encoding/jsonschema: cannot define scalar at top level

[rogpeppe]

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

v0.9.2

Does this issue reproduce with the latest stable release?

Yes

What did you do?

exec cue def jsonschema: schema.json
cmp stdout want-stdout

-- want-stdout --
@jsonschema(schema="http://json-schema.org/draft-07/schema#")
@jsonschema(id="https://test.example/foo")
string

-- schema.json --
{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "$id": "https://test.example/foo",
  "type": "string"
}

What did you expect to see?

Passing test.

What did you see instead?

> exec cue def jsonschema: schema.json
[stderr]
constraint not allowed because type object is excluded:
    ./schema.json:3:3
[exit status 1]
FAIL: /tmp/testscript1546751584/x.txtar/script.txtar:1: unexpected command failure

In the jsonschema implementation, the $id implementation constrains the result type to be
an object even though it doesn't have to be.