Fields with JsonldPredicate ID "@type" are not resolved to rdf:type in RDF

[stmbaier]

In the CWL schema definition files (like CommandLineTool.yml, Process.yml, etc.) multiple times fields are declared as having the predicate ID rdf:type like so:

fields:
    - name: class
      jsonldPredicate:
        "_id": "@type"
        "_type": "@vocab"

For CWL files, which are instances of the CWL schema, this works totally fine and leads to the desired instance-of relation in RDF, e.g. class: CommandLineTool in CWL YAML gets transformed to a cwl:CommandLineTool in RDF.

But if the CWL schema itself is transformed to RDF with SALAD and the metaschema.yml the ID of JsonldPredicate object of the field is tranformed to sld:_id <file:///LOCAL/PATH/TO/CWL-SCHEMA-DIRECTORY/@type>, which is clearly ill-formed and should instead also lead to rdf:type.
According to the SALAD schema definition the _id of JsonldPredicate is resolved according to vocabulary resolution, which starts with link resolution. And as the link resolution rules do not state any special cases for URIs starting with @ the interpretation as relative reference seems legit and leads to the ill-formed predicate IRI.
So in my eyes the CWL schema files must not use '@type' for _id of JsonldPredicates, but instead should use rdf:type (+ rdf namespace definition).

Is this analysis correct and its a bug or have I missed anything from the definitions?

[tetron]

I think this is right, it isn't being handled correctly.

One way to fix this in the metaschema would be to introduce some new enum types. This would introduce @type, @id and @vocab into the vocabulary.

eg

        type: enum
        name: JsonldPredicateIdEnum
        symbols:
          - "sld:@type"

        type: enum
        name: JsonldPredicateTypeEnum
        symbols:
          - "sld:@id"
          - "sld:@vocab"

And then

- name: JsonldPredicate
  type: record
  doc: |
    Attached to a record field to define how the parent record field is handled for
    URI resolution and JSON-LD context generation.
  fields:
    - name: _id
      type: ["null", JsonldPredicateIdEnum, string]
      jsonldPredicate:
        _id: sld:_id
        _type: "@id"
        identity: true
      doc: |
        The predicate URI that this field corresponds to.
        Corresponds to JSON-LD `@id` directive.
    - name: _type
      type: ["null", JsonldPredicateTypeEnum, string]