module: "mod.com"
-- x.cue --
package x

#Guide: {
        Terminals: [string]: #Terminal

        Steps: [string]: #Step

        #TerminalName: or([ for k, _ in Terminals {k}])

        #Step: {
                Terminal: #TerminalName
                Cmd:      string
        }

        #Terminal: {
                Image: string
        }
}

g: #Guide & {
        Terminals: client: {
                Image: "golang"
        }

        Steps: {
                list: {
                        Terminal: "client"
                        Cmd:      "ls"
                }
        }
}

Run:

cue eval --out json

What did you expect to see?

JSON output.

What did you see instead?

Error:

#Guide.#TerminalName.for[]: comprehension value must be struct, found *ast.StructLit

Compare with v0.2.2 which does correctly output the JSON representation.

With 304b02e the --out json flag does work if you add -c:

$ cue eval -c --out json
{
    "g": {
        "Terminals": {
            "client": {
                "Image": "golang"
            }
        },
        "Steps": {
            "list": {
                "Terminal": "client",
                "Cmd": "ls"
            }
        }
    }
}

[cueckoo]

Original reply by @mpvl in cuelang/cue#473 (comment)

For documentation purposes: this turns out to be an export bug!!! What happens with the --out mode is that it triggers several translation steps exporting to and from CUE and then to JSON. The cause is a bug in the exporter, which caused the reading back to fail.

The back and forth should probably be eliminated as well.